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Reference request: Lipid composition in bacterial, yeast and human membranes

Reference request: Lipid composition in bacterial, yeast and human membranes



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I would like to know about the lipid composition of different kinds of cellular membranes. I remember going through such a table once in a paper, but I am unable to find it anymore. What I am looking for is a kind of tabular data which shows the percentage of phosphocholines, cholesterol, sphingolipids and other kinds of lipids organism-wise (and if possible tissue/organ/organelle wise wherever applicable).

So please share the papers/links/books etc where I may find this information. Thank you.


Frontiers in Celland Developmental Biology

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Tuning of a Membrane-Perforating Antimicrobial Peptide to Selectively Target Membranes of Different Lipid Composition

The use of designed antimicrobial peptides as drugs has been impeded by the absence of simple sequence-structure–function relationships and design rules. The likely cause is that many of these peptides permeabilize membranes via highly disordered, heterogeneous mechanisms, forming aggregates without well-defined tertiary or secondary structure. We suggest that the combination of high-throughput library screening with atomistic computer simulations can successfully address this challenge by tuning a previously developed general pore-forming peptide into a selective pore-former for different lipid types. A library of 2916 peptides was designed based on the LDKA template. The library peptides were synthesized and screened using a high-throughput orthogonal vesicle leakage assay. Dyes of different sizes were entrapped inside vesicles with varying lipid composition to simultaneously screen for both pore size and affinity for negatively charged and neutral lipid membranes. From this screen, nine different LDKA variants that have unique activity were selected, sequenced, synthesized, and characterized. Despite the minor sequence changes, each of these peptides has unique functional properties, forming either small or large pores and being selective for either neutral or anionic lipid bilayers. Long-scale, unbiased atomistic molecular dynamics (MD) simulations directly reveal that rather than rigid, well-defined pores, these peptides can form a large repertoire of functional dynamic and heterogeneous aggregates, strongly affected by single mutations. Predicting the propensity to aggregate and assemble in a given environment from sequence alone holds the key to functional prediction of membrane permeabilization.

Graphic Abstract

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Introduction

The Arctic is home to various microbial strains, which can provide information regarding survival in harsh conditions [ 1]. The existence of cryophilic and cold-tolerant bacteria and the mechanisms by which these organisms adapt to fluctuations in temperature and osmolarity have prompted studies on general temperature-related survival mechanisms [ 2– 6]. Previous studies have revealed several mechanisms of coping with different temperature changes, such as the expression of cold-shock proteins, changes in membrane fluidity, and the production of polyhydroxyalkanoates (PHAs) and exopolysaccharides (EPSs), and ongoing research aims to elucidate the different survival strategies [ 7, 8].

Phospholipid fatty acids (PLFAs) are a major component of the microbial cell membrane, and they can be analyzed to determine microbial community composition and to monitor dynamic changes in membrane properties. This is interesting, because the membrane is the essential first line of defense from the outer environment for both Gram-negative and Gram-positive bacteria [ 9]. Unlike total lipid analysis, PLFA analysis involves laborious fractionation steps and requires careful treatment of the highly unstable phospholipids [ 10]. However, it can provide direct information on the identity of membrane phospholipids, unlike total lipid analysis, which has difficulties in detecting some fatty acids that are sensitive to strong acid or high temperature [ 11, 12].

Pseudomonas is a widespread bacterial genus that exists in various temperature zones, produces PHAs and EPSs, and displays multiple hydrolase activities exploited for bioremediation. Therefore, Pseudomonas is a promising genus for industrial use as well as for investigating different temperature-related behaviors [ 7, 13]. Previous studies have shown that bacterial lipid metabolism is altered in response to temperature and have attempted to identify the key players involved in mediating such changes in membrane fatty acids [ 13, 14]. Some studies have identified the enzymes Δ-9-fatty acid desaturase (DesA) and cyclopropane-fatty acid-acyl-phospholipid synthase (Cfa) as regulators of the levels of unsaturated fatty acids (UFAs) and cyclopropane-fatty acids (CFAs), respectively [ 15– 17]. However, most studies concluded this on the basis of total lipid analysis, whose results can differ from those of PLFA analysis [ 18]. Moreover, a previous study has shown the function of desA in Escherichia coli through the addition of stearic and palmitic acid and total lipid analysis [ 19]. Therefore, although previous research provided some insights, there is a need to carry out in vivo experiments to verify the exact role of this desaturase in the remodeling of the cellular membrane.

In the current study, we characterized the newly identified Pseudomonas sp. B14-6 strain, isolated from arctic soil, which showed dynamic changes in membrane PLFA content, hydrophobicity, and fluidity upon changes in temperatures. We aimed to determine the function of the genes desA and cfa from this new strain using PLFA analysis and genomic sequencing. Our results are expected to provide insights into temperature-dependent membrane behavior and its key factors, and they might inspire the development of new methods for industrial use of microbes at harsh temperatures as well as shed some light on the ecological implications of global warming.


Reference request: Lipid composition in bacterial, yeast and human membranes - Biology

a Department of Physics, King's College London, London WC2R 2LS, UK
E-mail: [email protected]

b School of Electronics and Computer Science, and Institute for Life Sciences, University of Southampton, Southampton SO17 1BJ, UK

c Department of Chemistry, School of Life Sciences, University of Sussex, Brighton BN1 9RH, UK

Abstract

Cell membranes naturally contain a heterogeneous lipid distribution. However, homogeneous bilayers are commonly preferred and utilised in computer simulations due to their relative simplicity, and the availability of lipid force field parameters. Recently, experimental lipidomics data for the human brain cell membranes under healthy and Alzheimer's disease (AD) conditions were investigated, since disruption to the lipid composition has been implicated in neurodegenerative disorders, including AD [R. B. Chan et al., J. Biol. Chem., 2012, 287, 2678–2688]. In order to observe the effects of lipid complexity on the various bilayer properties, molecular dynamics simulations were used to study four membranes with increasing heterogeneity: a pure POPC membrane, a POPC and cholesterol membrane in a 1 : 1 ratio (POPC–CHOL), and to our knowledge, the first realistic models of a healthy brain membrane and an Alzheimer's diseased brain membrane. Numerous structural, interfacial, and dynamical properties, including the area per lipid, interdigitation, dipole potential, and lateral diffusion of the two simple models, POPC and POPC–CHOL, were analysed and compared to those of the complex brain models consisting of 27 lipid components. As the membranes gain heterogeneity, a number of alterations were found in the structural and dynamical properties, and more significant differences were observed in the lateral diffusion. Additionally, we observed snorkeling behaviour of the lipid tails that may play a role in the permeation of small molecules across biological membranes. In this work, atomistic description of realistic brain membrane models is provided, which can add insight towards the permeability and transport pathways of small molecules across these membrane barriers.


ALTERNATIVE MECHANISMS FOR LIPID-MEDIATED CELLULAR FUNCTION

Although the cytoskeleton-based model may accurately describe the determinants of plasma membrane organization, it does not explain how cellular levels of cholesterol and sphingolipids influence protein function. Functional assays that use drugs or chelating agents to alter cellular cholesterol or sphingolipid levels clearly demonstrate that cellular signaling and protein function are influenced by cholesterol and sphingolipid abundance (Zhao et al., 2006 Lasserre et al., 2008 Lingwood et al., 2011). These data are often cited as support for the lipid raft model because they are consistent with the hypothesis that cholesterol and sphingolipid depletion disperses lipid rafts and eliminates the functions that they perform. The sensitivity of cell signaling to sphingolipid levels, however, is already explained by the known role of sphingolipid metabolites as ligands that selectively bind to and activate or inhibit several kinases, phosphatases, and membrane receptors involved in cell signaling (Bartke and Hannun, 2009). Changes in the cellular sphingolipid levels affect the availability of the bioactive sphingolipid metabolites (i.e., ceramide and sphingosine-1-phosphate) and the signaling pathways they regulate.

Alternative hypotheses for nonraft mechanisms of cholesterol-regulated signaling are emerging. Some data support a recently proposed model in which the direct binding of cholesterol to a scaffold protein regulates signaling complex assembly and thus the functions it performs (Sheng et al., 2012). In this mechanism, either sterol binding or unbinding to the scaffold protein may serve as the activating event. Such cholesterol-dependent signaling complex formation could be modulated by the differential cholesterol concentrations present in the plasma membrane and intracellular compartments where scaffold proteins reside in the absence of lipid rafts. As an example, a cholesterol-regulated signaling complex consisting of a serine/threonine phosphatase (PP2A), a tyrosine phosphatase (HePTP), oxysterol-binding protein (OSBP), and cholesterol is involved in the extracellular signal-regulated kinase (ERK) signaling pathway (Wang et al., 2005). Cholesterol binding to the OSBP scaffold protein allows OSBP to bind to PPA2 and HePTP, forming the complex that cooperatively dephosphorylates pERK cholesterol depletion or the addition of 25-hydroxcholeserol causes the complex to disassemble and abolishes its phosphatase activity (Wang et al., 2008). As a second example, cholesterol binding to the PDZ domain on the NHERF1 scaffold protein is required for sustained colocalization of NHERF1 with the cystic fibrosis transmembrane conductance regulator (CFTR) and CFTR activity (Sheng et al., 2012). The cholesterol-dependent formation of other signaling complexes was also reported but often attributed to assembly in lipid rafts (Green et al., 1999 Kranenburg et al., 2001 Roitbak et al., 2005). Might the mechanism for cholesterol depletion-induced cytoskeleton reorganization involve a cholesterol-dependent scaffold protein? Many additional studies will be required to assess the generality of this mechanism and thus the validity of this model for lipid-mediated cellular function.

The possible alternative mechanisms for cholesterol- and sphingolipid-mediated protein organization and activity discussed here are in no way comprehensive and require substantial testing. Given the complexity of the numerous events that give rise to cell signaling, the mechanisms that produce the observed lipid-mediated cellular functions may be far more elaborate than those described here or proposed to date. A substantial increase in efforts to develop and test alternative mechanisms will be essential to achieving an accurate model of lipid-mediated cellular function. Based on the growing interest in the existence of rafts and raft-like domains in the membranes of yeast (Wachtler and Balasubramanian, 2006), plant cells (Grennan, 2007), and even prokaryotic cells (LaRocca et al., 2013), these efforts should not be restricted to mammalian cells.


Static and dynamic lipid asymmetry in cell membranes

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Plasma Membrane: Chemical Composition and Functions | Cell

The membrane is mainly composed of lipids, proteins and carbohydrates. Water makes about 29% of total weight. Robertson (1959) proposed that plasma membrane is three-layered structure where proteins form the outer and inner layers of membrane that encloses lipids to form a unit membrane.

The lipids identified in the plasma membrane consist of cholesterol, phospholipids and galactolipids. The phospholipids include phosphatidylcholine, phosphatidylethanolamine and sphingomyelin. The phospholipids are found to be associated with the outer protein shell in the plasma membrane. Glycerol and fatty acid constitute lipid molecules.

In the membrane the lipid molecules consist of two parts —a head and two tails. The head is composed of glycerol and is hydrophilic where as the tails are composed of fatty acids that are hydrophobic. The head and tail are usually designated as polar and nonpolar end respectively. The proteins, in the membrane, are present in two layers and the lipids occur in between them.

The lipid molecules are oriented in two (bimolecular) layers with their hydrophilic polar ends directed towards protein and the hydrophobic nonpolar ends face each other. Hydrogen bonds, ionic linkages or electrostatic forces bind the protein and lipid components.

In the membrane it is present as enzyme protein, carrier protein and structural protein. The enzyme proteins have catalytic activity. The carrier proteins help to transport materials in and out of the cell across membrane. The structural proteins play an important role to form the structure of membrane.

They occur in the form of glycolipids and glycoproteins. Both of these forms are confined exclusively to the external membrane surface. Bell (1962) is of opinion that polysaccharides confer some stability to the lipoprotein complex in the membrane.

Origin of Plasma Membrane:

Bell suggested that the phragmosomes, those appear during middle lamella formation and are composed of polysaccharides, help in the formation of new plasma membrane on reaction with lipoprotein.

Morphology of Plasma Membrane:

The plasma membrane is not readily visible under light microscope. Electron microscope shows that the plasma membrane is 6 nm to 10 nm thick. It consists of three layers—two outer dense layer, each about 2 nm thick and a middle less dense area of 3.5 nm across, for a total thickness of 7.5 nm.

On the basis of these findings Robertson (1959) proposed the unit membrane structure to the lipoprotein organization of plasma membrane. According to this concept the outer dense layers represent the protein and the enclosed less dense are the lipids. Very small pores, none of which are of greater diameter than 5 nm, interrupt the continuity of the membrane.

There are different models to explain the structure of plasma membrane. The unit membrane model of Robertson consisting of protein-lipid-protein layer is previously mentioned. The fluid mosaic model of the membrane as proposed by Singer and Nicolson (1972) is now generally accepted. This model assumes that the membrane is a semifluid structure where lipids and integral proteins are present.

They are arranged in a mosaic manner, i.e. the integral proteins are embedded to a greater or lesser extent in the continuous bilayers of phospholipids. The integral proteins are amphipathic molecules, i.e. within the same molecule hydrophilic and hydrophobic groups occur. The hydrophilic groups protrude from the surface while the hydrophobic groups remain embedded in the lipids (Fig. 1.12).

Schematic Representation of Molecular Organization of the Plasma Membrane

Functions of Plasma Membrane:

(i) It is selectively permeable membrane

(ii) Its principal role is to regulate the flow of materials in and out of the cell

(iii) Carrier proteins in the membrane are involved in the transport of certain materials across the plasma membrane


Lipids Types: Simple, Compound and Derived Lipids

(a) They are esters of fatty acids with glyc­erol.

(b) They are found in nature in large quanti­ties.

(c) They are the best reserve of food material in the human body.

(d) They act as insulator for the loss of body heat.

(e) They act as a padding material for pro­tecting internal organs.

The chemical structure of fat (triglyceride) consists of three different molecules of fatty acids with one molecule of glycerol.

The three different fatty acids (R1, R2, R3) are esterified with the three hydroxyl groups of glycerol:

Physical Properties of Fats:

(a) The fats are insoluble in water, but readily soluble in ether, chloroform, benzene, car­bon tetrachloride.

(b) They are readily soluble in hot alcohol but slightly soluble in cold.

(c) They are themselves good solvents for other fats, fatty acids, etc.

(d) They are tasteless, odorless, colourless and neutral in reaction.

(e) Several neutral fats are readily crystallized, e.g., beef, mutton.

(f) Their melting points are low.

(g) The specific gravity of solid fats is about 0.86. So fat people float in water more read­ily than thin ones.

(h) They spread uniformly over the surface of water so the spreading effect is to lower surface tension.

Identification of Fats and Oils:

1. Hydrolysis of triacylglycerol takes place by lipases producing fatty acids and glyc­erol.

2. Phospholipases attack the ester linkage of phospholipids.

(b) Saponification:

1. Boiling with an alcoholic solution of strong metallic alkali hydrolyses triglycerides into glycerol and fatty acids —this is called saponification.

2. The products are glycerol and the alkali salts of the fatty acids which are called soaps.

3. Fats, phospholipids, glycolipids and waxes are called saponifiable lipids.

4. Steroids, polyisoprenoids and higher alcohols are grouped as un-saponifiable lipids because they cannot give rise to soap.

(c) Saponification number:

1. The number of milligrams of KOH required to saponify 1 gram of fat or oil.

2. The amount of alkali needed to saponify a given quantity of fat will depend upon the number of-COOH group present. It is inversely proportional to the average mo­lecular weight of the fatty acids in the fat i.e. the fats containing short chain fatty acids will have more -COOH groups per gram than long chain fatty acids—this will take up more alkali and, hence, will have higher saponification number.

Butter—containing a larger proportion of short chain fatty acids such as butyric and caproic acids, has relatively high saponification number 220 to 230.

1. The number of milligrams of KOH required to neutralize the free fatty acids of 1 gram of fat.

2. Significance: The acid number indicates the degree of rancidity of the given fat.

(e) Iodine number:

1. This is the amount (in grams) of iodine absorbed by 100 grams of fat.

2. This is the measure of the degree of unsaturation of a fat.

3. Significance: If the fat contains higher number of unsaturated fatty acids, it be­comes essential for the protection of heart disease. These unsaturated fatty acids, combined with the cholesterol, are oxi­dized in the liver—producing bile acids, bile salts, vit., D, gonadotrophin hormones. They prevent atherosclerosis.

(f) Acetyl number:

1. The number of milligrams of KOH required to neutralize the acetic acid obtained by saponification of 1 gram of fat after it has been acetylated.

2. This is a measure of the number of hy­droxy acid groups in the fat.

(g) Polenske number:

1. The number of milliliters of 0.1 (N) KOH required to neutralize the insoluble fatty acids from 5 grams of fat.

(h) Reichert-Miessl number:

1. This is same as the Polenske number ex­cept that the soluble fatty acids are meas­ured by titration of the distillate obtained by steam distillation of the saponification mixture.

2. Significance:

It measures the amount of volatile soluble fatty acids.

1. Chlorine, bromine and iodine atoms may be added to the double bonds of unsatu­rated fatty acids containing fats.

1. Nearly all natural fats are oxidized when exposed to air, light, moisture, particularly, if warm, it develops an unpleasant odour and taste. The enzyme lipase—in the pres­ence of moisture and warm temperature— bring about hydrolysis rapidly.

2. This happens due to the formation of peroxides at the double bonds of unsatu­rated fatty acids.

3. Vitamin E is an important natural antioxi­dant and prevents development of rancid­ity.

1. Soaps are metallic salts of fatty acids.

2. Soaps are formed by adding alkalis to fatty acids.

3. Soaps of unsaturated fatty acids are softer and more water soluble than those of satu­rated fatty acids.

4. Potassium soap of an acid is more water-soluble and softer than the sodium soap calcium and magnesium soaps are far less soluble.

1. They are esters of fatty acids with higher alcohols other than glycerol.

2. In the human body, the commonest waxes are esters of cholesterol.

3. They are mainly three types:

(a) True waxes are esters of higher fatty acids with acetyl alcohol or other higher straight chain alcohols.

(b) Cholesterol esters are esters of fatty acid with cholesterol.

(c) Vitamin A and vitamin D esters are palmitic or stearic acid esters of vita­min A (Retinol) or vitamin D, respec­tively.

Type # 2. Compound Lipids:

A. Phospholipids (phosphatides):

(i) They are esters of fatty acids with glyc­erol containing an esterified phosphoric acid and a nitrogen base.

(ii) They are present in large amounts in nerve tissue, brain, liver, kidney, pancreas and heart.

Biological functions of phospholipids:

(i) They increase the rate of fatty acid oxida­tion.

(ii) They act as carriers of inorganic ions across the membranes.

(iii) They help blood-clotting.

(iv) They act as prosthetic group to certain en­zymes.

(v) They form the structures of membranes, matrix of cell wall, myelin sheath, microsomes and mitochondria.

It is based on the type of alcohol present in the phospholipid.

There are three types:

1. Glycerophosphatides — In this, glycerol is the alcohol group.

(i) Phosphatidyl ethanolamine (cephalin).

(ii) Phosphatidyl choline (Lecithin).

2. Phosphoinositides — In this, inositol is the alcohol.

Phosphatidyl inositol (Lipositol).

3. Phosphosphingosides — In this, sphingosine is an amino alcohol.

The phospholipids include the following groups:

1. Phosphatidic acid and phosphatidyl glycerol’s:

Phosphatidic acid is important as an intermediate in the synthesis of triacylglycerol’s and phospholipids.

(a) It is formed from phosphatidyl glyc­erol.

(b) Chemically, it is di-phosphatidyl glyc­erol.

(c) It is found in inner membrane of mi­tochondria and bacterial wall.

2. Lecithin (Phosphatidylcholine):

The lecithin’s contain glycerol and fatty acids, phosphoric acid and choline (nitrogenous base). Lecithin’s generally contain a satu­rated fatty acid at α position and an un­saturated fatty acid at β position. They can exist in α or β forms.

Physical Properties:

(i) Lecithin’s are waxy, white substances but become brown soon when exposed to air

(ii) They are soluble in ordinary fat solvents except acetone.

(iii) They decompose when heated.

(iv) They constitute valuable agents for the emulsifications of fats and oils.

Chemical Properties of Lecithin:

(i) When aqueous solution of lecithin’s are shaken with H2SO4, choline is split off, forming phosphatidic acid.

(ii) When lecithin’s are boiled with alkalis or mineral acids, not only choline is split off, but phosphatidic acid is further hydrolyzed to glycerophosphoric acid and 2 molecules of fatty acids:

Lecithin → H2SO4 Phosphatidic acid + choline.

Phosphatidic acid → Glycerophosphoric acid + fatty acids (2 mol)

Physiological Functions of Lecithin:

(i) It facilitate the combinations with proteins to from lipoproteins of plasma and cells.

(ii) Acetylcholine formed from choline has an important role in the transmission of nerv­ous impulses across synapses.

(iii) Choline is the most important lipotropic agent as it can prevent formation of fatty liver.

(iv) Lecithin lowers the surface tension of lung alveoli. Dipalmityl lecithin is a major constituent of “lung surfactant” which pre­vents the adherence of the inner surface of the alveoli of the lungs (preventing the collapse of the alveoli) by its surface ten­sion lowering effect. The absence of this in the alveolar membrane of some prema­ture infants causes the respiratory distress syndrome in them.

(v) It lowers the surface tension of water mol­ecule and helps in the emulsification of fat.

Difference of Lecithin and Cephalin:

Cadmium chloride compound of cephalin is soluble but cadmium chloride compound of leci­thin is insoluble.

3. Cephalitis (Phosphatidyl ethanolamine):

They always occur in the tissues in asso­ciation with lecithin’s and are very similar in properties. The only difference is the nitrogenous base.

4. Phosphatidyl Serine:

A cephaline like phospholipid is found in tissues.

5. Phosphatidyl inositol (Lipositol or Phosphoinositides):

(i) It acts as second messenger in Ca ++ dependent hormone action.

(ii) Some signals must provide commu­nication between the hormone receptor on the plasma membrane and intracellular Ca ++ reservoirs.

(iii) They are more acidic than the other phospholipids.

(i) These are phosphoacylglycerols con­taining only one acyl radical in a po­sition e.g., Lysolecithin.

(a) By the action of phospholipase.

(b) By interaction of lecithin and cho­lesterol in presence of the enzyme lecithin cholesterol acyl transferase, so lysolecithin and cholesterol ester are formed

(i) These are the contents of brain and muscle.

(ii) Structurally, these resemble lecithin’s and cephalins but give a positive re­action when tested for aldehydes with Schiff s reagent (fuchsin-sulphurous acid) after pretreatment of the phos­pholipid with mercuric chloride.

(iii) They possess an ether link in a posi­tion instead of ester link. The alkyl radical is an unsaturated alcohol.

(i) These are found in large quantities in brain and nerve tissue.

(ii) The concentrations of these phospholipids are increased in Niemann-Pick disease in the liver and spleen.

(iii) These contain sphingosine (18 car­bon) (amino alcohol) fatty acid, phos­phoric acid and choline. No glycerol is present.

(iv) In sphingosine molecule -NH2 group binds a fatty acid by an amide link­age to produce ceramide. When phos­phate group is attached to ceramide it is called ceramide phosphate.

(v) When choline is split off from sphin­gomyelin, ceramide phosphate is left.

Action of Phospholipase:

(a) Phospholipase A1 attacks the ester bond in position 1 of phospholipid.

(b) Phospholipase A2 attacks β position and form

Lysolecithin + one mol. fatty acid.

(c) Phospholipase B (lysophospholipase) at­tacks lysolecithin and hydrolyzes ester bond in α position and forms glyceryl phosphoryl choline + 1 mol. fatty acid.

(d) Phospholipase C hydrolyzes phosphate ester bond and produces α, β di-acyl glyc­erol + phosphoryl choline.

(e) Phospholipase D-splits off choline and phosphatidic acid is formed

These contain an amino alcohol (sphingosine or iso-sphingosine) attached with an amide linkage to a fatty acid and glycosidically to a carbohydrate moiety (sugars, amino sugar, sialic acid).

These are further classified into:

(a) Cerebrosides contain galactose, a high molecular weight fatty acid and sphingosine. Therefore, they may also be classified as sphingolipids.

(b) They are the chief constituent of my­elin sheath.

(c) They may be differentiated by the type of fatty acid in the molecule.

Kerasin—Containing lignoceric acid [CH, — (CH2)22 — COOH].

Cerebron—Containing a hydroxylignoceric acid (cerebronic acid).

Nervon—Containing an unsaturated homologue of lignoceric acid called nervonic acid. [CH, — (CH2)7 — CH = CH — (CH2)13 — COOH].

Oxynervon—Containing hydroxy-nervonic acid [CH3 — (CH2)7 — CH = CH — (CH2)12— CH(OH) — COOH].

(d) Stearic acid is a major component of the fatty acids of rat brain cerebrosides.

(e) Cerebrosides, specially cerebronic acid, increases in Gaucher’s disease and the Kerasin characterized by glu­cose replacing galactose.

(f) The cerebrosides are in much higher concentration in medullated than in non-medullated nerve fibers.

(a) These are glycolipids occurring in the brain.

(b) Gangliosides contain ceramide (sphingosine + fatty acids), glucose, galactose, N-acetylgalactosamine and sialic acid.

(c) Some gangliosides also contain di-hydro-sphingosine or gangliosine in place of sphingosine.

(d) Most of the gangliosides contain a glucose, two molecules of galactose, one N-acetylgalactosamine and up to three molecules of sialic acid.

Types of Ganglioside:

C. Other compound lipids:

(i) Triacylglycerol (45%), phospholipids (35%), cholesterol and cholesteryl esters (15%), free fatty acids (less than 5%) and also protein combine to form a hydrophilic lipoprotein complex.

(ii) Since pure fat is less dense than water, the proportion of lipid to protein in lipoproteins in plasma is by ultracentrifugation.

(iii) The density of lipoproteins increases as the protein content rises and the lipid con­tent falls and the size of the particle be­comes smaller.

(iv) Lipoproteins may be separated on the ba­sis of their electrophoretic properties and may be identified more accurately by means of immuno-electrophoresis.

(v) Four major groups of lipoproteins have been identified which are important physi­ologically and in clinical diagnosis in some metabolic disorders of fat metabo­lism.

(b) Very low density lipoproteins (VLDL or pre-β-lipoproteins).

(c) Low density lipoproteins (LDL or β-lipoproteins).

(d) High density lipoproteins (HDL or α-lipoproteins).

(vi) Chylomicrons and VLDL: Predominant lipid is triacylglycerol (50%) and choles­terol (23%). The concentrations of these are increased in atherosclerosis and coro­nary thrombosis etc.

Predominant lipid is cholesterol (46%) and phospholipids (23%). Increase in atherosclerosis and coro­nary thrombosis, etc.

Predominant lipid is phospholipid (27%) and proteins (45%).

(vii) The protein moiety lipoprotein is known as an apoprotein which constitute nearly 60% of some HDL and 1% of chylomi­crons. Many lipoproteins contain more than one type of apoprotein polypeptide.

(viii) The larger lipoproteins (such as chylomicrons and VLDL) consist of a li­pid core of nonpolar triacylglycerol and cholesteryl ester surrounded by more po­lar phospholipid, cholesterol and Apo proteins.

(i) To transport and deliver the lipids to tis­sues.

(ii) To maintain structural integrity of cell sur­face and subcellular particles like mito­chondria and microsomes.

(iii) The β-lipoprotein fraction increases in severe diabetes mellitus, atherosclerosis etc. Hence determination of the relative concentrations of α-and β-lipoproteins and pre-β- lipoproteins are of diagnostic im­portance.

Phosphatidyl ethanolamine and serines are amino lipids and sphingomyelins and gangliosides contain substituted amino groups.

3. Sulpholipids (Sulphatides):

(i) These have been isolated from brain and other animal tissues.

(ii) These are sulphate derivatives of the galactosyl residue in cerebrosides.

Type # 3. Derived Lipids:

(i) These are obtained by the hydrolysis of fats.

(ii) Fatty acids occurring in natural fats usu­ally contain an even number of carbon atoms because they are synthesized from 2-carbon units and are straight chain de­rivatives.

(iii) The straight chain may be saturated (con­taining no double bonds) or unsaturated (containing one or more double bonds).

(iv) Carbon atoms of fatty acids are numbered from the carboxyl carbon (carbon No.l). The carbon atom adjacent to the carboxyl carbon (Carbon No. 2) is also known as the α -carbon. Carbon atom No. 3 is the β- carbon and the end methyl carbon is known as the γ-carbon.

(v) Various conventions are used for indicat­ing the number and position of the dou­ble bonds, e.g., Δ 9 indicates a double bond between carbon atoms 9 and 10 of the fatty acid.

iii. Substituted (methyl substituted- cerebronic acid)

iv. Cyclic (chaulmoogric acid) used in lep­rosy.

(Straight chain even number fatty acid is common)

B. Saturated fatty acids:

General formula for saturated fatty acids is CnH2n+1 COOH. Other higher fatty acids occur in waxes. A few branched-chain fatty acids have also been isolated from both plant and animal sources.

Prostanoids include Prostaglandins (PG), and thromboxane’s (TX).

General characteristics of prostanoid

(a) All are 20 carbon compounds.

(b) Trans double bond at 13 positions.

(c) -OH group at 15 position.

Prostaglandins (PG):

(a) They virtually exist in every mammalian tissue and act as local hormones.

(b) They have important physiologic and pharmacologic activities.

(c) They are synthesized in vivo by cyclization of the center of the carbon chain of 20-C polyunsaturated fatty acids (e.g., arachidonic acid) to form a cyclopentane ring.

(d) Three different eicosanoic fatty acids give rise to three groups of eicosanoids charac­terized by the number of double bonds in the side chains, e.g., PG1, PG2, PG3. Varia­tions in the substituent groups attached to the rings give rise to different types in each series of prostaglandins, as for exam­ple, “E” type of Prostaglandin has a keto group in position 9, whereas the “F” type has a hydroxyl group in this position.

Prostacyclin’s (PGI):

(a) They are formed in vascular endothelium and continually formed in heart. They are also formed in kidneys.

(b) They are formed from cyclic endo-peroxide PGH2 by the action of microsomal Prostacyclin synthetase.

(c) They inhibit platelet aggregation and gas­tric secretion from the pyloric mucosa.

(d) They decrease blood pressure and protect coronary arteries.

(e) They increase renal blood flow and stimu­late renin production.

(f) They are inhibited by hyperlipemia, vit. E deficiency and radiation.

(a) They contract smooth muscles on blood vessels, GI Tract, uterus, bronchioles.

(b) They are discovered in platelets, and have the cyclopentane ring interrupted with an oxygen atom (Oxane ring).

(c) The substituent groups attached to the rings being varied give rise to different types in each series of thromboxane’s la­belled A, B, etc.

(d) They produce vasoconstriction and in­crease blood pressure.

(e) They cause release of serotonin and cal­cium ion (Ca ++ ) from platelet granules.

(f) Imidazole’s inhibit their synthesis.

(a) They are the third group of eicosanoid de­rivatives formed via the lipoxygenase pathway rather than cyclization of the fatty acid chain.

(b) They are first described in leukocytes.

(c) They are characterized by the presence of three conjugated double bonds.

(d) They are stimulators of mucus secretion and are responsible for vasoconstriction of bronchial muscles.

(e) They are inhibited by prolonged use of aspirin.

The group of compounds known as prostaglandins are synthesized from arachidonic acid in the body. They have pharmacologic and biochemical activity.

C. Many Other Fatty Acids:

(i) These have been detected in biologic ma­terial.

Fish oil contain 5 and 6 un­saturated fatty acids having carbon atoms 22.

(ii) Various other structures with hydroxy groups (ricinoleic acid) or cyclic groups have been found in nature.

Example of cyclic groups is chaulmoogric acid which was used many years ago in the treatment of leprosy.

Essential Fatty Acids:

Burr and Burr (1930) introduced the term “Essen­tial Fatty Acids” (EFA) on the basis that they are essential for the growth and health of young albino rats. These polyunsaturated fatty acids which are not synthesized in the body but are taken from natu­ral sources are called essential fatty acids.

They are (mentioned above):

Linolenic and arachidonic acids are formed from linoleic acids provided linoleic acids are avail­able in the body in sufficient quantities.

(i) The essential fatty acids of vegetable oils have low melting points and iodine number.

(ii) They become saturated fatty acids on hydrogenation and the oils become solid fats.

a. The essential fatty acids in high concen­tration along with the lipids constitute the structural elements of the tissues.

b. The lipids of gonads also contain a high concentration of polyunsaturated fatty acids which suggest the importance of re­productive function.

c. They effect the prolongation of clotting time and increase the fibrinolytic activity.

d. They retard atherosclerosis being esterified and emulsified with cholesterol and are incorporated into lipoproteins for transport to the liver for further oxidation.

f. The deficiency of these acids in the diet of babies causes eczema.

Isomerism in Unsaturated Fatty Acids:

Variations in the locations of the double bond in unsaturated fatty acid chains produce isomers. Oleic acid has 15 different positional isomers. Geometric isomerism depends on the orienta­tion of radicals around the axis of double bonds. If the radicals which are being considered are on the same side of the bond, the compound is called “cis”, if on opposite side, “trans”. This can be illustrated with maleic acid and fumaric acid.

There are more geometric isomers in case of acids with greater degree of unsaturation. The un­saturated long chain of fatty acids occurring in na­ture are nearly all in the ‘cis’ form and the mol­ecules are “bent” at the position of the double bond. Thus, arachidonic acid is U-shaped.

Refined and Hydrogenated Oils:

It is prepared in the following man­ner:

(i) Free fatty acids are removed by alkali treat­ment.

(ii) Colouring matter is removed by activated carbon.

(iii) Odour is removed by superheated steam.

The refined oils are hydrogenated under optimum temperature and pres­sure with hydrogen in the presence of nickel cata­lyst. Unsaturated fatty acids are converted into satu­rated fatty acids.

The liquid oil becomes solid fat and the un­saturated fatty acid content decreases. Vanaspati is hydrogenated refined groundnut oil.

Alcohols found in lipid molecules include glyc­erol, cholesterol and higher alcohols (acetyl alco­hol), usually found in the waxes.

The unsaturated alcohols are important pig­ments. Phytyl alcohol is a constituent of chloro­phyll and lycophyll (C40H56O2) a polyunsaturated dihydroxy alcohol occurs in tomatoes as a purple pigment.

The steroids are often found in association with fat. They have a similar cyclic nucleus resembling phenanthrene (rings A, B, C) to which a cyclopentane ring (D) is attached. The parent substance is better designated as cyclopentano perhydrophenanthrene. The position on the steroid nucleus are numbered as shown in Fig. 4.17.

Methyl side chains occur typically at posi­tions 10 and 13 (constituting C atoms 19 and 18). A side chain at position 17 is usual (as in choles­terol). If the compound has one or more hydroxyl groups and no carbonyl or carboxyl groups, it is a sterol, and the name terminates in -OL.

Steroids may be divided in the following man­ner:

Sterols—cholesterol, ergosterol, coprosterol. Bile acids—Glycocholic acid and taurocholic acid.

Sex hormones—Testosterone, Estradiol.

It is widely distributed in all cells of the body. It occurs in animal fats but not in plant fats. Its struc­ture is given below. The metabolism of cholesterol is discussed in the chapter of lipid metabolism.

(i) It occurs in ergot and yeast.

(ii) It is the precursor of vitamin D.

(iii) It acquires anti-rachitic properties with the opening of ring B when irradiated with ultraviolet light.

It occurs in feces as a result of the reduction by bacteria in the intestine of the double bond be­tween C5 and C6 of cholesterol.

Important tests:

A drop of oil placed over a piece of ordinary paper. A translucent spot is visible. This indicates the presence of fat.

2. Emulsification test:

2 ml water is taken in one test tube and 2 ml of diluted bile salt solution in another test tube. Add 3 drops of the given oil to each test tube and shake vigorously. Note the stability of the emul­sification formed.

3. Saponification test:

Take 10 drops of co­conut oil in a test tube. Add 20 drops of 40% NaOH and 2 ml of glycerol to it. Gen­tly boil for about 3 minutes until com­plete saponification occurs. If oil globules are visible, boiling must be continued. Di­vide the solution into 3 parts to carry the following experiments in test tube 1, 2, 3.

To test tube No. 1 add saturated solution of NaCl. Note that the soap separates out and floats to the surface (salting out process).

To test tube No. 2 add a few drops of conc. HCl. An oily layer of the fatty acids rises to the surface.

To test tube No. 3 add a few drops of CaCl2 solution. The insoluble calcium soap is precipi­tated.

Add 10 drops of Hubble’s iodine reagent to 10 ml of chloroform. The chloro­form assumes a pink colour due to the free iodine. The solution is divided equally into three test tubes as (a), (b) and (c) and three types of oil are added.

Add the oil No. 1 to the test tube (a) drop by drop shaking the tube vigorously after each addi­tion till the pink colour of the solution just disap­pears. The number of oil drops required is noted.

The experiment is repeated by adding oil 2 and 3 to test tubes (b) and (c), respectively. The more the number of drops required to discharge the pink colour, the less is the unsaturation.

Colour Reactions to Detect Sterols:

Liehermann-Rurehard Reaction:

A chloroform so­lution of a sterol when treated with acetic anhy­dride and sulphuric acid gives a green colour. This reaction is the basis of a colorimetric estimation of blood cholesterol.

A red to purple colour ap­pears when a chloroform solution of the sterol is treated with an equal volume of concentrated sul­phuric acid.

i. The high concentration of polyunsaturated fatty acids in the lipids of gonads are important in repro­ductive function.

ii. The essential fatty acid deficiency causes swelling of mitochondrial membrane resulting in the reduc­tion in efficiency of oxidative phosphorylation pro­ducing increased heat.

iii. Docosahexenoic acid formed from dietary linolenic acids enhances the electrical response of the photoreceptors to illumination. Therefore, linolenic acid of the diet is essential for optimal vision.

iv. The deficiency of essential fatty acids causes skin lesions, abnormal pregnancy and lactation in adult females, fatty liver, kidney damage.

v. The genetic deficiency of lecithin cholesterol acyl transferase (LCAT) causes Norum’s Disease.

vi. Sitosterol decreases the intestinal absorption of exogenous and endogenous cholesterol and thereby lowers the blood cholesterol level.

vii. The deficiency of the enzyme sphingomyelinase causes the large accumulations of sphingomyelins in brain, liver and spleen of children resulting in the Niemann-Pick. disease with the symptoms of en­larged abdomen, liver, spleen and mental deterio­ration.

viii. Absence of dipalmityl lecithin (DPL) in premature foetus produces respiratory distress syndrome (Hyaline-membrane disease).

ix. The inherited Gaucher’s Disease in infancy and childhood is caused by the deficiency of the en­zyme glucocerebrosidase involving the large ac­cumulations of glucocerebrosides (usually Kera­sin) in the liver, spleen, bone marrow, and brain with the manifestations of weight loss, failure in growth, and progressive mental retardation.

x. The autosomal recessive Tay-Sach’s Disease (GM2 Gangliosidosis) results in the accumulation of large amounts of gangliosides in the brain and nervous tissues due to the absence of the enzyme hexosaminidase A with the association of progres­sive development of idiocy and blindness in infants soon after birth.

Clinical Orientation:

i. The inherited disorder Metachromatic Leukodys­trophy (MLD) happens on the sulfatide, formed from galactocerebroside, accumulation in various tis­sues owing to the deficiency of the enzyme sulfatase (Aryl sulfatase) with the symptoms of weakness, ataxia, defects in locomotion, paralysis, difficulties in speech in children before three years of age and psychiatric manifestation including progressive dementia in adults.

ii. Obesity and atherosclerosis are distinctly related to the concentrations of cholesterol and polyun­saturated fatty acids in the body.


Introduction

Coenzyme Q (ubiquinone or CoQ) is a vital lipid component in mitochondrial energy metabolism. It is a two-part molecule containing a long polyisoprenyl tail of n isoprene units positioning the molecule in the mid-plane of membrane bilayer, and a fully substituted benzoquinone ring that undergoes reversible reduction and oxidation. The redox chemistry of CoQ and CoQH2 (ubiquinol, a hydroquinone) allows it to play its best-known role in mitochondrial respiration, accepting electrons and protons from Complex I or Complex II and donating them to Complex III, thereby establishing a proton gradient across the mitochondrial inner membrane. CoQ also serves as an essential electron and proton acceptor in other aspects of metabolism including fatty acid β-oxidation, uridine biosynthesis, and oxidation of sulphide, proline, glycerol-3-phosphate, choline, dimethylglycine, and sarcosine [1,2]. CoQH2 also serves a crucial antioxidant function, protecting membranes as a chain terminator of lipid peroxidation reactions, and in the maintenance of reduced forms of vitamin E [1,3]. CoQ/CoQH2 is a component of lipoproteins and is present in all cellular membranes including the plasma membrane where it functions in cellular redox regulation as part of the plasma membrane oxidoreductase system [1].

The focus of this review is on the biosynthesis of CoQ6 in the yeast Saccharomyces cerevisiae and the relevance of this model to the biosynthesis of CoQ10 in human cells. Readers are directed to other recent reviews that discuss the biosynthesis of CoQ in prokaryotes such as Escherichia coli [4], and in eukaryotes including Schizosaccharomyces pombe, plants, Caenorhabditis elegans, Mus musculus, and humans [5–7]. For an in-depth discussion of the effects of CoQ10 deficiencies and the clinical syndromes associated with these deficiencies, readers are directed to the article by Brea-Calvo and colleagues [8] in this issue of Essays in Biochemistry.

Overview of CoQ biosynthesis

S. cerevisiae is an extraordinarily useful model for understanding the biosynthesis of CoQ. Early yeast classic and molecular genetics combined with subcellular fractionation, biochemical assays, and lipid chemistry have helped to identify many of the steps required for CoQ biosynthesis. In particular, the collection of respiratory deficient coq mutants identified by Tzagoloff [9,10] set the stage for isolation and characterization of the yeast COQ genes. A particular advantage is that the CoQ-less coq mutants are viable when cultured on growth medium containing a fermentable carbon source, but are incapable of growth on medium containing a non-fermentable carbon source. In most cases, expression of the human COQ ( human polypeptide involved in CoQ10 biosynthesis) homolog restores function in the corresponding yeast coq mutant. This rescue of yeast coq mutants by human COQ genes is a powerful and simple functional assay still being used to ascertain the effects of human mutations or polymorphisms on human COQ gene function. Thus, what we have learned about the biosynthesis of CoQ6 in the yeast model is highly relevant to the biosynthesis of CoQ10 in humans (Figure 1).

CoQ biosynthetic pathways in the yeast S. cerevisiae and in humans

The CoQ biosynthetic pathway has been shown to involve at least 14 nuclear-encoded proteins that are necessary for mitochondrial CoQ biosynthesis in S. cerevisiae. Black dotted arrows denote more than one step. Solid arrows denote a single step attributed to the corresponding yeast polypeptide named above each arrow. The corresponding human homologs are named below each arrow. The main ring precursor used by both yeast and humans is 4-hydroxybenzoic acid (4HB). Yeasts synthesize 4HB de novo from chorismate or may obtain it from the metabolism of tyrosine. Humans rely on tyrosine to produce 4HB (or on phenylalanine and phenylalanine hydroxylase to produce tyrosine). Yeast and human cells produce isopentenyl pyrophosphate (IPP) and dimethylally pyrophosphate (DMAPP) as precursors to form hexaprenyl diphosphate (n=6) via Coq1 in yeast or decaprenyl diphosphate (n=10) via PDSS1/PDSS2 in humans. Yeast Coq2 and human COQ2 attach the polyisoprenyl tail to 4HB. Subsequent to this step, the next three intermediates are identified as yeast hexaprenyl-intermediates: HHB, 3-hexaprenyl-4HB DHHB, 3-hexaprenyl-4,5-dihydroxybenzoic acid HMHB, 3-hexaprenyl-4-hydroxy-5-methoxybenzoic acid. The next three intermediates are hydroquinones: DDMQH2, 2-hexaprenyl-6-methoxy-1,4-benzenediol DMQH2, 2-hexaprenyl-3-methyl-6-methoxy-1,4-benzenediol DMeQH2, 2-hexaprenyl- 3-methyl-6-methoxy-1,4,5-benzenetriol to ultimately produce the final reduced product (CoQnH2). Red text identifies para-aminobenzoic acid (pABA) as an alternate ring precursor utilized by yeast (but not by humans). The next three intermediates are identified as yeast hexaprenyl-intermediates: HAB, 4-amino-3-hexaprenylbenzoic acid HHAB, 4-amino-3-hexaprenyl-5-hydroxybenzoic acid HMAB, 4-amino-3-hexaprenyl-5-methoxybenzoic acid. The next two intermediates are: IDDMQH2, 4-amino-3-hexaprenyl-5-methoxyphenol IDMQH2, 4-amino-3-hexaprenyl-2-methyl-5-methoxyphenol. The step denoted by the red dotted arrow depends on yeast Coq6 and converts HHAB into DHHB. Interconversion of (CoQnH2) and (CoQn) is shown via a reversible two-electron reduction and oxidation. Steps indicated by ‘. ’ are catalyzed by as yet unknown enzymes. Alternative compounds that may serve as ring precursors in CoQ biosynthesis are shown at the bottom of the panel: p-coumaric acid, resveratrol, and kaempferol. Analogs of 4HB that can function to bypass certain deficiencies in the CoQ biosynthetic pathway include: 3,4-dihydroxybenzoic acid (3,4-diHB), vanillic acid and 2,4-dihydroxybenzoic acid (2,4-diHB). It is not yet known whether 2-methyl-4HB (2-methyl-4HB) may also serve a bypass function.

The CoQ biosynthetic pathway has been shown to involve at least 14 nuclear-encoded proteins that are necessary for mitochondrial CoQ biosynthesis in S. cerevisiae. Black dotted arrows denote more than one step. Solid arrows denote a single step attributed to the corresponding yeast polypeptide named above each arrow. The corresponding human homologs are named below each arrow. The main ring precursor used by both yeast and humans is 4-hydroxybenzoic acid (4HB). Yeasts synthesize 4HB de novo from chorismate or may obtain it from the metabolism of tyrosine. Humans rely on tyrosine to produce 4HB (or on phenylalanine and phenylalanine hydroxylase to produce tyrosine). Yeast and human cells produce isopentenyl pyrophosphate (IPP) and dimethylally pyrophosphate (DMAPP) as precursors to form hexaprenyl diphosphate (n=6) via Coq1 in yeast or decaprenyl diphosphate (n=10) via PDSS1/PDSS2 in humans. Yeast Coq2 and human COQ2 attach the polyisoprenyl tail to 4HB. Subsequent to this step, the next three intermediates are identified as yeast hexaprenyl-intermediates: HHB, 3-hexaprenyl-4HB DHHB, 3-hexaprenyl-4,5-dihydroxybenzoic acid HMHB, 3-hexaprenyl-4-hydroxy-5-methoxybenzoic acid. The next three intermediates are hydroquinones: DDMQH2, 2-hexaprenyl-6-methoxy-1,4-benzenediol DMQH2, 2-hexaprenyl-3-methyl-6-methoxy-1,4-benzenediol DMeQH2, 2-hexaprenyl- 3-methyl-6-methoxy-1,4,5-benzenetriol to ultimately produce the final reduced product (CoQnH2). Red text identifies para-aminobenzoic acid (pABA) as an alternate ring precursor utilized by yeast (but not by humans). The next three intermediates are identified as yeast hexaprenyl-intermediates: HAB, 4-amino-3-hexaprenylbenzoic acid HHAB, 4-amino-3-hexaprenyl-5-hydroxybenzoic acid HMAB, 4-amino-3-hexaprenyl-5-methoxybenzoic acid. The next two intermediates are: IDDMQH2, 4-amino-3-hexaprenyl-5-methoxyphenol IDMQH2, 4-amino-3-hexaprenyl-2-methyl-5-methoxyphenol. The step denoted by the red dotted arrow depends on yeast Coq6 and converts HHAB into DHHB. Interconversion of (CoQnH2) and (CoQn) is shown via a reversible two-electron reduction and oxidation. Steps indicated by ‘. ’ are catalyzed by as yet unknown enzymes. Alternative compounds that may serve as ring precursors in CoQ biosynthesis are shown at the bottom of the panel: p-coumaric acid, resveratrol, and kaempferol. Analogs of 4HB that can function to bypass certain deficiencies in the CoQ biosynthetic pathway include: 3,4-dihydroxybenzoic acid (3,4-diHB), vanillic acid and 2,4-dihydroxybenzoic acid (2,4-diHB). It is not yet known whether 2-methyl-4HB (2-methyl-4HB) may also serve a bypass function.

The yeast model also provided early evidence that the eukaryotic CoQ biosynthetic pathway was localized to mitochondria. The Coq (denotes S. cerevisiae polypeptide involved in CoQ6 biosynthesis) polypeptides are nuclear encoded, and amino-terminal mitochondrial targetting sequences are needed to direct their transport to the mitochondrial matrix (Coq1, Coq3–Coq11) or to the inner mitochondrial membrane (Coq2). Assembly of Coq3–Coq9 plus Coq11 polypeptides into a high molecular mass complex termed the CoQ synthome in yeast (Figure 2) and Complex Q in human cells is another conserved feature of CoQ biosynthesis [7,11]. These complexes are essential for the biosynthesis of CoQ in yeast and human cells, and may serve to enhance catalytic efficiency and to minimize the escape of intermediates that may be toxic due to their redox or electrophilic properties. The CoQ-intermediates are quite hydrophobic and at least some of them appear to be essential partners in the assembly of the membrane-bound CoQ synthome [12] and Complex Q [7,13].