Enhancement of the turnover number of thermostable malate dehydrogenase by deleting hydrogen bonds around the catalytic site.

In malate dehydrogenase from Thermus flavus, substitution of Ile for Thr189, which is involved in hydrogen-bond network around a loop containing catalytic His186, caused an increase in the turnover number at room temperatures. In order to examine the effect of disruption of the hydrogen bonds on the catalytic activity, residues participating in the network were replaced by others which cannot form hydrogen bond by site-directed mutagenesis. All the mutations caused increases in the turnover number at room temperatures. This result suggests that an increase in flexibility introduced around catalytic site enhanced the catalytic activity of the malate dehydrogenase.