Glutamic racemase

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). 

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. 

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-... 

Figure 12.11 Superposition of active sites from the different proteins. The active site of SmeHyuA model (gray) is compared with that of (a) aspartate racemase from Pyrococcus horikoshii (IF)LA, black) and that of (b) glutamate racemase from Aquifex pyrophilus (1B73A, black).

Figure 17-9. Mechanism of glutamate racemase reaction. Cys 70 and Cysl78 serve as the bases to abstract an a-proton from the substrate, and a carbanion intermediate is formed. Alternatively, the racemization may proceed through a concerted mechanism. Reprinted from Hwang et al. 951.

Synthesis of D-Amino Acids with Glutamate Racemase... 

Figure 17-10. Enzymatic synthesis of D-amino acids by combination of glutamate racemase, glutamate dehydrogenase, D-amino acid aminotransferase and formate dehydrogenase reactions.

Table 17-4. Production of various D-amino acids by means of four purified enzymes glutamate racemase, D-amino acid aminotransferase, glutamate dehydrogenase, and formate dehydrogenase3.

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