The pyridoxal-phosphate-dependent enzymes exclusively catalyzing reactions of beta-replacement

Theoretic and experimental arguments are surveyed which justify the setting up, within the family of pyridoxal-P-dependent lyases, of a special subgroup that comprizes several enzymes catalyzing exclusively beta-replacement reactions of alpha-aminoacids with electronegative substituents in the beta-position. The authors and their associates have studied the physico-chemical and catalytic properties of four high purity enzymes belonging to this subgroup, namely: cysteine lyase (EC 4.1.1.10) from embryonic chicken yolk-sac, serine sulfhydrase from chicken liver and the closely analogous or synonymic cystathionine beta-synthase (EC 4.4.1.8) from rat liver, and beta-cyanoalanine synthase (EC 4.4.1.9) from lupine seedlings, in comparison with some pyridoxal-P-requiring lyases differing in reaction specificity, for example, gamma-specific, alphabeta-eliminating or plurifunctional lyases such as gamma-cystathionase (EC 4.4.1.1) of animal tissues. The results of these studies, relating to subtrate and cosubstrate specificities of the enzymes mentioned, their interactions with some selective inhibitors, catalysis of isotopic exchange of hydrogen atoms in substrates and substrate analogs, etc., indicate that lyases of the exclusively beta-replacing type substantially differ in reaction mechanism from other subgroups of this enzyme family. Thus, it appears highly improbable that transient formation of an alphabeta-unsaturated, coenzyme-substrate imine, considered as an obligatory step in the action of lyases in the alphabeta-eliminating and other subgroups, should occur in the sequences of reaction intermediates in the case of beta-replacing lyases. Suggested features of the presumable catalytic mechanism of these lyases are discussed, such as : fixed conformation of the aminoacid substrate in the ES complex (protein-bound pyridoxal-P aldimine), with beta-substituent in orientation cis (rather than trans) to the Halpha atom ; role of the binding of appropriate cosubstrates (nucleophilic replacing agent, Cs) inducing essential electronic and/or steric transitions in the catalytic site of the ternanry CsES complexes, etc.