Metabolic regulation in C4 photosynthesis: Phosphoenol pyruvate carboxylase and 3C intermediates of the photosynthetic carbon reduction cycle.

The effects of 3C intermediates of the photosynthetic carbon reduction (PCR) cycle (glyceraldehyde 3 phosphate [G3P], dihydroxyacetone phosphate and 3 phosphoglyceric acid) on the activity of phosphoenol pyruvate (PEP) carboxylase from Pennisetum purpureum (a C4 plant) have been investigated. Addition of these compounds, at concentrations greater than about 3 mM, to standard assay mixtures (5 mM concentrations of PEP, HCO3 (-) and Mg(2+)) reduced the observed activity. The extent of inhibition was greater at lower (<5 mM) concentration of Mg(2+). G3P was the most potent inhibitor. During studies of saturation kinetics with PEP as variable substrate it was found that addition of these 3C compounds (2mM) reduced the sigmoid nature of Michaelis-Menten plots and reduced the value of n obtained from Hill plots from about 3 to near one. The results are disussed in terms of the possible regulation of C4 photosynthesis by intermediates of the PCR cycle which are easily leaked from chloroplasts.