Principles of Biochemistry, Volume 2 - A. Lehninger 1985

Bioenergetics and Metabolism
Lipid Biosynthesis
Chapter Summary

Long-chain saturated Fatty acids are synthesized from acetyl-CoA by a cytoplasmic enzyme complex that includes the acyl carrier protein (ACP), which contains phosphopantetheine as a prosthetic group. The 3-ketoacyl-ACP synthase molecule contains two —SH groups essential for its catalytic activity: one belonging to phosphopantetheine (Pp) and the other to a Cysteine (Cys) residue. ACP Functions as a carrier of fatty acid synthesis intermediates. Acetyl-S-Cys-ACP, formed from acetyl-CoA, reacts with malonyl-S-ACP (derived from malonyl-CoA) to yield acetoacetyl-S-ACP with the release of CO₂. This is followed by the reduction of acetoacetyl-S-ACP to the D-3-hydroxy derivative, which is then hydrated to trans-∆²-butenoyl-S-ACP; the double bond in the acyl group of this compound is reduced and saturated at the expense of NADPH, forming butyryl-S-ACP. Subsequently, six more molecules of malonyl-S-ACP are sequentially added to the carboxyl end of the growing fatty acid chain, resulting in palmitoyl-S—Cys-ACP—the final product of the Reactions Catalyzed by the fatty acid synthase complex. This is followed by the hydrolytic release of free palmitic acid. Elongation of palmitic acid yields the 18-carbon stearic acid. Mixed-function oxygenases introduce double bonds into the palmitic and stearic acid molecules, leading to The formation of palmitoleic and oleic acids, respectively. Mammals are unable to synthesize linoleic acid and must obtain it from plant-derived dietary sources; exogenous linoleic acid can be converted by mammals into arachidonic acid, which serves as a precursor for Prostaglandins.

Triacylglycerols are synthesized through the reaction of two molecules of fatty acid CoA derivatives with glycerol-3-phosphate, producing phosphatidic acid, which is then dephosphorylated to diacylglycerol. The latter is acylated by a third fatty acid CoA derivative to form triacylglycerol. Diacylglycerols also serve as the primary precursors of phosphoglycerols. The HEAD group of phosphatidylethanolamine is initially formed as cytidine diphosphate ethanolamine via the interaction of cytidine triphosphate (CTP) with phosphoethanolamine. The phosphoethanolamine group is then transferred to diacylglycerol, yielding phosphatidylethanolamine. Phosphatidylcholine is formed either by the methylation of phosphatidylethanolamine or through the reaction of diacylglycerol with cytidine diphosphate Choline. Cholesterol is likewise synthesized from acetyl-CoA through a highly complex sequence of reactions involving important intermediates such as hydroxymethylglutaryl-CoA, mevalonate, and the linear hydrocarbon squalene. Squalene undergoes cyclization to form the characteristic fused-ring steroid system with a side chain. Cholesterol Biosynthesis is inhibited by dietary cholesterol.

References

FATTY ACID BIOSYNTHESIS. Cunningham, E. B. Biochemistry: Mechanisms of METABOLISM, McGraw-Hill, New York, 1978. Chapter 12 provides extensive additional information regarding the enzymology and mechanism of fatty acid synthesis.

Jeffcoat, R. The biosynthesis of Unsaturated fatty acids and Its Control in Mammalian Liver, Essays Biochem., 15, 1-36 (1979).

Lipid Biosynthesis

Snyder, F. (ed.). Lipid Metabolism IN Mammals, vols. 1 and 2, Plenum, New York, 1977.

Cholesterol Metabolism

Bloch, K. S. The Biological Synthesis of Cholesterol, Science, 150, 19-28 (1965).

Brown, M. S., Goldstein, J. L. Receptor-Mediated Control of Cholesterol Metabolism, Science, 191, 150-154 (1976).

General Topics

Clarke, M. R. The Head of the Sperm Whale, Sci. Am., 240, 128-141, January 1979. For more detailed information on the biological function of spermaceti in sperm whales.

Nelson, R. A. Protein and Fat Metabolism in Hibernating Bears, Fed. Proc., 39, 2955-2958 (1980).

Genetic Disorders

of Lipid Metabolism

Allison, A. C. Lysosomes. In: J. J. Head (ed.). Carolina Biology Readers, Carolina Biological Supply Company, Burlington, N.C., 1977.

Brown, M. S., Goldstein, J. L. Familial Hypercholesterolemia: Defective Binding of Lipoproteins to Cultured Fibroblasts Associated with Impaired Regulation of 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase Activity, Proc. Natl. Acad. Sci. USA, 71, 788-792 (1974).

Dietschy, J. M., Gotto, A. M., Jr., Ontko, J. A. (eds.). Disturbances in Lipid and Lipoprotein Metabolism, American Physiological Society, 1978.

Stanbury, J. B., Wyngaarden, J. B., Fredrickson, D. S. (eds.). The Metabolic Basis of Inherited Disease, 4th ed., McGraw-Hill, New York, 1978.

Questions and Problems

1. The Role of carbon dioxide in fatty acid synthesis. Carbon dioxide is an obligate participant in fatty acid biosynthesis. What is the specific Role of CO₂? Will palmitate formed during the incubation of a soluble liver fraction with ¹⁴CO₂ and other components required for fatty acid biosynthesis contain ¹⁴C? Explain your answer.

2. The pathway of carbon in fatty acid synthesis. Based on your knowledge of fatty acid biosynthesis, explain the following experimental findings:

a) The addition of uniformly labeled ¹⁴C-acetyl-CoA to a soluble liver fraction leads to the formation of uniformly labeled ¹⁴C-palmitate.

b) However, the addition of trace amounts of uniformly labeled ¹⁴C-acetyl-CoA to a soluble liver fraction in the presence of excess malonyl-CoA leads to the formation of palmitate containing ¹⁴C only at positions 15 and 16.

3. Overall equation for fatty acid synthesis. Write the overall equation for the biosynthesis of palmitic acid in rat liver, starting from mitochondrial acetyl-CoA and cytosolic NADPH, ATP, and CO₂.

4. The pathway of hydrogen in fatty acid synthesis. Let us consider a preparation containing all the Enzymes and Cofactors required for the biosynthesis of a fatty acid from acetyl-CoA and malonyl-CoA.

a) How many deuterium atoms (the heavy isotope of hydrogen) will be incorporated into each palmitic acid molecule if deuterium-labeled acetyl-CoA is used as a substrate

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along with an excess of unlabeled malonyl-CoA? At which positions will they be located? Explain your answer, b) How many deuterium atoms will be incorporated into each palmitate molecule if unlabeled acetyl-CoA and deuterium-labeled malonyl-CoA are used as substrates

At which positions will they be located? Provide an explanation.

5. Generation of NADPH required for fatty acid synthesis. Because The inner mitochondrial membrane is impermeable to acetyl-CoA, acetyl groups enter the Cytosol via a shuttle mechanism (see the scheme in Fig. 21-3). The cytosol contains an NADP-dependent malate dehydrogenase that catalyzes the reaction

Assuming that the inner mitochondrial membrane is permeable to Pyruvate (as well as to citrate and malate), propose a shuttle mechanism for the generation of NADPH in the cytosol involving the NADP-dependent malate dehydrogenase and Other Enzymes known to be localized in the Cell/35.html">Mitochondria and cytosol. Write the overall reaction for The transport of acetyl groups from the mitochondria into the cytosol.

6. Regulation of acetyl-CoA carboxylase activity by modulators. Acetyl-CoA carboxylase is the key regulatory step in fatty acid biosynthesis. Some properties of this enzyme are listed below.

a) The addition of citrate or isocitrate increases the $V_{\max}$ of the enzyme more than 10-fold.

б) Фермент существует в двух резко различающихся по активности формах, которые могут превращаться друг в друга.

Citrate and isocitrate bind preferentially to the filamentous form of the enzyme, whereas palmitoyl-CoA binds to the protomeric form.

Explain how these properties relate to the regulatory role of acetyl-CoA carboxylase in fatty acid biosynthesis.

7. The energetic "cost" of triacylglycerol synthesis. Based on the overall equation for the biosynthesis of tripalmitin from glycerol and palmitic acid, calculate how many ATP molecules are required for the formation of a single tripalmitin molecule.

8. Energy Requirements during phosphatidylcholine synthesis. Write the sequential steps and the overall equation for the biosynthesis of phosphatidylcholine via the salvage pathway starting from oleic and palmitic acids, dihydroxyacetone phosphate, and choline. How many ATP molecules are required for the synthesis of phosphatidylcholine by this pathway when starting with oleic acid, palmitic acid, and dihydroxyacetone phosphate?

9. Treatment of hypercholesterolemia. Plants do not synthesize cholesterol; instead, they produce other sterols known as phytosterols. The Structure of one of these, $\beta$-sitosterol, is shown here. When patients with hypercholesterolemia ingest $\beta$-sitosterol in their diet, their plasma cholesterol levels decrease, which should reduce the risk of atherosclerosis. Propose potential Mechanisms of action for $\beta$-sitosterol.

10. Interrelationship between Amino acid metabolism and fatty acid metabolism. A rat was administered a preparation of 3-¹⁴C-Alanine

One hour later, the animal was sacrificed, and Lipids were extracted from the liver. The palmitate obtained from the extraction contained ¹⁴C. How can this be explained? At which position in the palmitate molecule is the ¹⁴C located? Can alanine serve as a precursor in the actual de novo synthesis of palmitate?

11. Differences between anabolic and Catabolic pathways of fatty acids.

A single turn of the anabolic and catabolic cycles for short-chain fatty acids can be represented by the equation

a) Compare the overall equations of the Catabolic and anabolic pathways. Are they simply reverse reactions of one another? What are the differences between them?

b) What specific factors allow these pathways to operate independently of each other?