Hydrogenase
 
Hydrogenases are enzymes that catalyze the reversible oxidation of molecular hydrogen and play an important role in the anaerobic metabolism of microorganisms. Hydrogen uptake and production occur in sulfate-reducing bacteria and fermentative anaerobes, as well as in aerobic nitrogen-fixing bacteria, photosynthetic bacteria, and cyanobacteria (1). Three distinct categories of hydrogenases have been described: 
• Iron hydrogenases. With two [4Fe-4 S] centers in addition to a special type of [4Fe-4 S] center.
• Nickel–iron hydrogenases. With a nickel center coordinated to a sulfur atom, two [4Fe-4 S]  centers, and a [3Fe-xS] center.
• Nickel–iron-selenium hydrogenases. With a nickel center coordinated to a selenium atom as well as two [4Fe-4 S] centers.
 
References
1. D. S. Patil (1994) Methods. Enzymol. 243, 68–94.