Tuesday, December 11^th 2012,17 c.t.

G2-104, CeBiTec Building

Prof. Dr. John Golbeck

Penn State Department of Chemistry - Pennsylvania State University, USA

A Solar Hydrogen-Producing Bio-Nanoconstruct that outperforms Natural Photosynthesis

The photosynthetic efficiency for solar conversion of H₂O and NADP⁺ to O₂ and NADPH is 11.8%. In contrast, the highest reported photosynthetic efficiency for solar conversion to biomass is 2.4% for C3 plants and 3.7% for C4 plants. Thus, there exist large downstream metabolic losses between the most immediate product of photosynthesis and a harvestable product. We asked whether it might be possible to increase the efficiency of solar to biofuel production by directly wiring a protein catalyst to the reaction center. The strategy used here employs a molecular wire to directly connect the redox centers of Photosystem I (PSI) and a protein catalyst such as a H₂ase enzyme. The molecular wire serves to tether the photochemical module to the catalytic module at a fixed distance so that the electron can quantum mechanically tunnel between the surface-located [4Fe-4S] clusters of PSI and a H₂ase at a rate faster than the competing charge recombination between P₇₀₀⁺ and F_B^–. The PSI—wire—[FeFe]-H₂ase bioconstruct evolves H₂ at a rate of 2850 µmoles mg Chl^-1 h^-1, which is an electron transfer throughput of 5700 µmoles mg Chl^-1 h^-1, or 142 µmoles e^– PSI^_-1 s^-1. Putting this into perspective, cyanobacteria evolve O₂ at a rate of ~400 µmoles mg Chl^-1 h^-1, which is an electron transfer throughput of 1600 µmoles mg Chl^-1 h^-1, or 49 e^– PSI^-1 s^-1, given a PSI to PSII ratio of 1.8 as in Synechococcus sp. PCC 7002. Hence, the bioconstruct outperforms natural photosynthesis by a factor of 3.5, an increase due entirely to the elimination of kinetic bottlenecks due to diffusion chemistry. In this talk, I will discuss potential solutions to increasing the efficiency of solar biofuels production by taking advantage of direct wiring of protein catalysts, including formic acid dehydrogenase, to Photosystem I.

Lubner, C., Applegate, A., Knörzer, P., Happe, T., Bryant, D. A., and Golbeck, J. H. (2011) “A solar hydrogen-producing bio-nanodevice that outperforms natural photosynthesis”, Proc. Natl. Acad. Sci. USA 108, 20988-20991.

Prof. Dr. Olaf Kruse