構造から見るジフテリア菌由来ヘム分解酵素のヘム取り込みと反応最終段階のメカニズム

抄録

Heme oxygenase（HO）is a unique enzyme that catalyzes the conversion of heme to biliverdin,carbon monoxide and free iron. The enzyme is present in not only mammal but also plant, algae and pathogenic bacteria. In order to understand mechanisms of the substrate binding and the product release of bacterial HO, we have determined the crystal structures of the substratefree, Fe³⁺-biliverdin-bound, biliverdin-bound forms and reaction intermediates between the latter two states of HmuO, a heme oxygenase from Corynebacterium diphtheriae. In addition to these high resolution structures, we have conducted molecular dynamics simulation for the hemebinding and bilivedin-release. The substrate-free HmuO shows a widely open active site which is formed by a partially unwounded α-helix. The water molecule cluster is rearranged when the substrate is bound to HmuO. Upon reduction of Fe³⁺ to Fe²⁺, the axial histidine dissociates from Fe²⁺, followed by the relase of Fe²⁺ from the biliverdin group. The water molecule comes into the resulted space and forms hydrogen bonds between the axial histidine and the substrate biliverdin. From these results, we can discuss the molecular mechanism of HmuO at atomic level.