[Fe] variants based on the transition metals found in their active sites. However, the hydrogen gas oxidation and proton reduction activities varies greatly among the variants and even within their own subcategories. Ferredoxin hydrogenase participates in glyoxylate and dicarboxylate metabolism and methane metabolism. It has 3 cofactors: iron, Sulfur, and Nickel. Ferredoxin hydrogenase found in the green algae Chlamydomonas reinhardtii use supplied electrons from photosystem I to reduce protons into hydrogen gas. This electron supply transfer is possible through photosystem I interactions with photosynthetic electron transfer ferredoxin (PetF). The inter-conversion of protons and electrons with hydrogen gas allow organisms to modulate energy input and output, adjust organelle redox potential, and transduce chemical signals. Industrial Significance Hydrogen gas is a potential candidate for the partial replacement of fossil fuels as a clean energy carrier in fuel cells. However, because of the gas's weight, it often escapes the earth's atmosphere into space, making it a scarce resource on earth. The hydrogen photo-production in green algae, catalyzed by ferredoxin hydrogenase, is a potential source of hydrogen gas. Unfortunately, the inefficiency of the reaction and enzyme as whole to produce hydrogen places a barrier on the viability of this reaction on an industrial scale. In addition, ferredoxin hydrogenase is sensitive to oxygen; the typical half life for the enzyme under aerobic conditions is on the scale of seconds, rendering it difficult to cultivate and manage commercially. References EC 1.12.7 Iron enzymes Sulfur enzymes Nickel enzymes Enzymes of unknown structure