Pig heart mitochondrial NADP-dependent isocitrate dehydrogenase is the most extensively studied among the mammalian isocitrate dehydrogenases. The crystal structure of Escherichia cell isocitrate dehydpogenase and sequence alignment of porcine with E. coli isocitrate dehydrogenase suggests that the porcine Arg(101), Arg(110), Arg(120), and Arg(133) are candidates for roles in substrate binding. The four arginines were separately mutated to glutamine using a polymerase chain reaction method. Wild type and mutant enzymes were each expressed in E. coli, isolated as maltose binding fusion proteins, then cleaved with thrombin, and purified to yield homogeneous porcine isocitrate dehydrogenase. The R120Q mutant has a specific activity, as well as K-m values for isocitrate, Mn2+, and NADP(+) similar to wild type enzyme, indicating that Arg(120) is not needed for function. The specific activities of R101Q, R110Q, and R133Q are 1.73, 1.30, and 19.7 mu mols/min/mg, respectively, as compared with 39.6 units/mg for wild type enzyme. The R110Q and R133Q enzymes exhibit K-m values for isocitrate that are increased more than 400- and 165-fold, respectively, as compared with wild type. The K-m values for Mn2+, but not for NADP(+), are also elevated indicating that binding of the metal-isocitrate complex is impaired in these mutants. It is proposed that the positive charges of Arg(110) and Arg(133) normally strengthen the binding of the negatively charged isocitrate by electrostatic attraction. The R101Q mutant shows smaller, but significant increases in the K-m values for isocitrate and Mn2+; however, the marked decrease in k(cat) suggests a role for Arg(101) in catalysis The V-max of wild type enzyme depends on the ionized form of an enzymic group of pK 5.5, and this pK(aes) is similar for the R101Q and R120Q enzymes. In contrast, the pK(aes) for R110Q and R133Q enzymes increases to 6.4 and 7.4, respectively, indicating that the positive charges of Arg(110) and Arg(133) normally lower the pK of the nearby catalytic base to facilitate its ionization. These results may be understood in terms of the structure of the porcine NADP-specific isocitrate dehydrogenase generated by the Insight Modeler Program, based on the x-ray coordinates of the E. coli enzyme.
