Nitrogenase-Catalyzed H2 Production

Hydrogen can still be produced where the normal substrate N2 is absent, and the enzyme is functional. Under these conditions only 4 moles of ATP are required per mole of H2 evolved. 

Nitrogenases are also extremely labile in an O2 environment. Cyanobacteria have adopted two strategies to overcome the O2 sensitivity of the nitrogenase reaction, either temporal or spatial separation of O2 evolution and H2 production. Temporal separation has been achieved mainly in non-heterocystous unicellular cyanobacteria such as Gloeothece sp. strain ATCC 27152 and Plectonema boryanum. Nitrogen fixation occurs only in darkness, during which respiration of the carbohydrates, syn- 

Maria L. Ghirardi, Pin Ching Maness, and Michael Seibert 

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Cyanobacteria may possess several enzymes directly involved in H2 metabolism nitrogenase(s) catalyzing the production of H2 concomitantly with the reduction of nitrogen to ammonia, an uptake hydrogenase, catalyzing the consumption of H2 produced by the nitrogenase, and a bidirectional hydrogenase, which has the capacity to both take up and produce H2 [3, 5],... 

Pig. 7. Substrate reduction catalyzed by Mo-nitrogenase. Mo-nitrogenase is depicted as the box containing the Fe protein (Fe-p) and MoFe protein (MoFe-p). The required inputs for catalyzed reduction are shown at left. The electron donor is usually sodium dithionite (8204 ) in vitro and either ferredoxin (Fd) or flavodoxin (Fid) in vivo. Other mandatory inputs are protons (H" ) and magnesium adenosine triphosphate (MgATP). The most commonly used substrates (above the horizontal line) and their products (below the hne) are shown at right. The numbers in parentheses are the number of electrons required to effect each reduction shown. The horizontal arrow indicates that CO is an inhibitor of aU catalyzed reactions except for the production of H2 from protons. 

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