The importance of hydrogen to biological systems is emphasized by its central position in the spiral periodic table. Hydrogen is a constituent of water and nearly all organic molecules. Cells use H⁺ gradients to drive ATP synthesis, solute transport, and flagella rotation. Hydrogenase enzymes (reviewed by Vignais et al, 2001) mediate the redox interconversion of H⁺ and H₂. Some aerobic (reviewed by Friedrich and Schwarz, 1993) and anaerobic (Gross and Simon, 2003) chemolithotrophic prokaryotes can derive energy for growth from the oxidation of H₂, catalyzed by [NiFe]-hydrogenases. [Fe]-Hydrogenases are typically associated with the reduction of H⁺ leading to the evolution of H₂ (reviewed by Frey, 2002), and are found in both prokaryotes (reviewed by Nandi and Sengupta, 1998) and lower eukaryotes (some fungi, algae, and protozoa) (reviewed by Horner et al, 2002). Metal-free hydrogenases have been discovered in methanogenic archaea (reviewed by Berkessel, 2001).

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