Hydrogen evolution by direct electron transfer from photosystem I to hydrogenases.

H2 evolution by direct electron transfer from the dithionite-reduced photosystem I (PSI) complex to both hydrogenase I and hydrogenase II from Clostridium pasteurianum was observed. Evidence indicates that the electron carriers on PSI that transfer electrons to hydrogenase in this system are the FA/FB iron-sulfur clusters on the PsaC polypeptide, the terminal bound electron acceptors in PSI. Light-dependent H2 evolution was also observed, using high potential electron donors to PSI, from a combination of hydrogenase I and either solubilized purified PSI or thylakoids. Mediators capable of transferring electrons from the PSI complex to hydrogenase were not necessary for H2 evolution, indicating again that the mechanism of H2 evolution is direct electron transfer from PSI to hydrogenase, and that this can occur with light-reduced as well as chemically reduced PSI, and with PSI in thylakoids as well as the solubilized complex. Light-dependent H2 evolution was also observed from a mixture of thylakoids and the oxygen-resistant hydrogenase of Rhodococcus sp. MR11. These results suggest that direct electron transfer from PSI to hydrogenase could be engineered to occur in vivo in a photosynthetic organism to create an organism that would efficiently produce H2 from H2O.