Elucidation of the reaction mechanism of a PCB-degrading enzyme BphyC based on three-dimensional structural information.

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The so-called BphC enzyme is a dioxygenase which cleaves the catechol ring moiety situated in biphenyl and its derivatives. The cleaving point is located adjacent to the two neighboring hydroxyl groups in a catechol moiety, thus the enzyme works in an extradiol fashion. This kind of enzyme performs such reactions making use of molecular oxygen and incorporates both the atomic oxygen atoms under the catalytic influence of Fe (2+) or ferrous ion.The present investigators solved the three-dimensional structure of a BphC enzyme from Pseudomonas sp. stran KKS102 by X-ray structure analysis (J.Mol. Biol. 255,735-752 (1996)). The structural information coupled with various enzyme kinetic data on wild and mutant forms of the enzyme revealed the following points.As for the catalytic mechanism :1) The site of ring cleavage appears to be specifically determined by the fact that the molecular oxygen binding site is fixed relative to the poisition and orientation of the bound substrate molecules.2) One of the conserved residues, His 194, plays a role of ctalytic base abstracting a hydrogen from the susceptible hydroxyl group.As for the substrate specificity :1) The substrate binding site is essentially hydrophobic exhibiting very high complimentarity to the catechool ring moiety of the substrates.2) The Fe ion situated in the active site is not necessarily essential for substrate binding (but essential for catalysis, of course).