Racemase, alanine glutamic acid

Racemases are enzymes capable of interconverting D- to L-amino acids. Pyridoxal phosphate has been claimed to play a role as a cofactor in bacterial racemases for alanine, glutamic acid, and methionine, but not in others. It has also been claimed that in mammals the administration of pyridoxine facilitates the use of D-amino acids. 

The existence of enzymes in microorganisms which catalyze the interconversion of D- and L-amino acids is of considerable interest, since the intramolecular transfer of an amino group is apparently involved. The term racemase has been proposed for such enzymes. Two racemases have been reported. Alanine racemase has been shown to be present in a large number of microorganisms and has been partially purified from extracts of S. faecalis. Glutamic acid racemase has been demonstrated in acetone powders of Lactobacillus arabinosus. Both enzymes catalyze the interconversion of the n- and l- forms of their respective substrates. Alanine racemase requires pyridoxal phosphate as coenzyme. Pyri-doxamine phosphate under the conditions employed was not active. Glutamic acid racemase was found not to be affected by the addition of pyridoxal phosphate. However, further studies with purified preparations are necessary before pyridoxal phosphate can be excluded as cofactor for the glutamic acid racemase. Examination of animal tissues under conditions favorable for the demonstration of bacterial alanine racemase failed to reveal any activity. 

T Although D-amino acids do not generally occur in proteins, they do serve some special functions in the structure of bacterial cell walls and peptide antibiotics. Bacterial peptidoglycans (see Fig. 20-23) contain both D-alanine and D-glutamate. D-Amino acids arise directly from the l isomers by the action of amino acid racemases, which have pyridoxal phosphate as cofactor (see Fig. 18-6). Amino acid racemization is uniquely important to bacterial metabolism, and enzymes such as... 

Racemization. A proton can be added back to the original alpha position but without stereospecificity. A racemase which does this is important to bacteria. They must synthesize D-alanine and D-glu-tamic acid from the corresponding L-isomers for use in formation of their peptidoglycan envelopes.153-1543 The combined actions of alanine racemase plus D-alanine aminotransferase, which produces D-glutamate as a product, provide bacteria with both d amino acids. 

Bacteria utilize both D-alanine and D-glutamate in the synthesis of their peptidoglycan layers (Fig. 8-29). Both D-amino acids are formed by racemases. That of... 

Pyruvate is transaminated with hypotaurine by ca-ainino acid aminotransferase to form alanine, and acetaldehyde and sulinate, which are formed irreversibly from sulfinoace-toaldehyde and primarily produced [4R- H]NADH in a high yield. In contrast, [4S- H]NADH is produced with LeuDH (pro-S stereospecific) and amino acid racemase with low substrate specificity in a sunilar manner. [4S- H]NADPH can also be synthesized by means of NADP-dependent GluDH (pro-S stereosj ific) and glutamate racemase in a similar method [93]. 

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