Biochemistry: A Short Course

Transcription

1 Tymoczko Berg Stryer Biochemistry: A Short Course Second Edition CHAPTER 28 Fatty Acid Synthesis 2013 W. H. Freeman and Company

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3 Chapter 28 Outline

4 1. The first stage of fatty acid synthesis is transfer of acetyl CoA out of the mitochondria into the cytoplasm. Citrate is transported into the cytoplasm and cleaved into oxaloacetate and acetyl CoA. 2. The second state is the activation of acetyl CoA to form malonyl CoA. 3. The third stage is the repetitive addition and reduction of two carbon units to synthesize C 16 fatty acid. Synthesis occurs on an acyl carrier protein, a molecular scaffold.

5 Citrate, synthesized in the mitochondria, is transported to the cytoplasm and cleaved by ATP citrate lyase to generate acetyl CoA for fatty acid synthesis.

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7 Fatty acid synthesis requires reducing power in the form of NADPH. Some NADPH can be formed from the oxidation of oxaloacetate, generated by ATPcitrate lyase, by the combined action of cytoplasmic malate dehydrogenase and malic enzyme.

8 Pyruvate formed by malic enzyme enters the mitochondria where it is converted into oxaloacetate by pyruvate carboxylase. The sum of the reactions catalyzed by malate dehydrogenase, malic enzyme, and pyruvate carboxylase is: Additional NADPH is synthesized by the pentose phosphate pathway.

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10 Malonyl CoA is synthesized by acetyl CoA carboxylase, a biotin requiring enzyme. The formation of malonyl CoA occurs in two steps:

11 Fatty acid synthase, a complex of enzymes, catalyzes the formation of fatty acids. Fatty acid synthesis occurs on the acyl carrier protein (ACP), a polypeptide linked to CoA. Intermediates are linked to the sulfhydryl group of the CoA attached to ACP. Acetyl transacylase and malonyl transacylase attach substrates to the ACP.

12 β Ketoacyl synthase catalyzes the condensation of acetyl ACP and malonyl ACP to form acetoacetyl ACP. The next three steps a reduction, dehydration, and another reduction convert the keto group at carbon 3 to a methylene group ( CH 2 ), forming butyryl ACP. NADPH is the source of reducing power.

13 The second round of synthesis begins with the condensation of malonyl CoA with the newly synthesized butyryl ACP, forming C 6 β ketoacyl ACP. The reduction, dehydration, reduction sequence is repeated. Synthesis continues until C 16 acyl ACP, which is cleaved by thioesterase to yield palmitate.

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17 Enoyl reductase, the enzyme that catalyzes the reduction of the double bond in fatty acid synthesis, is inhibited by triclosan, a broad spectrum antibacterial compound that is added to a variety of household products.

18 The stoichiometry for the synthesis of palmitate is: The synthesis of the required malonyl CoA is described by the following reaction: Thus, the stoichiometry for the synthesis of palmitate from acetyl CoA is:

19 The reactions of fatty acid synthesis are similar in E. coli and animals. In animals, all of the enzymes required for fatty acid synthesis are components of a single polypeptide chain. The functional enzyme is composed of two identical chains. The enzyme consists of two distinct compartments. 1. The selecting and condensing compartment, which binds the acetyl and malonyl substrates and condenses them. 2. The modification compartment, which carries out the reduction and dehydration activities required for elongation.

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22 Tumors require large amounts of fatty acid synthase to produce precursors for membrane synthesis. Fatty acid synthase inhibitors retard tumor growth. Mice treated with fatty acid synthase inhibitors also showed dramatic weight loss, suggesting that such drugs may be used to treat obesity.

23 β Hydroxybutyric acid, when attached to ACP or CoA, is a substrate in fatty acid synthesis and degradation, and is a ketone body as well. An isomer of this key biochemical, γ hydroxybutyric acid is a potent, illegal drug.

24 Fatty acid synthase cannot generate fatty acids longer than C 16 palmitate. Longer fatty acids are synthesized by enzymes attached to the endoplasmic reticulum. These enzymes extend palmitate by adding two carbon units, using malonyl CoA as a substrate.

25 Enzymes bound to the endoplasmic reticulum introduce double bonds into saturated fatty acids. For instance: Mammals lack the enzymes that introduce double bonds beyond carbon 9. Linoleate and linolenate are essential fatty acids that must be obtained in the diet.

26 Arachidonate, a 20 carbon fatty acid with four double bonds, is derived from linoleate. Arachidonate is a precursor for a variety of signal molecules 20 carbons long, collectively called the eicosanoids. These signal molecules, which include prostaglandins, are local hormones because they are short lived and only affect nearby cells.

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28 Acetyl CoA carboxylase is subject to regulation on several levels. Carboxylase is inhibited when phosphorylated by AMP dependent kinase (AMPK). Inhibition due to phosphorylation is reversed by protein phosphatase 2A. Citrate actives carboxylase by facilitating the formation of active polymers of the carboxylase. Citrate mitigates inhibition due to phosphorylation. Palmitoyl CoA, the end product of fatty acid synthase, inhibits carboxylase by causing depolymerization of the enzyme. Carboxylase inhibits fatty acid degradation because its product, malonyl CoA, prevents the entry of fatty acyl CoA into the mitochondria by inhibiting carnitine acyl transferase 1.

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33 Glucagon and epinephrine inhibit carboxylase by enhancing AMPK activity. Insulin stimulates the dephosphorylation and activation of carboxylase. The enzymes of fatty acid synthesis are regulated by adapative control. If adequate fats are not present in the diet, the synthesis of enzymes required for fatty acid synthesis is enhanced.

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35 One pathway for ethanol processing consists of two steps and leads to excess production of NADH: Excess NADH inhibits gluconeogenesis and enhances lactate production, which may result in lactic acidosis. Excess NADH inhibits fatty acid degradation and stimulates fatty acid synthesis, leading to the accumulation of fats in the liver.

36 Liver can convert some of the acetate generated by aldehyde dehydrogenase into acetyl CoA, but the acetyl CoA cannot be processed by the citric acid cycle because of the paucity of NAD +. The build up of acetyl CoA can lead to ketone body secretion by the liver, which exacerbates the acidosis caused by lactate accumulation. If acetate cannot be processed, acetaldehyde accumulates. Acetaldehyde is very reactive and modifies reactive groups of proteins, causing a loss of protein function. As protein damage accumulates, liver function can fail.

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