Investigation of substrate activation by 4-chlorobenzoyl-coenzyme A dehalogenase.

4-Chlorobenzoyl-coenzyme A (4-CBA-CoA) dehalogenase catalyzes the hydrolysis of 4-CBA-CoA to 4-hydroxybenzoyl-coenzyme A (4-HBA-CoA), using the carboxylate side chain of aspartate 145 to displace the chloride from C(4) of the benzoyl ring. Previous UV-visible, Raman, and 13C NMR studies of enzyme-bound substrate analog or product ligand indicated that the environment of the enzyme active site induces a significant reorganization of the benzoyl ring pi-electrons. This observation was interpreted as evidence for electrophilic catalysis [viz. active-site-induced polarization of electron density away from the ring C(4)] [Taylor, K. L., Liu, R.-Q., Liang, P.-H., Price, J., Dunaway-Mariano, D., Tonge, P. J., Clarkson, J., & Carey, P. R. (1995) Biochemistry 34, 13881]. The recent crystal structure of the dehalogenase-4-HBA-CoA complex reveals two hydrogen bonds contributed to the benzoyl C=O by the backbone amide protons of Gly114 and Phe64 and a possible dipolar interaction with the positive pole of the 114-121 alpha-helix. Residues closely surrounding the benzoyl ring include W137, D145, W89, F64, F82, and H90. In the present study, the mutants D145A, H90Q, W137F, W89F, W89Y, F64L, F82L, and G114A were prepared to examine the effect of amino acid substitution on catalysis and on perturbation of the UV-visible spectral properties of the substrate benzoyl ring. Substitution of the two catalytic residues D145 and H90 inhibited catalysis but not ligand binding or the induction of the red shift in the benzoyl ring absorption. These two residues do not appear to contribute to substrate benzoyl ring binding or polarization. The F64L, F82L, W89F, and W137F mutants retained substantial catalytic activity and the ability to induce the red shift. The W89Y mutant, on the other hand, is inhibited in catalysis and ligand binding, suggesting that hydrophobicity more than packing may be critical for the benzoyl ring binding/activation. The G114A mutant was shown to be strongly inhibited in both substrate binding and activation, indicating that H-bonding and/or interaction with the dipole of the 114-121 alpha-helix may be crucial.