ABSTRACT
It is well known that the atmosphere of the prebiotic earth was hydrogen rich. Therefore, it is not unreasonable to assume that the early species were capable of metabolizing molecular hydrogen. However, the capacity of these microorganisms to metabolize molecular hydrogen was discovered only at the end of the 19th century. Later, the hydrogen-metabolizing enzyme was identied and established as hydrogenase by Stephenson and Stickland (1931) (Tamagnini et al., 2002). Most of the current researches on hydrogenase focus on prokaryotes, which have helped establish the structural and functional insight into the enzyme. The enzyme catalyzes the simplest reversible reaction (Equation 5.1), the reversible reductive formation of hydrogen from protons and electrons:
2H+ + 2e− → H2 (5.1)
The direction of the reaction mostly depends on the redox potential of the components which interact with the enzyme. In the presence of an electron acceptor, hydrogenase will act as a H2 uptake enzyme, while in the presence of an electron donor, the enzyme will produce H2. The hydrogen uptake reaction results in protons and electrons, which are subsequently used to generate ATP and reducing power while the hydrogen-producing reaction involves the reduction of protons by low oxidation electrons generated by fermentation (Nicolet et al., 2000).
