Tuesday, July 31, 2007 - 3:30 PM
S111
Overcoming the biological barrier to increased hydrogen production by investigating protein complexes involved in reverse electron transport
Johannes C.M. Scholten, Mary S. Lipton, and Eric J. Ackerman. Pacific Northwest National Laboratory, PO Box 999, MS P7-50, Richland, WA 99354
Enzymes of the pentose phosphate cycle can be coupled to a NADP+-dependent hydrogenase to briefly generate 11.6 mol H2 per mol glucose-6-phosphate (Woodward et al 2000). This remarkable yield of H2 (98%) means that the overall reaction occurred because hydrogenase oxidized NADPH as soon as it was formed (evolved H2 was flushed from the vessel preventing product inhibition). However, major obstacles have to be overcome, even when utilizing a cell-free approach for H2 production, which include requirements for cofactors and H2 removal. Cell-free and biological systems must dispose of reducing equivalents generated by the reactions. Specifically cofactors such as NADPH must be re-oxidized to sustain catabolic processes. When H2 partial pressure is excessive, H2 formation from a reduced cofactor becomes unfavorable due to thermodynamic reasons. Therefore, disposal of reducing equivalents towards H2 production needs an appropriate kind of molecular machinery to facilitate this removal. Syntrophic bacteria have a homologous molecular apparatus that can overcome such an unfavorable step by utilizing an energy-consuming (ATP) reversed electron transport (RET) process. In other words, the RET molecular machinery is coupling an energetically unfavorable redox reaction to the expenditure of a membrane ion gradient. This principle might offer a solution for H2 formation from a reduced cofactor despite elevated H2 concentrations and eliminates the need for a highly-flushed vessel. Current genome sequences of syntrophic bacteria reveal essential protein complexes required for the RET machinery. Thus integration of the RET machinery into the pentose phosphate system can be used to optimize H2 production from carbohydrates.