Hydrogen metabolism in organisms with oxygenic photosynthesis: hydrogenases as important regulatory devices for a proper redox poising?

Three different hydrogenases and the nitrogenase putatively participate in the hydrogen metabolism of micro-organisms carrying out oxygenic photosynthesis. Hydrogenases either produce hydrogen or split hydrogen into protons and electrons depending on their redox partners, whereas the nitrogenase produces hydrogen unidirectionally as a byproduct during the reduction of nitrogen to ammonia. Hydrogenases are well-characterized enzymes on the enzymatic, structural and genetic level, especially in prokaryotic micro-organisms. They can be classified regarding the metal composition of their active site (Fe-only, NiFe or metal-free), their preferential direction of reaction (uptake only or bidirectional/reversible) and their in vivo electron donors or accepters. The main physiological role of the uptake hydrogenase in cyanobacteria is probably recapturing the hydrogen produced by nitrogenase. The role of the bidirectional hydrogenase in phototrophs is still a matter of debate. Based on recent results which showed it to be of the NAD(P)-reducing type, a model for its physiological function is suggested. This model includes that this type of hydrogenase is linked to complex I of the respiratory electron-transport chain and might be an important electron valve during photosynthesis under rapidly changing light conditions. The existence of an Fe-only hydrogenase as well as an NiFe-hydrogenase in green algae is still enigmatic and is discussed as hydrogenases either participating in the production of hydrogen or during fermentation. (C) 1998 Elsevier Science S.A. All rights reserved.