Theoretical considerations about carbon isotope distribution in glucose of C₃ plants

The origin of the non-statistical intramolecular distribution of ¹³C in glucose of C₃ plants is examined, including the role of the aldolisation of triose phosphates as proposed by Gleixner and Schmidt (1997). A modelling approach is taken in order to investigate the relationships between the intramolecular distribution of ¹³C in hexoses and the reactions of primary carbon metabolism. The model takes into account C-C bond-breaking reactions of the Calvin cycle and leads to a mathematical expression for the isotope ratios in hexoses in the steady state. In order to best fit the experimentally-observed intramolecular distribution, the values given by the model indicate that (i), the transketolase reaction fractionates against ¹³C by 4-7‰ and (ii), depending on the photorespiration rate used for estimations, the aldolase reaction discriminates in favour of ¹³C by 6‰ during fructose-1,6-bisphosphate production; an isotope discrimination by 2‰ against ¹³C is obtained when the photorespiration rate is high. Additionally, the estimated fractionations are sensitive to the flux of starch synthesis. Fructose produced from starch breakdown is suggested to be isotopically heavier than sucrose produced in the light, and so the balance between these two sources affects the average intramolecular distribution of glucose derived from stored carbohydrates. The model is also used to estimate photorespiratory and day respiratory fractionations that appear to both depend only weakly on the rate of ribulose-1,5-bisphosphate oxygenation.