Electron pathways involved in H 2 -metabolism in the green alga Scenedesmus obliquus

Abstract

The green alga Scenedesmus obliquus is capable of both uptake and production of H(2) after anaerobic adaptation (photoreduction of CO(2) or photohydrogen production). The essential enzyme for H(2)-metabolism is a NiFe-hydrogenase with a [2Fe-2S]-ferredoxin as its natural redox partner. Western blot analysis showed that the hydrogenase is constitutively expressed. The K(m) values were 79.5 microM and 12.5 microM, determined with ferredoxin and H(2), respectively, as electron donor for the hydrogenase. In vitro, NADP(+) was reduced by H(2) in the presence of the hydrogenase, the ferredoxin and a ferredoxin-NADP reductase. From these results and considerations on the stoichiometry we propose that this light-independent electron transfer is part of the photoreduction of CO(2) in vivo. For ATP synthesis, necessary for the photoreduction of CO(2), light-dependent cyclic electron transfer around Photosystem (PS) I accompanies this 'dark reaction'. PS II fluorescence data suggest that (a) in S. obliquus H(2)-reduction might function as the anaerobic counterpart of the O(2)-dependent Mehler reaction, and (b) the presence of either a ferredoxin quinone-reductase or NAD(P)-dehydrogenase (complex I) in S. obliquus chloroplasts.

Photosynthetic H2 metabolism in Chlamydomonas reinhardtii (unicellular green algae)

“…This photosynthetic H 2 -evolution entails H 2 O-oxidation and a light-dependent transfer of electrons via photosystem-II (PSII) and photosystem-I (PSI) to the chloroplast ferredoxin (Randt and Senger 1985;Wünschiers et al 2001b). Ferredoxin eYciently binds to the [Fe]-hydrogenase and donates electrons to its catalytic site, known as the "hydrogen cluster" (HC).…”

Hydrogenases and Hydrogen Metabolism of Cyanobacteria

“…Since then hydrogenases have been observed and characterized in many microorganisms, including some algae, trichomonads, anaerobic ciliates, and chytrid fungi (4,5,59,77,86,94,100,176,222,223,225). The enzyme catalyzes the simplest of chemical reactions, the reversible reductive formation of hydrogen from protons and electrons:…”

Photoautotrophic hydrogen production by eukaryotic microalgae under aerobic conditions

“…Under special conditions (anaerobic conditions plus light), photosynthetic electrons are delivered to [FeFe]-H 2 ase instead of NADPH via ferredoxin, and [FeFe]-H 2 ase utilizes them to reduce protons to molecular hydrogen. This reaction is catalysed by [FeFe]-H 2 ase, which is extremely sensitive to oxygen and does not require any extra energy in the form of ATP 10 . In the meantime, cyanobacteria may produce hydrogen either as a byproduct of nitrogen fixation using nitrogenase 11 or by a reversible NAD(P)H-dependent NiFe-H 2 ase 12 .…”