Diatomic molecules are mostly not too different in size and shape, yet they can be vital nutrients, such as O2, or inimical to aerobic energy metabolism, as when CO blocks O2 binding to the heme Fe in hemoglobin or when CN− poisons mitochondrial cytochrome c oxidase. Nevertheless, both CO and CN− can be found as stable Fe ligands in the [NiFe] hydrogenases, which use a bimetallic cluster to extract energy from the oxidation of H2. Most hydrogenases operate only in the absence of oxygen, but Vincent et al. use protein film voltammetry to show that the membrane-bound hydrogenase of the bacterium Ralstonia eutropha is essentially insensitive to CO and can still effect H2 oxidation at ambient oxygen levels. As a preliminary indication of its potential use in fuel cells, two electrodes, one coated with the Ralstonia hydrogenase and the other with laccase, immersed in aqueous solution and flushed with H2 and air, work together to convert H2 into H2O with an open-circuit voltage of almost 1 V. — GJC
Proc. Natl. Acad. Sci. U.S.A. 10.1073/pnas.0504499102 (2005).