A lysine-tyrosine pair carries out acid-base chemistry in the metal ion-dependent pyridine dinucleotide-linked beta-hydroxyacid oxidative decarboxylases.

This work reviews published structural and kinetic data on the pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases. The family of metal ion-dependent pyridine nucleotide-linked beta-hydroxyacid oxidative decarboxylases can be divided into two structural families with the malic enzyme, which has an (S)-hydroxyacid substrate, comprising one subfamily and isocitrate dehydrogenase, isopropylmalate dehydrogenase, homoisocitrate dehydrogenase, and tartrate dehydrogenase, which have an (R)-hydroxyacid substrate, comprising the second subclass. Multiple-sequence alignment of the members of the (R)-hydroxyacid family indicates a high degree of sequence identity with most of the active site residues conserved. The three-dimensional structures of the members of the (R)-hydroxyacid family with structures available superimpose on one another, and the active site structures of the enzymes have a similar overall geometry of residues in the substrate and metal ion binding sites. In addition, a number of residues in the malic enzyme active site are also conserved, and the arrangement of these residues has a similar geometry, although the (R)-hydroxyacid and (S)-hydroxyacid family sites are geometrically mirror images of one another. The active sites of the (R)-hydroxyacid family have a higher positive charge density when compared to those of the (S)-hydroxyacid family, largely due to the number of arginine residues in the vicinity of the substrate alpha-carboxylate and one fewer carboxylate ligand to the divalent metal ion. Data available for all of the enzymes in the family have been considered, and a general mechanism that makes use of a lysine (general base)-tyrosine (general acid) pair is proposed. Differences exist in the mechanism for generating the neutral form of lysine so that it can act as a base.