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Monomeric isocitrate dehydrogenases from psychrophilic bacteria, Colwellia maris andColwellia psychrerythraea (CmIDH-II and CpIDH-M, respectively) are cold-adapted enzymes and show a high degree of amino acid sequential identity to each other (77%).However, maximum activity of CpIDH-M at optimum temperature is much less than that of 5 CmIDH-II. In the C-terminal Region 3 of these enzymes, which was suggested from previous study to be responsible for their distinct catalytic ability, several sequential differences of amino acid residue are present. Among them, ten amino acid residues were exchanged between them by site-directed mutagenesis and several properties of the mutated enzymes were examined in this study. The mutated enzymes of CmIDH-II substituted its 10 Gln671, Leu724 and Phe735 residues with the corresponding residues of CpIDH-M (termed Q671K, L724Q and F735L, respectively) showed lower specific activity and thermostability for activity than the wild-type enzyme. Furthermore, the decreased specific activity was also observed in L693F. In contrast, the corresponding mutants of CpIDH-M, F693L, Q724L and L735F, showed the increased specific activity and thermostability for activity. The catalytic 15 efficiency (k cat /K m ) values of these mutated CmIDH-II and CpIDH-M were lower and higher than those of their wild-type IDHs, respectively. These results suggest that the Gln671, Leu693, Leu724 and Phe735 residues of CmIDH-II are important for exerting its high catalytic ability.

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