Implementation of photobiological H2 production: the O2 sensitivity of hydrogenases

The search for the ultimate carbon-free fuel has intensified in recent years, with a major focus on photoproduction of H-2. Biological sources of H-2 include oxygenic photosynthetic green algae and cyanobacteria, both of which contain hydrogenase enzymes. Although algal and cyanobacterial hydrogenases perform the same enzymatic reaction through metallo-clusters, their hydrogenases have evolved separately, are expressed differently (transcription of algal hydrogenases is anaerobically induced, while bacterial hydrogenases are constitutively expressed), and display different sensitivity to O-2 inactivation. Among various physiological factors, the sensitivity of hydrogenases to O-2 has been one of the major factors preventing implementation of biological systems for commercial production of renewable H-2. This review addresses recent strategies aimed at engineering increased O-2 tolerance into hydrogenases (as of now mainly unsuccessful), as well as towards the development of methods to bypass the O-2 sensitivity of hydrogenases (successful but still yielding low solar conversion efficiencies). The author concludes with a description of current approaches from various laboratories to incorporate multiple genetic traits into either algae or cyanobacteria to jointly address limiting factors other than the hydrogenase O-2 sensitivity and achieve more sustained H-2 photoproduction activity.