Pyruvate degradation

In the reaction that bridges glycolysis and the TCA cycle, for each pyruvate degraded to acetyl CoA, one CO2 is released and a further NAD+ is reduced to NADH + H+. 

Acetyl-GoA can be formed either by P-oxidation of fatty acids or by decarboxylation of pyruvate. (Degradation of certain amino acids also produces acetyl-GoA see Section 23.6.) Most of these reactions take place in the mitochondria, requiring a transport mechanism to export acetyl-GoA to the cytosol for fatty-... 

The acetyl-CoA derived from amino acid degradation is normally insufficient for fatty acid biosynthesis, and the acetyl-CoA produced by pyruvate dehydrogenase and by fatty acid oxidation cannot cross the mitochondrial membrane to participate directly in fatty acid synthesis. Instead, acetyl-CoA is linked with oxaloacetate to form citrate, which is transported from the mitochondrial matrix to the cytosol (Figure 25.1). Here it can be converted back into acetyl-CoA and oxaloacetate by ATP-citrate lyase. In this manner, mitochondrial acetyl-CoA becomes the substrate for cytosolic fatty acid synthesis. (Oxaloacetate returns to the mitochondria in the form of either pyruvate or malate, which is then reconverted to acetyl-CoA and oxaloacetate, respectively.)... 

Pyruvate, produced by catabolism of glucose (and by degradation of several amino acids), can undergo several further transformations depending on the conditions and on the organism. In the absence of oxygen, pyruvate can be either reduced by NADH to yield lactate [CHjCHfOHjCO - or, in yeast,... 

Biomolecules are synthesized as well as degraded, but the pathways for anabolism and catabolism are not the exact reverse of one another. Fatty acids are biosynthesized from acetate by an 8-step pathway, and carbohydrates are made from pyruvate by the 11-step gluconeogenesis pathway. 

In summary, and regarded at its simplest, A. rtiger degrades suitable carbon sources by glycolysis which is occurring at an uncontrolled rate. There is inevitably an overflow at the pyruvate level which is converted to dtric acid. 

Pyruvate-dependent lyases serve catabolic functions in vivo in the degradation of sialic acids and KDO (2-keto-3-deoxy-manno-octosonate), and in that of 2-keto-3-deoxy aldonic acid intermediates from hexose or pentose catabolism. 

N-Acetylneuraminic acid aldolase (or sialic acid aldolase, NeuA EC 4.1.3.3) catalyzes the reversible addition of pyruvate (2) to N-acetyl-D-mannosamine (ManNAc (1)) in the degradation of the parent sialic acid (3) (Figure 10.4). The NeuA lyases found in both bacteria and animals are type I enzymes that form a Schiff base/enamine intermediate with pyruvate and promote a si-face attack to the aldehyde carbonyl group with formation of a (4S) configured stereocenter. The enzyme is commercially available and it has a broad pH optimum around 7.5 and useful stability in solution at ambient temperature [36]. 

The KDO aldolase (KdoA, EC 4.1.2.23) is involved in the catabolism of the eight-carbon sugar d-KDO, which is reversibly degraded to D-arabinose (15) and pyruvate (Figure 10.10). The enzyme has been partially purified from bacterial sources and studied for synthetic applications [71,74]. It seems that the KdoA, similar to... 

Attention is drawn to the dechlorination by anaerobic bacteria of both chlorinated ethenes and chlorophenolic compounds that serve as electron acceptors with electron donors including formate, pyruvate, and acetate. This is termed dehalorespiration and is important in the degradation of a range of halogenated compounds under anaerobic conditions, and is discussed further in Chapter 3, Part 2 and Chapter 7, Part 3. 

This enzyme catalyzes the conversion of pyruvate to formate and acetyl CoA and is a key enzyme in the anaerobic degradation of carbohydrates in some Enterobacteriaceae. Using an enzyme selectively C-labeled with glycine, it was shown by EPR that the reaction involves production of a free radical at C-2 of glycine (Wagner et al. 1992). This was confirmed by destruction of the radical with O2, and determination of part of the structure of the small protein that contained an oxalyl residue originating from gly-734. 

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