Enzymes of the Hatch-Slack Cycle in C3 Plants

Abstract

Phosphoenolpyruvate carboxylase (PEPC, EC 4.1.1.31), NADP-dependent malic enzyme (NADP-ME, EC 1.1.1.40) and pyruvate phosphate dikinase (PPDK, EC 2.7.9.1) are the enzymes that participate in reactions of the Hatch-Slack cycle in C₄ plants. The isoforms of these enzymes, which do not participate in photosynthesis, are present in all plants including the C₃ plants. This review is focused on the occurrence, gene families, regulation and possible functions of the above mentioned enzymes. PEPC is inhibited by L-malate, activated by D-glucose 6-phosphate and regulated by phosphorylation catalysed with specific PEPC kinase. Phosphorylated PEPC is characterized by a higher reaction rate, higher sensitivity to activation and lower sensitivity to inhibition. Similarly, PPDK is regulated by a specific PPDK regulatory protein, a most unusual bifunctional protein kinase/protein phosphatase. The function of PEPC in C₃ plants is replenishment of the citric acid cycle with intermediates, thus providing carbon skeletons for amino acid synthesis. PEPC in cooperation with malate dehydrogenase and NADP-ME produces NADPH for biosynthesis of fatty acids and specific defence compounds such as phytoalexins. The enhanced activities of PEPC, NADP-ME and PPDK under stress of various types could be a sign of their involvement in plant response to stress.