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As an enzyme, hydrogenase plays an important role in hydrogen metabolism in microorganisms. The main function of hydrogenase is to catalyze the reversible oxidation of molecular hydrogen to protons as shown by the formula: H2 = 2H+ + 2e- Based on the metal content, hydrogenases are divided into four classes: Ni-Fe hydrogenase, the Fe-only hydrogenase, Ni-Fe-Se hydrogenase and non-metal hydrogenase. In this paper, I will focus on the metal center of nickel-iron hydrogenase.1,2, 3 The crystal structure of the nickel-iron hydrogenases isolated from Desulfovibrio gigas was determined by Volbeda and his colleagues in 1995.4,5 The structure shows that nickel-iron hydrogenase is a heterodimeric protein which contains a large subunit and a small subunit. The small subunit contains two [4Fe-4S]2+/1+ clusters and one [3Fe-4S]1+/0 cluster. "These three Fe-S clusters are distributed almost along a straight line and the [3Fe-4S] cluster is located in the middle of the two [4Fe4S] clusters."1 The function of iron-sulfur clusters is to facilitate electron transfer. The large subunit consists of a bimetallic center― nickel-iron cluster. Ni binds to four cysteinethiolate and two of them bind with the Fe. Fe binds to three