On the relationship between affinity for molecular hydrogen and the physiological directionality of hydrogenases.

The physiological significance of the generic reaction H(2)<-->2[H] is not always clear because hydrogenases may function in the breakdown of molecular hydrogen or in its synthesis or in both directions. Fe-hydrogenases have nevertheless been most often associated with proton reduction and NiFe-hydrogenases with hydrogen oxidation. A re-determination of the K(M) of H(2) oxidation by Pyrococcus furiosus NiFe-hydrogenase-I and by Desulfovibrio vulgaris Fe-hydrogenase suggests that affinity for hydrogen has been seriously underestimated and that the kinetics of hydrogen activation in relation to the directionality of hydrogenases should be re-evaluated.