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Figure 2.2. A simplified (above) and detailed (below) description of the photosynthetic carbon reduction (PCR) cycle. The fixation of CO₂ by Rubisco to the acceptor molecule RuBP initiates the cycle with the production of two molecules of PGA. The subsequent, enzyme catalysed, generation of cycle intermediates are cycled to either regenerate RuBP or produce triose phosphates which are precursors for carbohydrate synthesis. The cycle is powered by the co-factors NADPH and ATP that are synthesised from the chloroplast electron transport chain. Enzymes include: PGK, phosphoglycerate kinase; GAPDH, glycraldehyde-3- phosphate dehydrogenase; TIM, triose phoshphate isomerase; ALD, aldolase; FBPase, fructose-1,6-bisphosphatase; TKL, transketolase, SBPase, seduheptulose-1,7-bisphosphatase; RPI, ribose-5-phosphate isomerase; RPE, ribose-5-phosphate epimerase and PRK, phosphoribulose kinase. (Courtesy Robert Sharwood).

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Figure 2.2b

Ribulose bisphosphate (RuBP) is consumed in the carboxylating step of carbon fixation. If such fixation is to continue, RuBP must be regenerated, and in this case via the PCR cycle. The PCR cycle operates within the stroma of chloroplasts, and consists of a sequence of 11 steps where a three-carbon compound (3-phosphoglycerate) is phosphorylated, reduced to glyceraldehyde 3-phosphate and isomerised to dihydroxyacetone phosphate. Condensation of this three-carbon compound with glyceraldehyde 3-phosphate yields a six- carbon compound (fructose bisphosphate). Following a series of carbon shunts, involving four-, five- and seven-carbon compounds, RuBP is regenerated.

Important features of the PCR cycle include: (1) for every step of the cycle to occur once, three carboxylations must occur via ribulose bisphosphate carboxylase thus generating six moles of phosphoglycerate (18 carbons); (2) for one turn of the cycle, three molecules of RuBP participate (15 carbons) and thus a net gain of three carbons has occurred for the plant; (3) in regenerating three molecules of RuBP, nine ATP and six NADPH are consumed.