Structure of 6-oxo camphor hydrolase His122A mutant bound to its natural product, (2S-4S)-a-campholinic acid

The crotonase homolog, 6-oxo camphor hydrolase (OCH), catalyzes the desymmetrization of bicyclic {beta}-diketones to optically active keto acids via an enzymatic retro-Claisen reaction, resulting in the cleavage of a carbon-carbon bond. We have previously reported the structure of OCH (Whittingham, J. L., Turkenburg, J. P., Verma, C. S., Walsh, M. A., and Grogan, G. (2003) J. Biol. Chem. 278, 1744–1750), which suggested the involvement of five residues, His-45, His-122, His-145, Asp-154, and Glu-244, in catalysis. Here we report mutation studies on OCH that reveal that H145A and D154N mutants of OCH have greatly reduced values of kcat/Km derived from a very large increase in Km for the native substrate, 6-oxo camphor. In addition, H122A has a greatly reduced value of kcat, and its Km is five times that of the wild-type. The location of the active site is confirmed by the 1.9-Å structure of the H122A mutant of OCH complexed with the minor diastereoisomer of (2S,4S)-{alpha}-campholinic acid, the natural product of the enzyme. This shows the pendant acetate of the product hydrogen bonded to a His-145/Asp-154 dyad and the endocyclic carbonyl of the cyclopentane ring hydrogen bonded to Trp-40. The results are suggestive of a base-catalyzed mechanism of C-C bond cleavage and provide clues to the origin of prochiral selectivity by the enzyme and to the recruitment of the crotonase fold for alternate modes of transition state stabilization to those described for other crotonase superfamily members.