Serine 339 in the Catalysis of γ- and β-Elimination Reactions

Serine 339 of the active site of Citrobacter freundii methionine -lyase (MGL) is a conserved amino acid in most pyridoxal 5-phosphate-dependent enzymes of the cystathionine -lyase subclass, to which MGL belongs. The reaction mechanism of the MGL-catalyzed -elimination reaction is poorly explored. We replaced serine 339 with alanine using site-directed mutagenesis. The replacement of serine 339 with alanine led to a significant (by two orders of magnitude) decrease in efficiency in the catalysis of the - and -elimination reactions by the mutant form of the enzyme. The exchange rates of the C-- and C--protons in the amino acids in complexes consisting of the enzyme and competitive inhibitors decreased by one-two orders of magnitude. The spectral characteristics of the mutant form indicated that the replacement did not lead to significant changes in the conformation and tautomerism of MGL internal aldimine. We crystallized the holoenzyme and determined its spatial structure at 1.7 resolution. The replacement of serine 339 with alanine did not affect the overall course of the polypeptide chain of the MGL subunit and the tetrameric enzyme structure. An analysis of the obtained kinetic and spectral data, as well as the known spatial structures of C. freundii MGL, indicates that serine 339 is necessary for efficient catalysis of - and -elimination reactions at the stage of C--proton abstraction from the external aldimine, the -elimination reaction at the stages of coenzyme C4-atom protonation, and C--proton abstraction from a ketimine intermediate.