Soundar S, O'Hagan M, Fomulu KS, Colman RF. Identification of Mn2+-binding aspartates from alpha, beta, and gamma subunits of human NAD-dependent isocitrate dehydrogenase.
J Biol Chem 2006;
281:21073-21081. [PMID:
16737955 DOI:
10.1074/jbc.m602956200]
[Citation(s) in RCA: 20] [Impact Index Per Article: 1.1] [Reference Citation Analysis] [Abstract] [MESH Headings] [Grants] [Track Full Text] [Journal Information] [Subscribe] [Scholar Register] [Indexed: 11/06/2022] Open
Abstract
The human NAD-dependent isocitrate dehydrogenase (IDH), with three types of subunits present in the ratio of 2alpha:1beta:1gamma, requires a divalent metal ion to catalyze the oxidative decarboxylation of isocitrate. With the aim of identifying ligands of the enzyme-bound Mn(2+), we mutated aspartates on the alpha, beta, or gamma subunits. Mutagenesis target sites were based on crystal structures of metal-isocitrate complexes of Escherichia coli and pig mitochondrial NADP-IDH and sequence alignments. Aspartates replaced by asparagine or cysteine were 206, 230, and 234 of the alpha subunit and those corresponding to alpha-Asp-206: 217 of the beta subunit and 215 of the gamma subunit. Each expressed, purified mutant enzyme has two wild-type subunits and one subunit with a single mutation. Specific activities of WT, alpha-D206N, alpha-D230C, alpha-D234C, beta-D217N, and gamma-D215N enzymes are 22, 29, 1.4, 0.2, 7.3 and 3.7 micromol of NADH/min/mg, respectively, whereas alpha-D230N and alpha-D234N enzymes showed no activity. The K(m,Mn(2+)) for alpha-D230C and gamma-D215N are increased 32- and 100-fold, respectively, along with elevations in K(m,isocitrate). The K(m,NAD) of alpha-D230C is increased 16-fold, whereas that of beta-D217N is elevated 10-fold. For all the mutants K(m,isocitrate) is decreased by ADP, indicating that these aspartates are not needed for normal ADP activation. This study demonstrates that alpha-Asp-230 and alpha-Asp-234 are critical for catalytic activity, but alpha-Asp-206 is not needed; alpha-Asp-230 and gamma-Asp-215 may interact directly with the Mn(2+); and alpha-Asp-230 and beta-Asp-217 contribute to the affinity of the enzyme for NAD. These results suggest that the active sites of the human NAD-IDH are shared between alpha and gamma subunits and between alpha and beta subunits.
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