The PEP-pyruvate-oxaloacetate node: variation at the heart of metabolism

At the junction between the glycolysis and the tricarboxylic acid cycle-as well as various other metabolic pathways-lies the phosphoenolpyruvate (PEP)-pyruvate-oxaloacetate node (PPO-node). These three metabolites form the core of a network involving at least eleven different types of enzymes, each with numerous subtypes. Obviously, no single organism maintains each of these eleven enzymes; instead, different organisms possess different subsets in their PPO-node, which results in a remarkable degree of variation, despite connecting such deeply conserved metabolic pathways as the glycolysis and the tricarboxylic add cycle. The PPO-node enzymes play a crucial role in cellular energetics, with most of them involved in (de)phosphorylation of nucleotide phosphates, while those responsible for malate conversion are important redox enzymes. Variations in PPO-node therefore reflect the different energetic niches that organisms can occupy. In this review, we give an overview of the biochemistry of these eleven PPO-node enzymes. We attempt to highlight the variation that exists, both in PPO-node compositions, as well as in the roles that the enzymes can have within those different settings, through various recent discoveries in both bacteria and archaea that reveal deviations from canonical functions.

Automated summary

The phosphoenolpyruvate (PEP)-pyruvate-oxaloacetate node (PPO-node) is a crucial intersection between glycolysis and the tricarboxylic acid cycle, involving at least eleven different types of enzymes, with most of them involved in (de)phosphorylation of nucleotide phosphates. Different organisms possess different subsets of enzymes in their PPO-node, reflecting the different energetic niches they can occupy. Variations in PPO-node composition highlight the remarkable degree of variation in roles that enzymes can play within different settings.