Electron Transport as a Limiting Factor in Biological Hydrogen Production

Abstract

Increasing the capability of biological systems for hydrogen production requires an understanding of the fundamental level of potential limiting factors. Degradation of various wastes with the concomitant evolution of hydrogen by photosynthetic bacteria has long been proposed as a possible biohydrogen source. Here we present evidence, using growth studies with different nitrogen sources, which indicates that the maximal in vivo activity of the nitrogenase (N₂ase) system of the photosynthetic bacterium, Rhodobacter capsulatus, is probably restricted at the level of electron flow. Thus, an increase in catalytic efficiency of at least threefold is possible. We are presently investigating the physiological mechanisms responsible for this metabolic gating. Although very little is presently known in general about the determinants for reaction specificity and efficiency between low-potential redox carriers and enzymes functioning in biological hydrogen production, these results suggest that a consideration of these factors may be extremely important. As a model system, we are undertaking studies of the role of electron carriers in controlling reductant flux through the pyruvate oxidoreductase (POR), N₂ase system. The methods we are using and results obtained should be applicable to other hydrogen-producing systems, for example, POR and hydrogenase.