ABSTRACT
Dehydrogenases catalyze the reduction of a substrate molecule with NAD+ as the reducing coenzyme. Most dehydrogenases share common structural features, their NAD binding domains and their folding topology, but a family of dehydrogenases has been recognized that does not contain this folding topology. Three members of this family-tartrate dehydrogenase, 3-isopropylmalate dehydrogenase (IPMDH), and iso citrate dehydrogenase (ICDH), all require a divalent metal ion (Mg2-1-or Mn2+). Manganese redox chemistry is not involved in the dehydrogenation process; the metal ion serves to bind, orient, and activate the substrate. IPMDH and ICDH are discussed below. Tartrate dehydrogenase has not been structurally characterized and is discussed in Sect. 3.1.1. 2.1.1.1. Isocitrate dehydrogenase (EC 1.1.1.41) ICDH catalyzes the decarboxylation of isocitrate to a-ketoglutarate and C02 via an oxalosuccinate intermediate [65]. Mammalian tissues have two forms of the enzyme: EC 1.1.1.41, which occurs in the mitrochondria and requires Mn2+ or Mg2+ and NAD+ as cofactors, and EC 1.1.1.42 which occurs in both the cytosol and the mito chondria and requires Mg2+ and NADP+ as cofactors. A crystal structure of the Mn2+-dependent enzyme from T. thermophilus with Mn2+ and isocitrate bound has been determined. Isocitrate and Mn2+ bind in a pocket between two major domains in which both subunits of the dimer participate. Mn2+ is bound to Asp-283 and Asp-307, two water molecules, and the isocitrate via the carboxylate and an OH group [66]. Structure determinations of mutant forms of the E. coli enzyme have been deposited in the Protein Data Bank [67]. 2.1.1.2. Isopropylmalate dehydrogenase (EC 1.1.1.85) IPMDH catalyzes the third step in the biosynthesis of the amino acid leucine in microorganisms and plants [68]. The reaction involves the conversion of 3-isopropylmalate to 2-oxoisocaproate with the reduction of NAD+ and the E. coli enzyme requires Mn2+ and K+ for optimum activity. Structures of the enzyme from E. coli, Salmonella typhimurium, and Thermus thermophilus have been determined. In the S. typhimurium enzyme the asymmetric unit contains a dimer with the two mono mers in different, “ open” and “ closed” conformations. In the open conformation, Mn2+ is coordinated to three aspartate residues, two waters, and a sulfate ion.
