Metabolic regulation in C4 photosynthesis: PEP-carboxylase and energy charge.

The effects of ATP, ADP and AMP on the activity of PEP carboxylase from Pennisetum purpureum (a C4 plant) have been investigated. AMP caused slight changes in activity. Both ATP and ADP were inhibitory. The extent of inhibition was related to the concentration of Mg(2+). Kinetics of inhibition (with PEP as variable substrate) were investigated at limiting concentrations of Mg(2+) (2 mM); with excess Mg(2+) (5 mM); or with the concentration of Mg(2+) buffered at about 8 mM. At low Mg(2+) parabolic competitive-kinetics were observed. With higher Mg(2+) concentration linear competitive kinetics of inhibition were observed with a K i (slope) of 1.25 mM ATP. This was increased to 2.03 mM on addition of the allosteric activator glucose-6-P (5 mM). With Mg(2+) buffered ATP was an activator at low concentrations of PEP.Response to enegery charge was investigated using either calculated concentrations of all adenylates or mixtures of AMP and ATP pre-incubated with adenylate kinase. Inhibition at high charge could be reduced by increasing the concentration of Mg(2+) or by addition of the activator glucose-6-P. With enzyme-generated charge series and Mg(2+) buffered at 8 mM an increase in activity was observed at high charge values.These results are discussed in terms of a possible regulatory role of adenylates in the C4 dicarboxylic acid pathway of photosynthesis.