ANALYSIS OF THE QUATERNARY STRUCTURE, SUBSTRATE-SPECIFICITY, AND CATALYTIC MECHANISM OF VALINE DEHYDROGENASE

The solution of the three-dimensional structure of Bacillus sphaericus leucine dehydrogenase has enabled us to undertake a homology-based mo deling exercise on the sequence differences between the families of le ucine (LeuDH) and valine (ValDH) dehydrogenases. This analysis indicat es that the secondary structure elements in the core of the two domain s of a single subunit of these enzymes are conserved, as are residues directly implicated in the recognition of the nucleotide cofactor and in catalysis. Comparison of the sequences indicates that the residues in the pocket accommodating the side chain of the amino acid substrate are conserved be tween these two enzymes, suggesting that the small d ifferences in specificity arise from minor changes in molecular struct ure, possibly associated with shifts of the main chain rather than mut ation of residues in the pocket itself, While B. sphaericus LeuDH is a n octamer, both Streptomyces cinnamonensis and Streptomyces coelicolor ValDHs are dimers, The differences in quaternary structure can be und erstood in terms of the deletion in the latter of a C terminal loop, w hich forms important interactions around the four-fold axis in LeuDH.