Hydrogen evolution by chloroplast-hydrogenase systems: improvements and additional observations

Rapid electrocatalytic procedure for hydrogenase kinetic determination in the H2 evolution direction

The linear sweep voltammetric method is used as a new approach for kinetic determination with enzymes accepting reversible redox couples as cosubstrate. A monolayer of hydrogenase molecules is grafted onto a glassy carbon electrode which is both the support of the enzyme and the detector of the activity. Reduced viologen concentration in the enzyme microenvironment is controlled by the electrode potential. The catalytic current produced by the enzyme allows an easy kinetic constant determination without the classical constraints found in hydrogenase kinetic measurements.

Light induced H2 evolution in a hydrogenase-TiO2 particle system by direct electron transfer or via rhodium complexes

Three different hydrogenases (isolated from Clostridium pasteurianum, Desulfovibrio desulfuricans strain Norway 4 and D. baculatus 9974) added to a suspension of TiO2 (anatase) powder are able to catalyze H2 evolution under band gap illumination of the semiconducting particles, and in the presence of EDTA or methanol as electron donor. This H2 production can be obtained by the direct electron transfer from the conduction band of the TiO2 particles to the active site of the enzyme at pHs higher than 7. This mediator-independent charge transfer is more efficient with C. pasteurianum and D. baculatus 9974 hydrogenases, and in the presence of methanol. Rhodium tris- and bis-bipyridyl complexes can act efficiently as electron carriers from the supporting particles to the adsorbed enzyme molecules in cases where the direct transfer is inefficient.

Application of hydrogenase for photoinduced hydrogen evolution

Various attempts have been made to develop suitable redox systems for the photochemical utilization of solar energy. Recent work has shown that the three component systems containing a photosensitizer, an electron donor, and an electron acceptor can be used to evolve hydrogen when a suitable catalyst is present. The reactions are quite general and have been demonstrated for a wide range of photosensitizers and electron carriers. Hydrogenase or colloidal platinum are widely used as catalysts, for they can catalyze the reduction of protons in the presence of a suitable electron donating agent such as reduced methylviologen. Some properties of these components and the efficiency of the photoinduced hydrogen evolution are discussed.

The redox properties of the iron-sulphur cluster in hydrogenase from Chromatium vinosum, strain D

The midpoint potentials of the changes in the electron spin resonance (ESR) spectra in the region of g = 2 in hydrogenase II from Chromatium vinosum were estimated by redox titrations. As the enzyme was progressively reduced, the g = 2.02 signal increased, while the satellite lines at g = 1.98 etc. decreased. At still lower potentials the signal at g = 2.02 decreased. The midpoint potentials of the two processes were estimated to be + 100 mV and - 20 mV, respectively, at pH 8.5. The first potential showed significant pH-dependence. The titration data fitted to n = 1 curves with reasonable reversibility. The enzyme activity showed no significant changes in this potential range. The results are discussed in relation to the interaction of the iron-sulphur cluster with nickel.

Bidirectional measurement of hydrogenase activity by the pH-stat method

The protons produced by the catalytic activity of hydrogenase in H2 evolution from dithionite-reduced methyl viologen or through benzyl viologen reduction by H2 gas are automatically titrated by a pH-stat device. This approach allows the measurement of hydrogenase activity and ensures the constancy of pH during the reaction in absence of buffers. Kinetic assays and pH and temperature-dependence experiments with Desulfovibrio gigas hydrogenase performed by this method basically confirm the results obtained with customary manometric assay.

Production of superoxide radicals by soluble hydrogenase from Alcaligenes eutrophus H16

The soluble hydrogenase (hydrogen-NAD+ oxidoreductase, EC 1.12.1.2) of Alcaligenes eutrophus H16 was shown to be stabilized by oxidation with oxygen and ferricyanide as long as electron donors and reducing compounds were absent. The simultaneous presence of H2, NADH and O2 in the enzyme solution, however, caused an irreversible inactivation of hydrogenase that was dependent on the O2 concentration. The half-life periods of 4 degrees C under partial pressures of 0.1, 5, 20 and 50% O2 were 11, 5, 2.5 and 1.5 h respectively. Evidence has been obtained that hydrogenase produces superoxide free radical anions (O2-.), which were detected by their ability to oxidize hydroxylamine to nitrite. The correlation between O2 concentration, nitrite formation and inactivation rates and the stabilization of hydrogenase by addition of superoxide dismutase indicated that superoxide radicals are responsible for enzyme inactivation. During short-term activity measurements (NAD+ reduction, H2 evolution from NADH), hydrogenase activity was inhibited by O2 only very slightly. In the presence of 0.7 mM-O2 an inhibition of about 20% was observed.

Efficiency of ferredoxins and flavodoxins as mediators in systems for hydrogen evolution

1. The efficiencies of ferredoxins and flavodoxins from a range of sources as mediators in systems for hydrogen evolution were assessed. 2. In supporting electron transfer from dithionite to hydrogenase of the bacterium Clostridium pasteurianum, highest activity was shown by the ferredoxin from the cyanobacterium Chlorogloeopsis fritschii and flavodoxin from the bacterium Megasphaera elsdenii. The latter was some twenty times as active as comparable concentrations of Methyl Viologen. Ferredoxins from the cyanobacterium Anacystis nidulans and the red alga Porphyra umbilicalis also showed high activity. 3. In mediating electron transfer from chloroplast membranes to Clostridium pasteurianum hydrogenase the flavodoxin from Anacystis nidulans proved the most active with Nostoc strain MAC flavodoxin and Porphyra umbilicalis ferredoxin also being appreciably more active than other cyanobacterial and higher plant ferredoxins. 4. In both hydrogenase systems the ferredoxin and flavodoxin from the red alga Chondrus crispus and the ferredoxin from another red alga Gigartina stellata showed very low activity. 5. There appeared to be no apparent correlation of efficiency in supporting hydrogenase activity with midpoint redox potential (Em) of the mediators, though some correlation of Em with the efficiency of the mediators in supporting NADP+ photoreduction by chloroplasts, or pyruvate oxidation by a Clostridium pasteurianum system, was evident. 6. Activity of the mediators in the hydrogenase systems therefore primarily reflects differences in tertiary structure conferring differing affinities for the other components of the systems.

Biological activity of synthetic molybdenum-iron-sulphur, iron-sulphur and iron-selenium analogues of ferredoxin-type centres

The molybdenum-iron-sulphur cluster [Fe6Mo2S8(SCH2CH2OH)9]3-, which contains two Fe3MoS4 cubane-like centres, is the best plausible analogue available to date for the molybdenum site of the nitrogenase enzymes. The iron-sulphur cluster [Fe4S4(S . CH2CH2OH)4]2- and the iron-selenium cluster [Fe4Se4(S . CH2CH2OH)4]2- are structural analogues of the ferredoxin Fe4S4 active centre. All three clusters would replace ferredoxin and mediate electron transfer to Clostridium pasteurianum hydrogenase in a H2-evolving system with sodium dithionite as the electron donor. The clusters would not replace hydrogenases which themselves are unable to evolve H2 from reduced ferredoxins. The molybdenum-iron-sulphur cluster would also replace ferredoxin in a chloroplast-ferredoxin-hydrogenase H2 evolving system.