Cofactor, acetylation fatty acid oxidation

Although the fatty acid oxidation scheme works neatly for even-numbered chain lengths, it can t work completely for fatty acids that contain an odd number of carbons. P-oxidation of these compounds leads to propionyl-CoA and acetyl-CoA, rather than to two acetyl-CoA at the final step. The propionyl-CoA is not a substrate for the TCA cycle or other simple pathways. Propionyl-CoA undergoes a carboxylation reaction to form methylmalonyl-CoA. This reaction requires biotin as a cofactor, and is similar to an essential step in fatty acid biosynthesis. Methylmalonyl-CoA is then isomerized by an epimerase and then by methylmalonyl-CoA mutase—an enzyme that uses Vitamin Bi2 as a cofactor—to form succinyl-CoA, which is a TCA-cycle intermediate. 

The rate of fatty acid oxidation is most likely regulated by the carnitine-dependent transport of acyl residues across the mitochondrial inner membrane, the supply of fatty acids to the cell, and the concentration of cofactors such as CoA and carnitine. Moreover, feedback inhibition is exerted by NADH on 3-hydroxyacyl-CoA dehydrogenase and by acetyl-CoA on 3-ketothiolase. Interestingly, malonyl-CoA, an intermediate in the fatty acid biosynthetic pathway, is a strong inhibitor ofCAT-I. 

The studies of the individual enzymes of fatty acid synthesis in higher plants has shown that the two reductive steps, p-ketoacyl ACP reductase and enoyl ACP reductase have different cofactor requirements. As a result the synthesis of fatty acids depends on the availability of both NADH and NADPH. While the provision of NADPH can be attributed to the photosynthetic reactions, the source of NADH in the chloroplast is less certain. Takahama etal (8) have demonstrated that the content of NADPH in the chloroplast is influenced by illumination as expected, but there is no such fluctuation of the oxidation state of NAD/NADH. The production of NADH to be utilized in fatty acid synthesis would therefore appear to depend on dark reactions. One possibility would be by the action of pyruvate dehydrogenase, which would generate not only the NADH required for reduction in fatty acid synthesis but also the precursor acetyl CoA. 

It is, however, better known that flavoenzymes (i.e., enzymes utilizing the flavin adenine dinucleotide [FAD FADH2] redox system) mediate the introduction of a,P carbon-carbon double bonds into carboxylic acids and into acetyl Coenzyme A (acetyl CoA) thioesters of long-, medium-, and short-chain fatty acids. In carboxylic acids, such as those of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11) the oxidation is affected by the enzyme sucdnate dehydrogenase (fumerate reductase— EC 1.3.99.1), which utilizes the cofactor flavin adenine dinucleotide (FAD) The latter is reduced to FADH2 and an ( )-double bond is introduced. The process shown in Scheme 9.105, for the conversion of succinate (1,4-butanedioic acid) to fumerate [(E)-l,4-butenedioic acid], is a fragment of the tricarboxylic acid (citric acid, TCA, or Krebs) cycle (Chapter 11), which is the pathway commonly utilized for oxidative degradation of acetate to carbon dioxide. 

Evidence was obtained that fatty acid synthesis proceeds in the cytoplasmic fraction rather than in the mitochondria. A supernatant enzyme system was able to synthesize palmitic acid starting from acetyl-CoA in the presence of NADPH, Mn++, HCOg- and ATP. The purified enzyme system was free of oxidation enzymes. HCO3" was not incorporated into the fatty acid but had a cofactor role. Subsequent experiments showed that acetyl-CoA is carboxylated to malonyl-CoA by acetyl-CoA-carboxylase, which contains d-biotin as coenzyme. Early investigations gave strong evidence for the participation of biotin in a number of carboxylation (COg-fixation) reactions (Lardy et al. 1956). The enzyme acetyl-CoA-carboxylase has been isolated (Brady et al. 1958, Lynen et al. 1959, Wakil et al. 1958). COg forms the free carboxy-group of malonyl-CoA. The equation of the malonyl-CoA synthesis is ... 

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