Abstact

L-Methionine deprivation for cancer treatment requires pyridoxal 5'-phosphate (PLP)-dependent L-methionine γ-lyases (MGL) with sustained activity in plasma. We investigated the MGL from the alkaliphilic thermophile Thermobrachium celere (TcMGL), which was compared to that from Pseudomonas putida (PpMGL) as a reference. Catalysis was limited to L-methionine, L-homocysteine, and L-cysteine, with highest catalytic efficiency towards L-methionine. Binding of apo-TcMGL to PLP results from an endothermic entropy-driven process, contrasting with apo-PpMGL that binds to cofactor following an exothermic enthalpy-driven reaction, as demonstrated with isothermal titration calorimetry. The tetrameric crystal structure of TcMGL revealed a mobile domain adopting open and closed conformations, regulating access of substrate and PLP to the active site. TcMGL exhibited great stability in human plasma in vitro. Half-life of the active enzyme increased from 9.2 h to 66.7 h with increasing PLP concentration, in much greater magnitude than that of PpMGL. Two-way ANOVA demonstrated a highly significant effect on half-lives of enzyme type and PLP concentration. The enzyme eliminated L-methionine from human plasma and exerted potent cytotoxicity against human carcinoma cells in vitro. TcMGL favourable characteristics make it a suitable candidate for treatment of cancer.