Hydrogenases and Hydrogen Photoproduction in Oxygenic Photosynthetic Organisms^*

The photobiological production of H2 gas, using water as the only electron donor, is a property of two types of photosynthetic microorganisms: green algae and cyanobacteria. In these organisms, photosynthetic water splitting is functionally linked to H2 production by the activity of hydrogenase enzymes. Interestingly, each of these organisms contains only one of two major types of hydrogenases, [FeFe] or [NiFe] enzymes, which are phylogenetically distinct but perform the same catalytic reaction, suggesting convergent evolution. This idea is supported by the observation that each of the two classes of hydrogenases has a different metallo-cluster, is encoded by entirely different sets of genes (apparently under the control of different promoter elements), and exhibits different maturation pathways. The genetics, biosynthesis, structure, function, and O2 sensitivity of these enzymes have been the focus of extensive research in recent years. Some of this effort is clearly driven by the potential for using these enzymes in future biological or biohybrid systems to produce renewable fuel or in fuel cell applications.