Saturation of cyanobacterial photoevolution of molecular hydrogen by photosynthetic redox components

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Abstract Light-induced hydrogen-gas exchange in the filamentous non-heterocystous cyanobacterium Oscillatoria chalybea has been recorded directly by means of mass spectrometry in an H/D collector at high sensitivity. The mixed hydrogen-uptake/-evolution signal that has been described in a previous paper (R. Abdel-Basset, K.P. Bader, J. Photochem. Photobiol. B: Biol. 43 (1998) 146-151) can be converted into a net evolution signal by the addition of carbonyl cyanide m-chlorophenylhydrazone (CCCP) and under these conditions the reaction is stable for at least 30 min. The effect is saturated at the low protonophore concentration of 2 μM. The participation of photosystem II in the electron supply for proton reduction has been substantiated by detailed analyses of the sensitivity of hydrogen evolution to 3-(3,4-dichlorophenyl)-N-N′ -dimethylurea (DCMU). The inhibitory effect of DCMU, which is only minimal under steady-state anaerobic conditions (when photolysis of water is inhibited in cyanobacteria), can be revealed and is shown to suppress three quarters of the evolution rate during the transition phase from aerobic to anaerobic conditions. In these early phases of the experiments when the time-dependent hydrogen evolution is suboptimal, electron-donor systems like methylviologen/dithionite and dichlorophenolindophenol/ascorbate(DCPIP/asc) significantly enhance the hydrogen evolution. Dark evolution of molecular hydrogen is also stimulated under the conditions of DCPIP/asc-dependent electron supply for proton reduction. The concentration dependence of the electron-donor system DCPEP/asc reveals a clear sigmoidal curve. From this and from the specificities of the resulting Lineweaver-Burk plot, it can be concluded that Michaelis-Menten kinetics do not appear to be operational in this case but rather that cooperative interactions must be taken into account. Fitting the data to a log(V/Vmax — V) versus logS Hill plot yields values of about three, which might be interpreted in terms of the cooperative binding of three (or four) substrate molecules to, e.g., the iron-sulfur clusters of the heterotetrameric reversible hydrogenase system in Oscillatoria chalybea. Addition of water molecules containing the stable isotope D2 (D2O) to the cyanobacterial reaction assays in the dark, results in the light-induced evolution of D2; thus deuterons that immediately come into exchange equilibrium with protons and the complex water clusters have been reduced by the analysing light. The resulting photoevolution of molecular deuterium (m/e = 4) in this case is by no means smaller than that observed following a 30 min pre-illumination, which excludes the substantial (and inevitable) participation of reduced electron-transport carriers in the proton supply for hydrogenase activity. Protons (and deuterons) from the supramolecular structure of the bulk water clusters appear to be preferentially used in the reaction of cyanobacterial hydrogen evolution.