Fumarase substrate specificity

S. Except for oxido-reductases, transferases, and hydrolases, most ligases (enzymes that catalyze bond formation) are entirely substrate specific. Thus, fumarate hydratase (or fumarase) reversibly and stereospecifically adds water to fumaric acid to produce (S)-( — )-malic acid only (8) (Figure 1), and another enzyme, mesaconase, adds water to mesaconic acid to form (+ )-citramalic acid (9) (Figure 2). Although no extensive studies are available, it appears that neither fumarase nor mesaconase will add water stereospecifically to any other a,(3-unsaturated acid. 

The stereospecific addition of water to fumaric add catalyzed by the enzyme fumarase yields optically pure L-malic add (Fig. 28). A Brevibac-terium flctvum strain with high fumarase activity has been used industrially for the commerdal production of L-malic add (97). The substrate specificity of fumarase is narrow and hence its broader application in organic synthesis has been somewhat limited. However, it has been shown to synthesize L-fferco-chloromalic add in very high optical purity (98). 

Isotopic Studies with Fumarase. The kinetic data reported above have led to preliminary attempts to interpret the nature of the binding of substrate to enzyme in terms of specific amino acid side chains. Another approach to the nature of the reaction was made in studies with D20. It was found that the hydrogens of fumarate do not equilibrate with... 

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