Characterization of site-directed mutants of residues R58, R59, D116, W340 and R372 in the active site of E. coli cystathionine beta-lyase.

Cystathionine beta-lyase (CBL) catalyzes the hydrolysis Of L-cystathionine (L-Cth) to produce L-homocysteine, pyruvate, and ammonia. A series of active-site mutants of Escherichia coli CBL (eCBL) was constructed to investigate the roles of residues R58, R59, D116, W340, and R372 in catalysis and inhibition by aminoethoxyvinylglycine (AVG). The effects of these mutations on the k(cat)/K-m(L-Cth) for the P-elimination reaction range from a reduction of only 3-fold for D116A and D116N to 6 orders of magnitude for the R372L and R372A mutants. The order of importance of these residues for the hydrolysis Of L-Cth is: R372 >> R58 > W340 approximate to R59 > D116. Comparison of the kinetic parameters for L-Cth hydrolysis with those for inhibition of eCBL by AVG demonstrates that residue R58 tethers the distal carboxylate group of the substrate and confirms that residues W340 and R372 interact with the alpha-carboxylate moiety. The increase in the pK(a) of the acidic limb and decrease in the pK(a) of the basic limb of the k(cat)/K-m(L-Cth) versus pH profiles of the R58K and R58A mutants, respectively, support a role for this residue in modulating the pK(a) of an active-site residue.