A highly specific monomeric isocitrate dehydrogenase from Corynebacterium glutamicum

The monomeric isocitrate dehydrogenase (IDH) of Corynebacterium glutamicum is compared to the topologically distinct dimeric IDH of Escherichia coli, Both IDHs have evolved to efficiently catalyze identical reactions with sim ilar pH optimum as well as striking specificity toward NADP and isocitrate, However, the monomeric IDH is 10-fold more active (calculated as k(cat)/ K -m.isocitrate/K-m.NADP) and 7-fold more NADP-specific than the dimeric enzy me, favoring NADP over NAD by a factor of 50,000, Such an extraordinary coe nzyme specificity is not rivaled by any other characterized dehydrogenases, In addition, the monomeric enzyme is 10-fold more specific for isocitrate. The spectacular substrate specificity may be predominantly attributed to t he isocitrate-assisted stabilization of catalytic complex during hydride tr ansfer. No significant overall sequence identity is found between the monom eric and dimeric enzymes. However, structure-based alignment leads to the i dentification of three regions in the monomeric enzyme that match closely t he three motifs located in the central region of dimeric IDHs and the homol ogous isopropylmalate dehydrogenases, The role of Lys253 as catalytic resid ue has been demonstrated by site directed mutagenesis, Our results suggest that monomeric and dimeric forms of IDHs are functionally and structurally homologous, (C) 2000 Academic Press.