Photoproduction of Hydrogen by Purple Bacteria: A Critical Evaluation of the Rate Limiting Enzymatic Steps

Abstract

Abstract The enzymatic mechanisms and energetics of nitrogenase-catalyzed photoproduction of hydrogen from organic C-compounds by purple bacteria are discussed in respect to the question of which of the following three enzymes or enzyme systems are rate limiting for H2-production: (a) the nitrogenase-complex; (b) the enzymes and electron transport proteins involved in hydrogen transfer from the organic substrate(s) to nitrogenase; and (c) the system of photosynthetic ATP-regeneration. Calculations of maximum in vivo rates of photosynthetic ATPregeneration (gATP-values derived from growth rates), and of ATP-consumption by nitrogenase- catalyzed H2-production, and comparison of these rates with the specific activities of the enzymes of hydrogen or electron transfer from the C-substrate to the nitrogenase-complex, make it very likely that, in Rhodospirillum rubrum, nitrogenase-catalyzed H2-formation is limited by the availability of ATP and, possibly, of reducing power. In two other nonsulfur purple bacterial species (Rhodobacter capsulatus and Rhodobacter sphaeroides), H2-photoproduction is probably not energy-limited.