Mechanism of Ergothioneine Synthase(EgtB) from Mycobacterium thermoresistibile: A QM/MM Study

【作者】 田歌； 刘永军；

【机构】 山东大学理论化学研究所；

【摘要】 Ergothioneine synthase（EgtB） catalyzes the formation of C-S bond betweenγ-glutamyl cysteine and N-α-trimethyl histidine, which is the pivotal step for the ergothioneine biosynthesis.^[1-2] In this work, the QM/MM approach was employed to investigate the detailed mechanism of EgtB from Mycobacterium thermoresistibile.Two binding modes of O₂（end-on and side-on） and different spin states of the reactant have been identified. The FeⅡ-O₂ complex can be described as a hybridization of iron（Ⅱ）-superoxo and iron（Ⅲ）-peroxo complexes, in which electron transfer occurs from the activated γ-glutamyl cysteine to O₂ via the Fe center. The strong covalent interaction of the substrate with the Fe-O₂ complex facilitates the attacking of the distal oxygen atom of Fe-O₂ complex to the sulfur atom of cysteinate to form a ring structure. The cleavage of dioxygen bond leads to the formation of a sulfoxide. The abstraction of the C2-H of imidazole group by the FeIV-oxo species is calculated to be the rate-limiting step with an energy barrier of26.4 kcal/mol. The sulfoxide binds to C2 radical to complete the formation of C-S bond with an energy barrier of 2.6 kcal/mol. The whole reaction is exothermic by42.7 kcal/mol. The conserved residue Tyr377 is not a good candidate for H atom transfer for the rate-limiting step.更多还原

【Abstract】 Ergothioneine synthase（EgtB） catalyzes the formation of C-S bond betweenγ-glutamyl cysteine and N-α-trimethyl histidine, which is the pivotal step for the ergothioneine biosynthesis.^[1-2] In this work, the QM/MM approach was employed to investigate the detailed mechanism of EgtB from Mycobacterium thermoresistibile.Two binding modes of O₂（end-on and side-on） and different spin states of the reactant have been identified. The FeⅡ-O₂ complex can be described as a hybridization of iron（Ⅱ）-superoxo and iron（Ⅲ）-peroxo complexes, in which electron transfer occurs from the activated γ-glutamyl cysteine to O₂ via the Fe center. The strong covalent interaction of the substrate with the Fe-O₂ complex facilitates the attacking of the distal oxygen atom of Fe-O₂ complex to the sulfur atom of cysteinate to form a ring structure. The cleavage of dioxygen bond leads to the formation of a sulfoxide. The abstraction of the C2-H of imidazole group by the FeIV-oxo species is calculated to be the rate-limiting step with an energy barrier of26.4 kcal/mol. The sulfoxide binds to C2 radical to complete the formation of C-S bond with an energy barrier of 2.6 kcal/mol. The whole reaction is exothermic by42.7 kcal/mol. The conserved residue Tyr377 is not a good candidate for H atom transfer for the rate-limiting step.更多还原