Hydrogenase is found in both cyanobacteria and algae [1]. Two types of hydrogenase exist: the bidirectional enzyme and the uptake hydrogenase. The uptake hydrogenase can solely perform the reverse of the reaction shown below (i.e. generate protons and electrons from hydrogen). In strains used to produce hydrogen via hydrogenase or nitrogenase, this uptake hydrogenase must be eliminated [1]. The bidirectional enzyme may perform the reaction in either direction. Hydrogenases contain a unique active site: it contains a bimetal ligand, carbon monoxide, and cyanide [2]. The latter two compounds are usually quite toxic to biological systems; therefore it is novel to find an enzyme that requires them to function. The metal ligand system is also unique for this type of chemistry. Either a di-iron or a nickel-iron moiety, this metal system is unique for the generation of hydrogen [1,2]. The mechanism of synthesis is different from the traditional platinum catalyzed chemical process. First a hydride is formed, then a proton is added to generate hydrogen gas by hydrogenase [2].  Hydrogenase reactions occur through both photobiological production and fermentation processes.  In photobiological production of hydrogen, hydrogenase ultimately derives the electron and protons from photosynthesis [1], whereas the fermentative process requires an organic acid or a sugar to donate its electrons. Organisms that produce hydrogen include Chlamydomonas reinhardtii, Rhodobacter spheroides, Clostridium butyricum, and Enterobacter cloacae.

[1] Prince, R.C., Kheshgi, H.S., 2005, Crit Rev Microbiol 31, 19-31.
[2] Alper, J., 2003, Science 299, 1686-1687.
[3] Kapden, I.K., Kargi, F., 2006, Enzyme Microb Technol 38, 569-582.
[4] Nath, K., Das, D., 2004, Appl Microbiol Biotechnol 65, 520-529.