Glutamic acid racemase

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. 

D-glutamic acid L-glutamic acid Glu decarboxylase + Glu racemase iMctohac. brevis 207... 

Racemization also occurs in the presence of microbial racemase. As for other amino acids, the racemase that is specific for glutamic acid is found in... 

The reaction mechanism for glutamate racemase has been studied extensively. It has been proposed that the key for the racemization activity is that the two cysteine residues of the enzyme are located on both sides of the substrate bound to the active site. Thus, one cysteine residue abstracts the a-proton from the substrate, while the other detivers a proton from the opposite side of the intermediate enolate of the amino acid. In this way, the racemase catalyzes the racemization of glutamic acid via a so-called two-base mechanism (Fig. 15). 

Among the numerous enzymes that utilize pyridoxal phosphate (PLP) as cofactor, the amino acid racemases, amino acid decarboxylases (e.g., aromatic amino acids, ornithine, glutamic acid), aminotransferases (y-aminobutyrate transaminase), and a-oxamine synthases, have been the main targets in the search for fluorinated mechanism-based inhibitors. Pharmaceutical companies have played a very active role in this promising research (control of the metabolism of amino acids and neuroamines is very important at the physiological level). 

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... 

A number of other racemases and epimerases may function by similar mechanisms. While some amino acid racemases depend upon pyridoxal phosphate (Chapter 14), several others function without this coenzyme. These include racemases for aspartate,113 glutamate,114-1153 proline, phenylalanine,116 and diamino-pimelate epimerase.117 Some spiders are able to interconvert d and l forms of amino acid residues in intact polypeptide chains.118119... 

Amino acid racemases have long been known to be important in bacterial metabolism, because several u-amino acids are required for the synthesis of cell wall mucopolysaccharides. u-Serine is found in relatively large amounts in mammalian brain, where it acts as an agonist of the N-methyl-n-aspartate (NMDA) glutamate receptor. Serine racemase has been purified from rat brain and cloned fromhuman brain (Wolosker et al., 1999 De Miranda et al., 2000). 

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. 

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]. 

The tertiary structure of glutamate racemase has already been resolved and it has also been shown that a substrate analog glutamine binds between two cysteine residues. These data enabled us to predict that the new proton-donating amino acid residue should be introduced at position 74 instead of Gly for the inversion of enantioselectivity of the decarboxylation reaction. 

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... 

The amino acid sequences of known hydantoin racemases present two highly conserved cysteines around positions 75 and 180 (see the asterisks in Figure 12.4). The enzymes involved in the racemization/epimerization of different substrates such as glutamate racemase and diaminopimelate epimerase present two cyste-... 

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