2byl: Difference between revisions

==Overview==

Ornithine aminotransferase and 4-aminobutyrate aminotransferase are, related pyridoxal phosphate-dependent enzymes having different substrate, specificities. The atomic structures of these enzymes have shown (i) that, active site differences are limited to the steric positions occupied by, two tyrosine residues in ornithine aminotransferase and (ii) that, uniquely among related, structurally characterized aminotransferases, the, conserved arginine that binds the alpha-carboxylate of alpha-amino acids, interacts tightly with a glutamate residue. To determine the contribution, of these residues to the specificities of the enzymes, we analyzed, site-directed mutants of ornithine aminotransferase by rapid reaction, kinetics, x-ray crystallography, and 13C NMR spectroscopy. Mutation of one, tyrosine (Tyr-85) to isoleucine, as found in aminobutyrate, aminotransferase, decreased the rate of the reaction of the enzyme with, ornithine 1000-fold and increased that with 4-aminobutyrate 16-fold, indicating that Tyr-85 is a major determinant of specificity toward, ornithine. Unexpectedly, the limiting rate of the second half of the, reaction, conversion of ketoglutarate to glutamate, was greatly increased, although the kinetics of the reverse reaction were unaffected. A mutant in, which the glutamate (Glu-235) that interacts with the conserved arginine, was replaced by alanine retained its regiospecificity for the delta-amino, group of ornithine, but the glutamate reaction was enhanced 650-fold, whereas only a 5-fold enhancement of the ketoglutarate reaction rate, resulted. A model is proposed in which conversion of the enzyme to its, pyridoxamine phosphate form disrupts the internal glutamate-arginine, interaction, thus enabling ketoglutarate but not glutamate to be a good, substrate.

==About this Structure==

[[Category: transit peptide]]