The term hydrogenase refers to enzymes that catalyse Hz consumption or
evolution according to the reaction:
H2e2H + + 2e- (1)
All hydrogenases are bidirectional to some extent in vitro, but the enzyme
appears to catalyse either H2 oxidation or production in uivo. Hydrogen
evolution usually occurs in anaerobic micro-organisms, and serves to get rid
of excess reductant when protons are the only available oxidant (Schlegel and
Schneider, 1978), whereas H2 utilization can occur in aerobic and anaerobic
bacteria, and is linked to ATP-producing electron transport systems.
Anaerobic bacteria oxidize H2 using sulphate, sulphur, C02, nitrate or
fumarate as the terminal electron acceptor, and the photosynthetic bacteria
use H2 and other compounds, rather than H20, as the reductant for COz
fixation (Knaff, 1978). Aerobic Hz-oxidizing bacteria can grow with H2 and
COZ as the sole energy and carbon sources, respectively. Among these
bacteria, some have a soluble and a membrane-bound hydrogenase; the
soluble enzyme catalyses Hz-dependent NAD+ reduction which is used for
COZ fixation, whereas the membrane-bound hydrogenase is linked to electron
transport, and is therefore involved in energy production (Adams et al., 1981).
The aerobic N2-fixing bacteria evolve and consume HI, and among this group
are the rhizobia, the azotobacter and the cyanobacteria. Hydrogen evolution
by these micro-organisms is catalysed by nitrogenase, and an uptake
hydrogenase is responsible for H2 oxidation. Whereas the cyanobacteria have
hydrogenase activity even in cells not fixing NZ (Tel-Or et al., 1977; Eisbrenner
et al., 1978), hydrogenase is generally derepressed under Nz-fixing conditions
in Azotobacter and in hydrogen uptake positive (Hup+) strains of Rhizobium.
Autotrophic growth of R. japonicurn on H2 has been demonstrated in the
laboratory (Hanus et al., 1979), and thus H2 oxidation by this bacterium,
without concomitant Nz fixation, may conceivably occur in nature under
some conditions.