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 non-protein diatomic ligands, which are two CN- and one CO. Also, there is an additional bridge X between Ni and Fe, which is thought to be oxygen and associated with the inactive form of the enzyme. The structure of the Ni-Fe center is shown in figure 1. Above all, every nickel-iron hydrogenase molecular consists of 12 irons, 12 sulfides, and 1 nickel. According to recent structure and spectroscopic studies, researchers have found that the active site of enzyme is at the Ni-Fe center.1,3,4.
Figure 1. Crystal structure of nickel-iron center isolated from D. gigas.
In order to investigate the Ni-Fe hydrogenase catalytic mechanism, an active site of Ni-Fe complex was studied by EPR.