Serine 51 residue of Citrobacter freundii tyrosine phenol-lyase assists in C-α-proton abstraction and transfer in the reaction with substrate

In the spatial structure of tyrosine phenol-lyase, the Ser51 residue is located in the active site of the enzyme. The replacement of Ser51 with Ala by site-directed mutagenesis led to a decrease of the k_cat/K_m parameter for reactions with L-tyrosine and 3-fluoro-L-tyrosine by three orders of magnitude, compared to wild type enzyme. For the elimination reactions of S-alkylcysteines, the values of k_cat/K_m decreased by an average of two orders of magnitude. The results of spectral studies of the mutant enzyme gave evidence for a considerable change of the chiral properties of the active site as a result of the replacement. Fast kinetic studies for the complexes of the mutant form with competitive inhibitors allowed us to conclude that the Ser51 residue interacts with the side chain amino group of Lys257 at the stage of C-α-proton abstraction. This interaction ensures the correct orientation of the side chain of Lys257 accepting the C-α-proton of the external aldimine and stabilizes its ammonium form. Also, it is probable that Ser51 takes part in formation of a chain of hydrogen bonds which is necessary to perform the transfer of the C-α-proton to the C-4′-position of the leaving phenol group in the reaction with the natural substrate.