Tyr‐139 in Thermus thermophilus 3‐isopropylmalate dehydrogenase is involved in catalytic function

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Abstract
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.