Long-chain saturated fatty acids are synthesized from acetyl-CoA by a cytosolic complex of six enzymes plus acyl carrier protein (ACP), which contains phosphopantetheine as its prosthetic group. The fatty acid synthase, which in some organisms consists of multifunctional polypeptides, contains two types of -SH groups (one furnished by the phosphopantetheine of ACP and the other by a Cys residue of the enzyme a-ketoacyl-ACP synthase) that function as carriers of the fatty acyl intermediates. Malonyl-ACP, formed from acetyl-CoA (shuttled out of mitochondria) and CO2, condenses with an acetyl bound to the Cys -SH to yield acetoacetyl-ACP with release of CO2. Reduction to the D-β-hydroxy derivative and its dehydration to the trans-Δ2-unsaturated acyl-ACP is followed by reduction to butyryl-ACP. For both reduction steps, NADPH is the electron donor. Six more molecules of malonyl-ACP react successively at the carboxyl end of the growing fatty acid chain to form palmitoyl-ACP, the end product of the fatty acid synthase reaction. Free palmitate is released by hydrolysis. Fatty acid synthesis is regulated at the level of malonyl-CoA formation.
Palmitate may be elongated to yield the 18carbon stearate. Palmitate and stearate in turn can be desaturated to yield palmitoleate and oleate, respectively, by the action of mixed-function oxidases. Mammals cannot make linoleate and must obtain it from plant sources. Mammals convert exogenous linoleate into arachidonate, the parent compound of a family of very potent hormonelike eicosanoids (prostaglandins, thromboxanes, and leukotrienes).
Triacylglycerols are formed by reaction of two molecules of fatty acyl-CoA with glycerol-3-phosphate to form phosphatidate, which is dephosphorylated to a diacylglycerol then acylated by a third molecule of fatty acyl-CoA to yield a triacylglycerol. This process is hormonally regulated. Triacylglycerols are carried in the blood in chylomicrons. Diacylglycerols are also the major precursors of glycerophospholipids. In bacteria, phosphatidylserine is formed by the condensation of serine with CDP-diacylglycerol, and decarboxylation of phosphatidylserine produces phosphatidylethanolamine. Phosphatidylglycerol is formed by condensation of CDP-diacylglycerol with glycerol-3phosphate followed by removal of the phosphate in monoester linkage. Yeasts use similar pathways in the synthesis of phosphatidylserine, phosphatidylethanolamine, and phosphatidylglycerol; phosphatidylcholine is formed by methylation of phosphatidylethanolamine. Mammalian cells have somewhat different pathways for synthesizing phosphatidylcholine and phosphatidylethanolamine. The head group alcohol (choline or ethanolamine) is activated as the CDP-derivative, then condensed with diacylglycerol. Phosphatidylserine is derived only from phosphatidylethanolamine. The synthesis of plasmalogens involves formation of their characteristic double bond by a mixedfunction oxidase. The head groups of sphingolipids are attached by unique mechanisms. Phospholipids are moved to their intracellular destinations by transport vesicles or specific proteins.
Cholesterol is formed from acetyl-CoA in a complex series of reactions through the intermediates β-hydroxy-β-methylglutaryl-CoA, mevalonate, and two activated isoprenes, dimethylallyl pyrophosphate and isopentenyl pyrophosphate. Condensation of isoprene units produces the noncyclic squalene, which is cyclized to yield the steroid ring system and side chain. Cholesterol synthesis is inhibited by elevated intracellular cholesterol. Cholesterol and cholesteryl esters are carried in the blood as plasma lipoproteins. Very low-density lipoprotein (VLDL) carries cholesterol, cholesteryl esters, and triacylglycerols from the liver to other tissues, where the triacylglycerols are degraded by lipoprotein lipase, converting VLDL to low-density lipoprotein (LDL). The LDL, rich in cholesterol and its esters, is taken up by receptor-mediated endocytosis, in which the apolipoprotein B-100 of LDL is recognized by LDL receptors in the plasma membrane. High-density lipoprotein (HDL) serves to remove cholesterol from the blood, carrying it to the liver. Dietary conditions or genetic defects in cholesterol metabolism may lead to atherosclerosis and heart disease.
The steroid hormones (glucocorticoids, mineralocorticoids, and sex hormones) are produced from cholesterol by alteration of the side chain and the introduction of oxygen atoms into the steroid ring system. In addition to cholesterol, a very wide variety of isoprenoid compounds are derived from mevalonate through condensations of isopentenyl pyrophosphate and dimethylallyl pyrophosphate. Prenylation of certain proteins targets them for association with cell membranes and is essential for their biological activity.
The general references in Chapters 9 and 16 will also be useful.
General
Gotto, A.M., Jr. (ed) (1987) Plasma Lipoproteins, New Comprehensive Biochemistry, Vol. 14 (Neuberger, A. & van Deenen, L.L.M., series eds), Elsevier Biomedical Press, Amsterdam.
Tiuelue reuiews couering the structure, synthesis, and metabolism of lipoproteins, regulation of cholesterol synthesis, and the enzymes LCAT and lipoprotein lipase.
Hawthorne, J.N. & Ansell, G.B. (eds) (1982) Phospholipids, New Comprehensive Biochemistry, Vol. 4 (Neuberger, A. & van Deenen, L.L.M., series eds), Elsevier Biomedical Press, Amsterdam.
A collection of reuiews that includes excellent couerage of the biosynthetic pathways to glycerophospholipids and sphingolipids, phospholipid transfer proteins, cznd bilayer assembly.
Mead, J.F., Alfin-Slater, R.B., Howton, D.R., & Popjak, G. (1986) Lipids: Chemistry, Biochemistry, and Nutrition, Plenum Press, New York. Chapters 8 (fatty acid synthesis), 9 (desaturation of fatty acids), 12 (digestion and absorption), 15 (cholesterol synthesis), 17 (glycerophospholipid metabolism), and 18 (sphingolipid metabolism) are especially germczne to the topics in this chapter.
Numa, S. (ed) (1984) Fatty Acid Metabolism and Its Regulation, New Comprehensive Biochemistry, Vol. 7 (Neuberger, A. & van Deenen, L.L.M., series eds), Elsevier Biomedical Press, Amsterdam.
An extremely helpful collection of reuiews; Chapters 1 (acetyl-CoA carboxylase), 2 (bacterial and animal fatty acid synthase), 4 (fatty acid desaturation), and 6 (fatty acid synthesis in plants) are particularly reluctant.
Biosynthesis of Fatty Acids and Eicosanoids
Capdevila, J.H., Falck, J.R., & Estabrook, R.W. (1992) Cytochrome P450 and the arachidonate cascade. FASEB J. 6, 731-736.
This issue of the FASEB J. contains 20 articles on the structure and function of uarious cytochrome P-450s.
Harwood, J.L. (1988) Fatty acid metabolism. Annu. Reu. Plant Physiol. Plant Mol. Biol. 39, 101138.
A good account of the compartmentcction and enzymology of fatty acid synthesis in plants; also discusses the desaturase systems.
Kim, K.-H., Lopez-Casillas, F., Bai, D.H., Luo, X., & Pape, M.E. (1989) Role of reversible phosphorylation of acetyl-CoA carboxylase in longchain fatty acid synthesis. FASEB J. 3, 22502256.
An advanced discussion of hormonal regulation of this enzyme by covalent alteration.
Lands, W.E.M. (1991) Biosynthesis of prostaglandins. Annu. Reu. Nutr. 11, 41-60.
Discussion of the nutritional requirement for unsaturated fatty acids and recent biochemical work on pathways from arachidonate to prostaglandins; advanced level.
McCarthy, A.D. & Hardie, D.G. (1984) Fatty acid synthase-an example of protein evolution by gene fusion. Trends Biochem. Sci. 9, 60-63.
A short account of the structure of the proteins of fatty acid synthase in bacteria, yeast, and vertebrates and the likely evolutionary route to their formation.
Wakil, S.J., Stoops, J.K., & Joshi, V.C. (1983) Fatty acid synthesis and its regulation. Annu. Reu. Biochem. 52, 537-579.
An aduanced discussion.
Biosynthesis of Membrane Phospholipids
Bishop, W.R. & Bell, R.M. (1988) Assembly of phospholipids into cellular membranes: biosynthesis, transmembrane movement and intracellular translocation. Annu. Reu. Cell. Biol. 4, 579-610. The enzymology and cell biology of phospholipid synthesis and targeting; aduanced leuel.
Browse, J. & Somerville, C. (1991) Glycerolipid synthesis: biochemistry and regulation. Annu. Rev. Plant Physiol. Plant Mol. Biol. 42, 467-506.
A detailed reuiew of the pathways to glycerolcontaining phospholipids in higher plants.
Kennedy, E.P. (1962) The metabolism and function of complex lipids. Harvey Lectures 57, 143-171. A classic description of the role of cytidine nucleotides in phospholipid synthesis.
Kent, C., Carman, G.M., Spence, M.W., & Dowhan, W. (1991) Regulation of eukaryotic phospholipid metabolism. FASEB J. 5, 2258-2266. The genetics and biochemistry of phospholipid metabolism in yeast, and the factors that regulate synthesis and intracellular transport of phospholipids.
Raetz, C.R.H. & Dowhan, W. (1990) Biosynthesis and function of phospholipids in Escherichia coli. J. Biol. Chem. 265, 1235-1238.
A brief reuiew of bacterial biosynthesis of phospholipids and lipopolysaccharides.
Wirtz, K.W.A. (1991) Phospholipid transfer proteins. Annu. Rev. Biochem. 60, 73-99.
Discussion of the proteins that are belieued responsible for transport of newly synthesized phospholipids from their sites of formation to their intracellular targets; aduanced level.
Biosynthesis of Cholesterol, Steroids, and Isoprenoids
Benveniste, P. (1986) Sterol biosynthesis. Annu. Reu. Plant Physiol. 37, 275-308.
A detailed reuiew of sterol synthesis, with emphasis on the differences between the path to cholesterol and that to the plant sterols.
Bloch, K. (1965) The biological synthesis of cholesterol. Science 150, 19-28.
The author's Nobel address; a classic description of cholesterol synthesis in animals.
Brown, M.S. & Goldstein, J.L. (1984) How LDL receptors influence cholesterol and atherosclerosis. Sci. Am. 251 (November), 58-66.
An introduction to the role of low-density lipoproteins in cholesterol transport in health and disease.
Glickman, R.M. & Sabesin, S.M. (1988) Lipoprotein metabolism. In The Liuer: Biology and Pathobiology, 2nd edn (Arias, I.M., Jakoby, W.B., Popper, H., Schachter, D., & Shafritz, D.A., eds), pp. 331-354, Raven Press, New York.
A very useful description of the structure, composition, synthesis, and roles of plasma lipoproteins.
Goldstein, J.L. & Brown, M.S. (1990) Regulation of the mevalonate pathway. Nature 343, 425-430. The allosteric and coualent regulation of the enzymes of the mevalonate pathway; includes a short discussion of the prenylation of Ras and other proteins.
Kleinig, H. (1989) The role of plastids in isoprenoid biosynthesis. Annu. Reu. Plant Physiol. Plant Mol. Biol. 40, 39-59.
The emphasis is on the unique features of isoprenoid synthesis in plants.
Myant, N.B. (1990) Cholesterol Metabolism, LDL, and the LDL Receptor. Academic Press, Inc., New York.
This advanced book covers the genetics, biochemistry, and cell biology of cholesterol synthesis and uptake in healthy individuals and in patients with familial hypercholesterolemia.
Rine, J. & Kim, S.-H. (1990) A role for isoprenoid lipids in the localization and function of an oncoprotein. New Biol. 2, 219-226.
A discussion of the isoprenylation of the Ras protein.
1. Pathway of Carbon in Fatty Acid Synthesis Using your knowledge of fatty acid biosynthesis, provide an explanation for the following experimental observations:
(a) The addition of uniformly labeled [14C]acetyl-CoA to a soluble liver fraction yields palmitate uniformly labeled with 14C.
(b) However, the addition of a trace of uniformly labeled [14C]acetyl-CoA in the presence of an excess of unlabeled malonyl-CoA to a soluble liver fraction yields palmitate labeled with 14C only in C-15 and C-16.
2. Synthesis of Fatty Acids from Glucose After a person has consumed large amounts of sucrose, the glucose and fructose that exceed caloric requirements are transformed to fatty acids for triacylglycerol synthesis. This fatty acid synthesis consumes acetyl-CoA, ATP, and NADPH. How are these substances produced from glucose?
3. Net Equation of Fatty Acid Synthesis Write the net equation for the biosynthesis of palmitate in rat liver, starting from mitochondrial acetyl-CoA and cytosolic NADPH, ATP, and CO2.
4. Pathway of Hydrogen in Fatty Acid Synthesis Consider a preparation that contains all the enzymes and cofactors necessary for fatty acid biosynthesis from added acetyl-CoA and malonylCoA.
(a) If [2-2H]acetyl-CoA (labeled with deuterium, the heavy isotope of hydrogen):and an excess of unlabeled malonyl-CoA are added as substrates, how many deuterium atoms are incorporated into every molecule of palmitate? What are their locations? Explain.
(b) If unlabeled acetyl-CoA and [2-2H]malonylCoA: are added as substrates, how many deuterium atoms are incorporated into every molecule of palmitate? What are their locations? Explain.
5. Energetics of β-Ketoacyl-ACP Synthase In the condensation reaction catalyzed by β-ketoacylACP synthase (Fig. 20-5), a four-carbon unit is synthesized by the combination of a two-carbon unit and a three-carbon unit, with the release of C02. What is the thermodynamic advantage of this process over one that simply combines two twocarbon units?
6. Modulation of Acetyl-CoA Carboxylase AcetylCoA carboxylase is the principal regulation point in the biosynthesis of fatty acids. Some of the properties of the enzyme are described below:
(a) The addition of citrate or isocitrate raises the Vm~ of the enzyme by as much as a factor of 10.
(b) The enzyme exists in two interconvertible forms that differ markedly in their activities:
Protomer (inactive) <===>filamentous polymer (active)
Citrate and isocitrate bind preferentially to the filamentous form, and palmitoyl-CoA binds preferentially to the protomer.
Explain how these properties are consistent with the regulatory role of acetyl-CoA carboxylase in the biosynthesis of fatty acids.
7. Shuttling of Acetyl Groups across the Inner Mitochondrial Membrane The acetyl group of acetylCoA, produced by the oxidative decarboxylation of pyruvate in the mitochondrion, is transferred to the cytosol by the acetyl group shuttle outlined in Figure 20-11.
(a) Write the overall equation for the transfer of one acetyl group from the mitochondrion to the cytosol.
(b) What is the cost of this process in ATPs per acetyl group?
(c) In Chapter 16 we encountered an acyl group shuttle in the transfer of fatty acyl-CoA from the cytosol to the mitochondrion in preparation for β oxidation (see Fig. 16-6). One result of that shuttle was separation of the mitochondrial and cytosolic pools of CoA. Does the acetyl group shuttle also accomplish this?
8. Oxygen Requirement for Desaturases The biosynthesis of palmitoleate (Fig. 20-14), a common unsaturated fatty acid with a cis double bond in the O9 position, uses palmitate as a precursor. Can this be carried out under strictly anaerobic conditions? Explain.
9. Energy Cost of Triacylglycerol Synthesis Use a net equation for the biosynthesis of tripalmitoylglycerol (tripalmitin) from glycerol and palmitate to show how many ATPs are required per molecule of tripalmitin formed.
10. Turnouer of Triacylglycerols in Adipose Tissue When [14C]glucose is added to the balanced diet of adult rats, there is no increase in the total amount of stored triacylglycerols, but the triacylglycerols become labeled with 14C. Explain.
11. Energy Cost of Phosphatidylcholine Synthesis Write the sequence of steps and the net reaction for the biosynthesis of phosphatidylcholine by the salvage pathway from oleate, palmitate, dihydroxyacetone phosphate, and choline. Starting from these precursors, what is the cost in number of ATPs of the synthesis of phosphatidylcholine by the salvage pathway?
12. Salvage Pathway for Synthesis of Phosphatidylcholine A young rat maintained on a diet deficient in methionine fails to thrive unless choline is included in the diet. Explain.
13. Synthesis of Isopentenyl Pyrophosphate If 2-[14C]acetyl-CoA is added to a rat liver homogenate that is synthesizing cholesterol, where will the 14C label appear in 03-isopentenyl pyrophosphate, the activated form of an isoprene unit?
14. HMG-CoA in Ketone Body Synthesis The ratelimiting step in the early stages of cholesterol biosynthesis is the conversion of β-hydroxy-βmethylglutaryl-CoA to mevalonate, catalyzed by HMG-CoA reductase (Fig. 20-32). The liver of a fasting animal has decreased reductase activity. When the flow through this reaction is reduced, what is the effect on the formation of ketone bodies from acetyl-CoA? How does this explain increased ketosis during fasting? (Hint: See Figure 16-16.)
15. Actiuated Donors in Lipid Synthesis In the biosynthesis of complex lipids, components are assembled by transfer of the appropriate group from an activated donor. For example, the activated donor of acetyl groups is acetyl-CoA. For each of the following groups, give the form of the activated donor:
(a) phosphate;
(b) D-glucosyl;
(c) phosphoethanolamine;
(d) n-galactosyl;
(e) fatty acyl;
(f) methyl;
(g) the two-carbon group in fatty acid biosynthesis;
(h) Δ3-isopentenyl.
