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 similar pH optimum as well as striking specificity toward NADP and isocitrate. However, the monomeric IDH is 10-fold more active (calculated as kcat/Km.isocitrate/Km.NADP) and 7-fold more NADP-specific than the dimeric enzyme, favoring NADP over NAD by a factor of 50,000. Such an extraordinary coenzyme 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 the isocitrate-assisted stabilization of catalytic complex during hydride transfer. No significant overall sequence identity is found between the monomeric and dimeric enzymes. However, structure-based alignment leads to the identification of three regions in the monomeric enzyme that match closely the three motifs located in the central region of dimeric IDHs and the homologous isopropylmalate dehydrogenases. The role of Lys253 as catalytic residue has been demonstrated by site-directed mutagenesis. Our results suggest that monomeric and dimeric forms of IDHs are functionally and structurally homologous.
Chen
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R
Department of Biochemistry, College of Medicine, University of Saskatchewan, Saskatoon, Canada. ridong.chen@monsanto.com
Yang
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H
D000429:Alcohol Oxidoreductases / Q000737:chemistry; D000595:Amino Acid Sequence; D001665:Binding Sites; D002384:Catalysis; D003352:Corynebacterium / Q000201:enzymology*; D019281:Dimerization; D004591:Electrophoresis, Polyacrylamide Gel; D004926:Escherichia coli / Q000201:enzymology; D006863:Hydrogen-Ion Concentration; D007521:Isocitrate Dehydrogenase / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D008969:Molecular Sequence Data; D016297:Mutagenesis, Site-Directed; D011485:Protein Binding; D017386:Sequence Homology, Amino Acid; D013379:Substrate Specificity; D013696:Temperature
D000429:Alcohol Oxidoreductases; D007521:Isocitrate Dehydrogenase