The Physiology and Functional Genomics of Cyanobacterial Hydrogenases and Approaches Towards Biohydrogen Production

Abstract

Three different enzymes, the nitrogenase and two different hydrogenases, an uptake and a bidirectional enzyme, are involved in cyanobacterial hydrogen metabolism. In strains containing nitrogenase, H-2 is produced as a byproduct during nitrogen fixation. Many cyanobacterial strains additionally express an uptake hydrogenase that recycles these reducing equivalents. Since not every nitrogen-fixing strain encodes the genes of the uptake hydrogenase, it seems to be dispensable under some environmental conditions. Genome comparisons suggest that the cyanobacterial uptake hydrogenase requires the presence of five additional accessory genes for its maturation. The primary function of the bidirectional hydrogenase is to increase energetic yield during fermentation and light induced H-2 production in transition states, when cells shift from dark anaerobic conditions to those where light is present. Under oxidizing conditions, the bidirectional enzyme can also catalyze hydrogen uptake. Comparative genomics and database searches reveal the specific association of the pyruvate:ferredoxin/flavodoxin oxidoreductase with the bidirectional hydrogenase, indicating a functional association. This chapter summarizes what is known about the physiological function of the hydrogenases, how they are integrated in the overall metabolism, their phylogenetic ancestry, and their distribution in cyanobacterial genomes. Oxygenic phototrophs provide the framework that is needed for biological hydrogen production from sunlight and water, and they could be used as a blueprint for biomimetic systems for hydrogen generation. The genetic modifications that have been made to achieve higher production rates are described and future strategies for the use of cyanobacteria as rewarding H-2 producers are outlined.