О-Аcetylhomoserine sulfhydrylase from <i>Clostridioides difficile</i>: the role of tyrosine residues of the active center

Abstract

O-acetylhomoserine sulfhydrylase is one of the key enzymes in the biosynthesis of methionine in Clostridioides difficile. The mechanism of the γ-substitution reaction of O-acetyl-L-homoserine catalyzed by this enzyme is the least studied among pyridoxal-5′-phosphate-dependent enzymes involved in the metabolism of cysteine and methionine. To clarify the role of the active site residues Tyr52 and Tyr107, four mutant forms of the enzyme with replacements for phenylalanine and alanine were obtained. The catalytic and spectral properties of mutant forms were investigated. The rate of the γ-substitution reaction catalyzed by mutant forms with the replacement of the Tyr52 residue decreased by more than three orders of magnitude compared to the wild-type enzyme. Tyr107Phe and Tyr107Ala mutant forms practically did not catalyze this reaction. Replacements of the residues Tyr52 and Tyr107 led to a decrease in the affinity of the apoenzyme to the coenzyme by three orders of magnitude and changes in the ionic state of the internal aldimine of the enzyme. The obtained results allowed us to assume that Tyr52 is involved in ensuring the optimal position of the catalytic coenzyme-binding lysine residue at the stages of C-α-proton elimination and elimination of the side group of the substrate. Tyr107 can act as a general acid catalyst at the stage of acetate elimination.