2014 年 88 巻 7 号 p. 358-365
Homoserine dehydrogenase catalyzes the conversion of L-aspartate-4-semialdehyde to L-homoserine. Homoserine dehydrogenase is required for the biosynthesis of the three essential amino acids, i.e. lysine, methionine, and isoleucine, from aspartic acid. This enzyme attracts attention also as a promising antifungal drug target. We have determined the crystal structures of homoserine dehydrogenase from Thermus thermophilus HB8 in both substrate-free form and homoserine-binding form by X-ray diffraction. Crystallization conditions were surveyed at 293 K by using a hanging-drop vapor-diffusion method. The substrate-free form was solved to 1.4 Å resolution and the homoserine-binding form was solved to 2.0 Å resolution. The homoserine dehydrogenase was a dimer with each subunit composed of three distinct domains, nucleotide-binding, dimerization, and substrate-binding domain. In the homoserine-binding forms, the amino group of homoserine made a hydrogen bond to the side chain carboxylate of Glu180. This hydrogen bond induced the conformation change of Thr165-Pro183 loop, which made the loop close to the active site. The substrate homoserine was rigidly recognized by several amino acid residues in the enzyme active site, indicating that Lys99 or Lys195 would be an essential catalytic residue to facilitate hydride transfer.