CO2 Fixation in Rhodopseudomonas Blastica

Abstract

The Calvin reductive phosphate cycle constitutes the primary pathway for the generation of intermediates of central metabolism from CO2 in plants, algae and virtually all photosynthetic and autotrophic microbes (figure 1a) [1, 2]. Of the two key enzymes, phosphoribulokinase and D-ribulose 1,5 bisphosphate carboxylase-oxygenase (RuBisCO), the latter is estimated to be the most abundant protein in the biosphere with the greatest number of associated publications [3]. Such singular attention, primarily by plant biochemists, is attributable to the following: (a) RuBisCO is not only responsible for the fixation of CO2 via the enediol of ribulose bisphosphate (RuBP) to yield two molecules of D-3-phosphoglycerate but it is also the key enzyme of the competing photo respiratory pathway catalyzing the oxygenation of RuBP to yield one molecule each of 3-phosphoglycerate and 2-phosphogluconate (figure 1b). Consequently photosynthesis and photorespiration are opposing metabolic cycles - the former effects a net fixation of carbon from CO2 whilst the latter effects a net loss of carbon from central metabolism via glycolate, which is either metabolised or excreted, with an attendant reduction in growth yield [4].