The Iron-Hydrogenase of Thermotoga maritima Utilizes Ferredoxin and NADH Synergistically: a New Perspective on Anaerobic Hydrogen Production

The hyperthermophilic and anaerobic bacterium Thermotoga maritima ferments a wide variety of carbohydrates, producing acetate, CO2, and H-2. Glucose is degraded through a classical Embden-Meyerhof pathway, and both NADH and reduced ferredoxin are generated. The oxidation of these electron carriers must be coupled to H-2 production, but the mechanism by which this occurs is unknown. The trimeric [FeFe]-type hydrogenase that was previously purified from T. maritima does not use either reduced ferredoxin or NADH as a sole electron donor. This problem has now been resolved by the demonstration that this hydrogenase requires the presence of both electron carriers for catalysis of H-2 production. The enzyme oxidizes NADH and ferredoxin simultaneously in an approximately 1: 1 ratio and in a synergistic fashion to produce H-2. It is proposed that the enzyme represents a new class of bifurcating [FeFe] hydrogenase in which the exergonic oxidation of ferredoxin (midpoint potential, -453 mV) is used to drive the unfavorable oxidation of NADH (E-o' = -320 mV) to produce H-2 (E-o' = -420 mV). From genome sequence analysis, it is now clear that there are two major types of [FeFe] hydrogenases: the trimeric bifurcating enzyme and the more well-studied monomeric ferredoxin-dependent [FeFe] hydrogenase. Almost one-third of the known H-2-producing anaerobes appear to contain homologs of the trimeric bifurcating enzyme, although many of them also harbor one or more homologs of the simpler ferredoxin-dependent hydrogenase. The discovery of the bifurcating hydrogenase gives a new perspective on our understanding of the bioenergetics and mechanism of H-2 production and of anaerobic metabolism in general.