| Abstract: | We have investigated the electronic and steric effects of substituents in the aromatic moiety of the substrate on the two principal stages of the reaction catalyzed by tyrosine-phenol-lyase. The substrate specificity of the enzyme is controlled during the stage of elimination of the aromatic ring. The process may be formally considered as an electrophilic substitution in the aromatic nucleus and includes tautomerization of the phenol group into cyclohexadienone and subsequent beta-elimination with regeneration of aromaticity in the leaving group. The OH-group in the rho-position of the ring is the first necessary condition for the stage to proceed. The same stage is also sensitive to the steric parameters of the substituent in the ring which ensures the second factor of control. When the requirements of substrate specificity are fulfilled (L-tyrosine, 3-F-L-tyrosine) the "key" stage of elimination of phenol moiety is not the rate-limiting one, the velocity of the reaction being determined by the preceding stage of alpha-proton abstraction. |
