Antimicrobial Research Centre and Department of Biochemistry, McMaster University, 1200 Main Street West, Hamilton, Ontario, Canada L8N 3Z5.
The structure of the antifungal drug target homoserine dehydrogenase (HSD) was determined from Saccharomyces cerevisiae in apo and holo forms, and as a ternary complex with bound products, by X-ray diffraction. The three forms show that the enzyme is a dimer, with each monomer composed of three regions, the nucleotide-binding region, the dimerization region and the catalytic region. The dimerization and catalytic regions have novel folds, whereas the fold of the nucleotide-binding region is a variation on the Rossmann fold. The novel folds impose a novel composition and arrangement of active site residues when compared to all other currently known oxidoreductases. This observation, in conjunction with site-directed mutagenesis of active site residues and steady-state kinetic measurements, suggest that HSD exhibits a new variation on dehydrogenase chemistry.