General Reactions Involving Pyruvate

L-Lactate dehydrogenase (cytochrome) [Lactic acid dehydrogenase] (1.1.2.3) is present. 

Pyruvate decarboxylase [a-Carboxylase, a-Ketoacid carboxylase] (4.1.1.1) is present. 

---

Some catabolic reactions of amino acid carbon chains are easy transformations to and from TCA cycle intermediates—for example, the transamination of alanine to pyruvate. Reactions involving 1-carbon units, branched-chain, and aromatic amino acids are more complicated. This chapter starts with 1-carbon metabolism and then considers the catabolic and biosynthetic reactions of a few of the longer side chains. Amino acid metabolic pathways can present a bewildering amount of material to memorize. Perhaps fortunately, most of the more complicated pathways lie beyond the scope of an introductory course or a review such as this. Instead of a detailed listing of pathways, this chapter concentrates on general principles of amino acid metabolism, especially those that occur in more than one pathway. 

The decarboxylation of pyruvic acid is an example of a more general type of biochemical reaction the decarboxylation of a-keto acids. The reaction is complex and occurs in several consecutive steps. The intermediates have been identified, but little is known of the enzymes involved. The reaction starts with the complexion of pyruvic acid with one molecule of enzyme-bound thiamine pyrophosphate. This is followed by decarboxylation of pyruvic acid and the formation of an intermediate, 2-acetylthiamine pyrophosphate, in which the aldehyde carbon of the acetyl is bound to the carbon 2 of the thiozole ring of the thiamine pyrophosphate. In the second step, the aldehyde is oxidized, the disulfide bond of enzyme-bound lipoic acid is reduced, and the free enzyme-bound thiamine pyrophosphate is restored. The tWrd step of the reaction involves the transacylation from reduced lipoic acid to CoA. Finally, lipoic acid is reoxidized by the catalytic activity of an NAD-dependent flavoprotein, lipoic dehydrogenase (see Fig. 1-14). 

Although the metabolism of vanillate generally involves de-O-methylation to 3,4-dihy-droxybenzoate followed by intradiol ring hssion, in Acinetobacter Iwoffii vanillate is hydroxylated to 3-0-methyl gallate, which produces pyruvate and oxalacetate in reactions that have already been noted (Sze and Dagley 1987). 

In the hydrogenation of ethyl pyruvate in the presence of Pt-Al203 modified by 10,11-dihydrocinchonidine, alkaloid adsorption leads to a marked increase in reaction rate (44). The actual hydrogenation involves two kinds of reactive sites, chirally modified Pt (Ptm) and unmodified metal (Pt ). Accordingly, the reaction is analyzed in terms of a general two-cycle mechanism (Scheme 18). The first cycle is ligand-... 

CoA thioesters are also the products of the oxidative decarboxylation reactions of a-keto acids, especially pyruvate and a-ketoglutarate, from which acetyl-CoA and succinyl-CoA are formed, respectively (Equation (14)). Three distinct types of enzymes catalyze such reactions however, the mechanistic involvement of CoA is generally rather limited for two of these, and only a brief discussion of each will be provided here. For more detailed information on these enzymes, the reader is referred to the relevant chapters on thiamin and lipoic acid enzymology and on radical enzymes in this series (see Chapters 1.08 and 7.03). 

Deamination of amino acids in animal tissue is generally effected by transamination with an a-keto-acid. In the majority of cases, this is 2-oxoglutarate formed by the citric acid cycle. Aspartate aminotransferase and alanine aminotransferase are examples of this kind of reaction. In Figure 2.7, transamination involving these enzymes is depicted as it is known to occur in mammalian liver. Note that the scheme shown here requires participation of oxalacetate and pyruvate and thus is intimately connected with metabolic pathways considered earlier. Serine and glycine are readily interconvertible in animal tissue by the enzyme serine hydroxymethyltransferase. It is worth noting also that decarboxylation of serine to ethanolamine as mentioned above can be followed by A -methylation to yield choline. Choline is both an essential component of many... 

---