Photoreductive path of carbon fixation in green plant photosynthesis. Reaction pathway of six-carbon ribulose 1,5-bisphosphate carboxylation adduct intermediate

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, τ L = 0.4 s , which is nearly two orders of magnitude shorter than the corresponding value, τ 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 ligh-dark cycles of the NADP + NADPH redox couple and endogenous changes that accompany the 2-carboxy-D-arabinitoll-phosphate binding to the enzyme active site.