Identification of Protein Fold and Catalytic Residues of g-Hexachlorocyclohexane Dehydrochlorinase LinA
Nagata, Y., Mori, K., Takagi, M., Murzin, A.G., Damborsky, J.
PROTEINS 45: 471-477 (2001)
g-Hexachlorocyclohexane dehydrochlorinase (LinA) is a unique dehydrochlorinase that has no homologous sequence at the amino acid-sequence level, and for which the evolutionary origin is unknown. We here propose that LinA is a member of a novel structural superfamily of enzymes containing scytalone dehydratase, 3-oxo-delta-steroid isomerase, nuclear transport factor-2, and the a-subunit of naphthalene dioxygenase—all known structures with different functions. The catalytic and the active site residues of LinA are predicted based on its homology model. Nine mutants that carry substitutions in the proposed catalytic residues were constructed by site-directed mutagenesis. In addition to these, eight mutants that have a potential to make contact with the substrate were prepared by site-directed mutagenesis. These mutants were expressed in E. coli, and their activities in crude extract were evaluated. Most of the features of the LinA mutants could be explained on the basis of the present LinA model, indicating its validity. We conclude that LinA catalyses the proton abstraction via the catalytic dyad H73-D25 by the similar mechanism as described for scytalone dehydratase. The results suggest that LinA and scytalone dehydratase evolved from a common ancestor. LinA may have evolved from an enzyme showing a dehydratase activity.