Biological Energy Capture
The photosynthetic production of H2 by the fusion of a di-iron (FeFe) hydrogenase to photosystem I is a promising and long-sought solution to the problem of solar energy capture. By placing H2 production immediately after photon capture, metabolic losses are minimized. In addition, this scheme exploits the high activity of FeFe-hydrogenases to reduce the protein load on the cell. However, this scheme is complicated as the O2 generated by photosystem II inactivates every FeFe-hydrogenase that has ever been studied.
Linking photosynthesis to H2 production requires the evolution of new biological functionality: an O2-tolerant hydrogenase. Toward this, we are developing artificial metabolic pathways that tie microbial metabolism to hydrogenase activity. Our work on the directed evolution of hydrogenases fully opened our eyes to the potential of engineered organisms. At the same time, we have come to realize that much inefficiency of photosynthesis arises in the light harvesting and photon capture steps. Electroactive bacteria use specialized inner and outer membrane spanning extracellular electron transfer (EET) complexes to transfer electrons between metabolism and the exterior of the cell.