Enzymatic characterization and functional implication of two structurally different isocitrate dehydrogenases from Xylella fastidiosa.

Isocitrate dehydrogenase (IDH) is a key enzyme at the critical junction between the tricarboxylic acid cycle and the glyoxylate cycle. Most bacteria have only one IDH, while a few contain two IDH isozymes. The coexistence of two different type IDHs in one organism was little known. Xylella fastidiosa is a nutritionally fastidious plant pathogen that contains two structurally different IDHs, an NAD+ -dependent homodimeric IDH (diXfIDH) and an NADP+ -dependent monomeric IDH (monoXfIDH). Kinetic characterization showed that diXfIDH displayed 206-fold preferences for NAD+ over NADP+ , while monoXfIDH showed 13,800-fold preferences for NADP+ over NAD+ . The putative coenzyme crucial amino acids (Asp-268, Ile-269, and Ala-275 in diXfIDH, and Lys-589, His-590, and Arg-601 in monoXfIDH) were studied by site-directed mutagenesis. The coenzyme specificities of the three diXfIDH mutants (D268K, D268K/I269Y, and D268K/I269Y/A275V) were switched successfully from NAD+ to NADP+ . Meanwhile, the mutant monoXfIDHs (H590L/R601L and K589T/H590L/R601L) greatly reduced the affinity for NADP+ , but failed to improve the ability to use NAD+ and had similar affinity to NADP+ and NAD+ . The biochemical properties of diXfIDH and monoXfIDH were investigated in detail. This study gives a further insight into the determinants of the coenzyme specificity in both monomeric and dimeric forms of IDHs.