The Central Role of Phosphoenolpyruvate Metabolism in Developing Oilseeds

Abstract

Metabolite interconversion at the phosphoenolpyruvate (PEP)-pyruvate-oxaloacetate node involves a complex set of cytosolic and plastidial reactions that interconnect the major pathways of carbohydrate metabolism, thereby making a crucial contribution to the distribution of carbon flux among catabolism, anabolism, and ATP and NAD(P)H supply to plant cells. Enzymes involved in plant PEP metabolism catalyze a diverse array of reactions, including a major metabolic branchpoint between primary and secondary (shikimate pathway) metabolism. Carbon partitioning at the PEP branchpoint is complicated by an intricate network of posttranslational enzyme controls, including allosteric effectors and protein kinase mediated phosphorylation. Experiments on transgenic or mutant plants possessing altered amounts of PEP metabolizing enzymes or transporters are enhancing our understanding of the functional organization and control of oilseed PEP metabolism. Such experiments illustrate the highly flexible nature of plant PEP metabolism and the crucial biosynthetic function played by glycolysis and respiration beyond their role in catabolic ATP generation. This chapter summarizes what is known about the key PEP metabolizing enzymes and corresponding metabolic fluxes during the reserve deposition stage of oilseed development. PK and PEPC are of particular interest since they play an essential role in controlling the provision of: (i) pyruvate for mitochondrial ATP production via oxidative phosphorylation; (ii) tricarboxylic acid cycle intermediates needed for nitrogen assimilation and amino acid biosynthesis; and (iii) precursors and cofactors (e.g., pyruvate, acetyl-CoA, malate, ATP, and NAD(P)H) needed for plastidial fatty acid synthesis. Novel insights into the functions, and molecular and regulatory characteristics of oilseed PK and PEPC isozymes have arisen through their purification and detailed biochemical and molecular characterization, as well as advances in functional genomics, proteomics, and metabolic flux analysis. PK and PEPC are becoming important targets for metabolic engineering of the PEP branchpoint to modify levels of agronomically important end products, such as storage proteins and lipids in oilseeds.