固氮酶催化活性中心及其化学模拟

Catalytic site of nitrogenases and its chemical simulations

固氮酶是固氮微生物在常温常压下固氮成氨的催化剂,其催化机理和化学模拟一直是国际上长期致力研究的对象.钼铁蛋白高分辨1.0单晶X射线衍射分析表明,固氮酶催化活性中心铁钼辅基的结构为Mo Fe7S9C(R-homocit),其中,Mo原子和3个u2-硫配体、1个组氨酸和1个高柠檬酸配位,形成八面体构型.高柠檬酸以α-烷氧基氧和α-羧基氧与钼螯合形成双齿配位,氨基酸残基上的组氨酸咪唑氮和半胱氨酸巯基与钼和铁单齿配位.在固氮酶铁钼辅基的生物合成过程中,高柠檬酸和咪唑侧基是在最后的合成步骤插入铁硫碳簇前驱体中,其中高柠檬酸和咪唑侧基有可能对质子传递以及稳定Mo Fe7S9C簇起到重要作用.本文从固氮酶铁钼辅基结构出发,结合最近本课题组从化学模拟出发,将固氮酶催化活性中心铁钼辅基结构修订为加氢新结构Mo Fe7S9C(R-Hhomocit)的研究,着重介绍了近年来国内外固氮酶活性中心、生物合成和催化作用机理的研究进展,并展望了固氮酶的研究前景.

Nitrogenase catalyzes the reduction of dinitrogen to ammonia in the process of biological nitrogen fixation. In the past few decades, its catalytic mechanism and chemical simulation have been widely studied. The high resolution X-ray structural analysis of the Mo Fe protein in nitrogenase reveals the iron molybdenum cofactor（Fe Mo-co） as a cage structure, Mo Fe7S9C（R-homocit）. The molybdenum atom is coordinated by three sulfur atoms, a nitrogen atom from histidine residue and two oxygen atoms from R-homocitrate. Recently, the model has been modified as a protonated structure of Mo Fe7S9C（R-Hhomocit）. Homocitrate and imidazole sidechain may play important roles in delivering proton and stabilizing the Mo Fe7S9C cluster in the process of N2 reduction. However the mechanism of N2 reduction remains unclear. In this review, the chemistry of molybdenum with homocitrate and imidazole is discussed, including the iron molybdenum sulfur complexes, which will be helpful to understand the coordination environment of molybdenum atom in iron molybdenum cofactor.