Abstract
Most of water-soluble vitamins serve as coenzymes of enzymic catalysis. Recent advances in gene manipulation technologies and their introduction to enzymology have allowed us to solve many problems, particularly, involvement of the enzyme protein in the elementary steps in catalysis. This paper describes new insights into the understanding of the enzyme catalysis on the basis of X-ray crystallographic studies on the following three enzymes. 1)_D-Amino acid oxidase : Possible mechanism for the reductive-half reaction was provided by the evaluation of the substrate-flavin interaction prior to electron transfer from the substrate to flavin. The molecular basis underlying the electron-density augmentation at C(4a) of reduced flavin, which enhances markedly the reactivity of reduced flavin to oxygen in the oxidation-half reaction, was revealed. 2)Aspartate transaminase : A significant strain on the protonated pyridoxal-phosphate-Lys258 Schiff base in the unliganded enzyme and its successive relaxation during catalysis were revealed. This strain enhances the catalytic efficiency of the enzyme by increasing the energy level of "the starting state". 3)Diol dehydratase : The tight enzyme-coenzyme(adenosylcobalamin) interaction at both the cobalamin moiety and adenine ring of the adenosyl group would inevitably lead to cleave of the cobalt-carbon bond, generating 5'-carbon radical in the active site. Rotation of the ribose moiety around the glycoside linkage makes the 5'-carbon radical accessible stereochemically to the hydrogen atom of the substrate to be abstracted.
