The role of Tyr-139, which is thought to be located at the active site of Thermus thermophilus HB8 3-isopropylmalate dehydrogenase, has been investigated by site-specific replacement with phenylalanine. The replacement scarcely affected the Michaelis constant (K m) for 3-isopropylmalate, but caused a 13-fold decrease of that for NAD. The catalytic constant (k cat) showed a 14-fold decrease. Accordingly, the catalytic efficiency (k cat/K m) decreased for 3-isopropylmalate but not for NAD. The results suggest that Tyr-139 is involved in the catalytic function through interaction with 3-isopropylmalate.
