Release of enzyme strain during catalysis reduces the activation energy barrier.
Several mechanisms have been considered as principal factors in enhancing the catalytic reaction velocity of enzymes: approximation, covalent catalysis, general acid-based catalysis, and strain. Among them, the strain on the substrate and/or the enzyme is often found to be brought about on association of the substrate and the enzyme. If this strain is released in the transition state, it contributes to enhancing the k(cat) value, although it does not change the k(cat)/K(m) value. In aspartate aminotransferase, however, we found by analysis of the Schiff base pK(a) values that the unliganded enzyme carries a strain in the protonated Schiff base formed between the coenzyme pyridoxal phosphate and a lysine residue. This bond is cleaved in most of the reaction intermediates, including the transition state. As a result, the activation energy between the free enzyme plus substrate and the transition state is decreased by 16 kJ/mol, equal to the value of the strain energy. The net effect of this strain is enhancement (10(3)-fold) of the catalytic efficiency in terms of k(cat)/K(m), the more important indicator of the catalytic efficiency at low concentration of the substrate.
Kagamiyama
H
H
Department of Biochemistry, Osaka Medical College,2-7 Daigakumachi, Takatsuki 569-8686, Japan. med001@art.osaka-med.ac.jp
Hayashi
H
H
D001219:Aspartate Aminotransferases / Q000737:chemistry* / Q000378:metabolism*; D002384:Catalysis; D007700:Kinetics; D008958:Models, Molecular; D011487:Protein Conformation; D012545:Schiff Bases / Q000737:chemistry / Q000378:metabolism; D013816:Thermodynamics
D012545:Schiff Bases; D001219:Aspartate Aminotransferases