Transamination in pyridoxal phosphate systems. Kinetic studies of the influence of the side-chains of several amino-acids on 1,3-prototropic shift in zinc(II) aldimine complexes

Abstract

The rates of transamination of 15 amino-acids have been measured in the presence of zinc(II) and pyridoxal-5′-phosphate. On mixing the reactants, zinc(II) aldimine complexes are rapidly (ca. 5 min.) formed and these species subsequently transaminate in a slow, second step. Trends in the transamination rates are interpreted as resulting from various properties of the amino-acid side-chains. Side-chains of non-acid–base type appear to influence the rates by inductive and steric effects. The following order of reactivity was observed: alanine > methionine > phenylalanine > α-amino-n-butyric acid > glycine > serine > isoleucine > threonine > valine. The data indicate that electron donation by the side-chain to the α-carbon atom is rate enhancing and that bulky side-chains act to decrease the rates. The latter effect is probably due to steric hindrance either of base abstracting the α-proton or of acid donating a proton to the γ-carbon atom. Side-chains of acid–base type apparently do not effect intramolecular acid–base catalysis of the prototropic shift.