GEM-DIALKYL SUCCINIC ACIDS - A NOVEL CLASS OF INHIBITORS FOR CARBOXYPEPTIDASES

gem-Dimethylsuccinic add and its higher homolog, 2-methyl-2-ethylsucci nic acid (MESA) are highly potent inhibitors of both carboxypeptidase A (CPA) and B. The inhibition constant of MESA for CPA (0.11 mu M for the racemic mixture) is remarkable considering the relatively simple s tructure of the compound. The molecular feature which is crucial for h igh affinity binding to both carboxypeptidases appears to be the nonpo lar gem-dialkyl locus. The structure of the complex between MESA and C PA has been determined by X-ray crystallography to 2.0 Angstrom resolu tion and shows the R enantiomer of the inhibitor to be bound in a gene rally substrate-like manner, The carboxymethyl group is coordinated to the Zn ion in the active site, and the gem-dialkyl locus corresponds in position to the alpha-carbon of the C-terminal amino acid in a pept ide substrate, The methyl group of the inhibitor occupies a cavity in the enzyme which is apparently not filled upon substrate-binding, We p ostulate that this cavity (the alpha-methyl hole) is designed to allow the proximal Glu-270 residue to undergo a critical movement during ca talysis, The hydrophobic nature of the above cavity may play a role in modulating the reactivity of this residue. These results suggest that similar cenophilic (empty-loving) inhibitors may be found for other e nzymes.