Acid Catabolism

Lactate is catabolized aerobically. Lactate is catabolized anaerobically. 

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Fatty acids with odd numbers of carbon atoms are rare in mammals, but fairly common in plants and marine organisms. Humans and animals whose diets include these food sources metabolize odd-carbon fatty acids via the /3-oxida-tion pathway. The final product of /3-oxidation in this case is the 3-carbon pro-pionyl-CoA instead of acetyl-CoA. Three specialized enzymes then carry out the reactions that convert propionyl-CoA to succinyl-CoA, a TCA cycle intermediate. (Because propionyl-CoA is a degradation product of methionine, valine, and isoleucine, this sequence of reactions is also important in amino acid catabolism, as we shall see in Chapter 26.) The pathway involves an initial carboxylation at the a-carbon of propionyl-CoA to produce D-methylmalonyl-CoA (Figure 24.19). The reaction is catalyzed by a biotin-dependent enzyme, propionyl-CoA carboxylase. The mechanism involves ATP-driven carboxylation of biotin at Nj, followed by nucleophilic attack by the a-carbanion of propi-onyl-CoA in a stereo-specific manner. 

FIGURE 25.1 The citrate-malate-pyruvate shuttle provides cytosolic acetate units and reducing equivalents (electrons) for fatty acid synthesis. The shuttle collects carbon substrates, primarily from glycolysis but also from fatty acid oxidation and amino acid catabolism. Most of the reducing equivalents are glycolytic in origin. Pathways that provide carbon for fatty acid synthesis are shown in blue pathways that supply electrons for fatty acid synthesis are shown in red. 

Muscalure, structure of, 287 Mutarotation, 985-986 glucose and, 985-986 mechanism of. 986 Mycomycin, stereochemistry of, 330 Mylar, structure of, 819 n yo-Inositol, structure of, 135 Myrcene. structure of, 202 Mvristic acid, catabolism of, 1137 structure of. 1062... 

Figure 29-2. Overall flow of nitrogen in amino acid catabolism.

Lim CB et al Reduction in the rates of protein and amino acid catabolism to slow down the accumulation of endogenous ammonia a strategy potentially adopted by mudskippers during aerial exposure in constant darkness. J Exp Biol 2001 ... 

TRANSAMINATION TYPICALLY INITIATES AMINO ACID CATABOLISM... 

Figure 30-19. The analogous first three reactions in the catabolism of leucine, valine, and isoleucine. Note also the analogy of reactions and to reactions of the catabolism of fatty acids (see Figure 22-3). The analogy to fatty acid catabolism continues, as shown in subsequent figures.

The catabolism of leucine, valine, and isoleucine presents many analogies to fatty acid catabolism. Metabolic disorders of branched-chain amino acid catabolism include hypervalinemia, maple syrup urine disease, intermittent branched-chain ketonuria, isovaleric acidemia, and methylmalonic aciduria. 

Liu, C., Russell, R.M., and Wang, X.D., Exposing ferrets to cigarette smoke and a pharmacological dose of beta-carotene supplementation enhance in vitro retinoic acid catabolism in lungs via induction of cytochrome P450 enzymes, J. Nutr., 133, 173, 2003. 

Ganesan B, P Dobrowski, BC Weimer (2006) Identification of the leucine-to-2-methylbutyric acid catabolic pathway of Lactococcus lactis. Appl Environ Microbiol 72 4264-4273. 

Iwagami S, K Yang, J Davies (2000) Characterization of the protocatechuate acid catabolic gene cluster from Streptomyces sp. strain 2065. Appl Environ Microbiol 66 1499-1508. 

Nonessential Amino Acid Synthesis Essential Amino Acids Amino Acid Degradation Generalities of Amino Acid Catabolism Products of Amino Acid Degradation... 

The nitrogen contained in the amino acids is usually disposed of through the urea cycle. One of the early, if not the first, steps in amino acid catabolism involves a transamination using oxaloacetate or a-ketoglutarate as the amino-group acceptor. This converts the amino acid into a 2-keto acid, which can then be metabolized further. 

Teplyakov, A., Obmolova, G., Toedt, J., Galperin, M. Y., and Gilliland, G. L. (2005). Crystal structure of the bacterial YhcH protein indicates a role in sialic acid catabolism. 

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