Crystal structure of 2-hydroxyl-6-oxo-6-phenylhexa-2,4-dienoic acid (HPDA)hydrolase (BphD enzyme) from the Rhodococcus sp strain RHA1 of the PCB degradation pathway

2-Hydroxyl-6-oxo-6-phenylhexa-2, 2-dienoic acid (HPDA) hydrolase (the BphD enzyme) hydrolyzes a ring-cleavage product of an aromatic compound generate d in a biphenyl/polychlorinated biphenyl (PCB) degradation pathway of bacte ria. The crystal structure of the BphD enzyme has been determined at 2.4 An gstrom resolution by the multiple isomorphous replacement method. The final refined model of the BphD enzyme yields an R-factor of 17.5% at 2.4 Angstr om resolution with reasonable geometry. The BphD enzyme is an octameric enz yme with a 422 point-group symmetry. The subunit can be divided into core a nd lid domains. The active site of the enzyme is situated in the substrate- binding pocket, which is located between the two domains. The substrate-bin ding pocket can be divided into hydrophobic and hydrophilic regions. This f eature of the pocket seems to be necessary for substrate binding, as the su bstrate is composed of hydrophilic and hydrophobic parts. The proposed orie ntation of the substrate seems to be consistent with the general catalytic mechanism of alpha/beta -hydrolases. (C) 2001 Academic Press.