title: Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate.
abstract: In this paper we examine the six-carbon intermediate pathway of ribulose 1,5-bisphosphate (RuBP) carboxylation reaction in photosynthesis. Based on the observed reactions of purified RuBP carboxylase, mechanisms are described for carbon dioxide assimilation leading to the hydrolytic splitting of the six-carbon intermediate to two enzyme-bound glycerate-3-P (3-PGA) molecules. It is concluded that, under photosynthetic conditions, the reduction of enzyme-bound NADP+ by the chlorophyll is responsible for the rapid carboxylase turnover rate given by the lifetime, tau L = 0.4 s, which is nearly two orders of magnitude shorter than the corresponding value, tau D = 11 +/- 3 s, for the dark decay of enzyme-bound RuBP. The nocturnal inhibition and photoactivation of RuBP carboxylation are described in terms of the reversible light-dark cycles of the NADP+/NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitol-1-phosphate binding to the enzyme active site.
mesh_terms: D002244:Carbon / Q000378:metabolism; D002413:Cations, Divalent; D005988:Glyceric Acids / Q000378:metabolism; D008274:Magnesium / Q000378:metabolism; D009249:NADP / Q000378:metabolism; D010428:Pentosephosphates / Q000378:metabolism*; D010777:Photochemistry; D010788:Photosynthesis; D010944:Plants / Q000201:enzymology*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism*; D012274:Ribulosephosphates / Q000378:metabolism*
chemical_list: D002413:Cations, Divalent; D005988:Glyceric Acids; D010428:Pentosephosphates; D012274:Ribulosephosphates; C001933:ribulose-1,5 diphosphate; D009249:NADP; D002244:Carbon; C005156:3-phosphoglycerate; D012273:Ribulose-Bisphosphate Carboxylase; D008274:Magnesium