PEPC

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Introductions

Phosphoenolpyruvate carboxylase (PEPC) is a key enzyme in the primary metabolism of bacteria, algae and vascular plants and is thought to be present in the cytoplasm. Phosphoenolpyruvate carboxylase catalyzes the irreversible β-carboxylation of phosphoenolpyruvate (PEP) in the presence of HCO^3- and Mg²⁺ to produce oxaloacetate (OAA) and inorganic phosphate (Pi). PEPC is widely distributed in all photosynthetic organisms, including vascular plants, algae, cyanobacteria and photosynthetic bacteria, as well as non-photosynthetic bacteria and protozoa. In vascular plants, PEPC-catalyzed reactions are the major stationary steps in C4 photosynthesis and photosynthetic CO₂ assimilation in scenic acid metabolism (CAM). In most non-photosynthetic tissues and C3 leaves in plants, the primary function of PEPC is back-complementation, supplementing the tricarboxylic acid (TCA) cycle with intermediates that are extracted for various biosynthetic pathways and nitrogen assimilation. PEPC also provides malic acid in guard cells and legume rhizomes. Consistent with this broad role, PEPC is encoded by a small gene family and is expressed in a variety of plant tissues.

Structure

PEPC carboxylase is found in plants and some types of bacteria, but not in fungi or animals (including humans). The crystal structure of PEP carboxylase has been determined in a variety of organisms, including maize and Escherichia coli. The entire enzyme exists as a dimer of dimers: two identical subunits interact closely via a salt bridge between arginine (R438 - exact location may vary depending on genetic origin) and glutamate (E433) to form a dimeric residue. This dimer (more loosely) assembles with another of its kind to form a four-subunit complex. The monomeric subunit consists mainly of α-helices (65%), each with a mass of 106 kDa. The enzyme active site has not been fully characterized. It includes a conserved aspartate (D564) and a glutamate (E566) residue, which are non-covalently bound to the divalent metal cofactor ion via the carboxylic acid functional group.

Mechanism

Most PEPCs are metabolic enzymes and their activities are regulated by a variety of metabolic effectors. The activity of PEPC in vascular plants is further regulated by reversible phosphorylation of conserved Ser residues near the N terminus. Phosphorylation of PEPC becomes less sensitive to its feedback inhibitor malate and more sensitive to the activator glucose 6-phosphate (Glu6P), resulting in higher activity. phosphorylation of PEPC is catalyzed by a dedicated PEPC kinase, which is also encoded by a small gene family. Treatments that lead to elevated PEPC activity, such as enhanced nitrogen assimilation and Pi starvation in leaves of C3 plants, induce PEPC kinase gene expression and phosphorylation of PEPC. These mechanisms strictly regulate PEPC activity, which is essential for plants due to the irreversible nature of the response.

Function

Phosphoenolpyruvate carboxylase (PEPC) is an enzyme that catalyzes the irreversible reaction of phosphoenolpyruvate with carbon dioxide to form oxaloacetate. It is widely present in plants and bacteria, and is lacking in animals and filamentous mycobacteria. It is present in chloroplasts of plants and in soluble graded precipitates of bacteria. The enzyme in E. coli has a molecular weight of about 360,000 tetramers and can be influenced by many factors, such as activation for acetyl coenzyme A and inhibition by aspartate. This enzyme is a metameric enzyme and its main function is to supply the tricarboxylic acid cycle with oxaloacetate, and it is also related to the photosynthetic carbon dioxide fixation reaction of C4 plants (C4 dicarboxylic acid cycle) and malic acid formation in Sedum (Sedum acid metabolism).