Phosphoenolpyruvate carboxykinase

Enzyme

Phosphoenolpyruvate carboxykinase (, PEPCK) is an enzyme in the lyase family used in the metabolic pathway of gluconeogenesis. It converts oxaloacetate into phosphoenolpyruvate and carbon dioxide. It is found in two forms, cytosolic and mitochondrial.

In humans there are two isoforms of PEPCK; a cytosolic form (SwissProt P35558) and a mitochondrial isoform (SwissProt Q16822) which have 63.4% sequence identity. The cytosolic form is important in gluconeogenesis. However, there is a known transport mechanism to move PEP from the mitochondria to the cytosol, using specific membrane transport proteins. PEP transport across the inner mitochondrial membrane involves the mitochondrial tricarboxylate transport protein and to a lesser extent the adenine nucleotide carrier. The possibility of a PEP/pyruvate transporter has also been put forward. X ray structures of PEPCK provide insight into the structure and the mechanism of PEPCK enzymatic activity. The mitochondrial isoform of chicken liver PEPCK complexed with Mn2+, Mn2+ phosphoenolpyruvate (PEP), and Mn2+ GDP provides information about its structure and how this enzyme catalyzes reactions. Delbaere et al. (2004) resolved PEPCK in E. coli and found the active site sitting between a C terminal domain and an N terminal domain. The active site was observed to be closed upon rotation of these domains. Phosphoryl groups are transferred during PEPCK action, which is likely facilitated by the eclipsed conformation of the phosphoryl groups when ATP is bound to PEPCK.

PEPCKase converts oxaloacetate into phosphoenolpyruvate and carbon dioxide. As PEPCK acts at the junction between glycolysis and the Krebs cycle, it causes decarboxylation of a C4 molecule, creating a C3 molecule. As the first committed step in gluconeogenesis, PEPCK decarboxylates and phosphorylates oxaloacetate (OAA) for its conversion to PEP, when GTP is present. As a phosphate is transferred, the reaction results in a GDP molecule. Although this reaction is possible, the kinetics are so unfavorable that the mutants grow at a very slow pace or do not grow at all. The role that PEPCK C plays in gluconeogenesis may be mediated by the citric acid cycle, the activity of which was found to be directly related to PEPCK C abundance. PEPCK C levels alone were not highly correlated with gluconeogenesis in the mouse liver, as previous studies have suggested. Researchers at Case Western Reserve University have discovered that overexpression of cytosolic PEPCK in skeletal muscle of mice causes them to be more active, more aggressive, and have longer lives than normal mice; see metabolic supermice.

PEPCK is one of three decarboxylation enzymes used in the inorganic carbon concentrating mechanisms of C4 and CAM plants. The others are NADP malic enzyme and NAD malic enzyme. In C4 carbon fixation, carbon dioxide is first fixed by combination with phosphoenolpyruvate to form oxaloacetate in the mesophyll. In PEPCK type C4 plants the oxaloacetate is then converted to aspartate, which travels to the bundle sheath. In the bundle sheath cells, aspartate is converted back to oxaloacetate. PEPCK decarboxylates the bundle sheath oxaloacetate, releasing carbon dioxide, which is then fixed by the enzyme Rubisco. For each molecule of carbon dioxide produced by PEPCK, a molecule of ATP is consumed. PEPCK acts in plants that undergo C4 carbon fixation, where its action has been localized to the cytosol, in contrast to mammals, where it has been found that PEPCK works in mitochondria. Although it is found in many different parts of plants, it has been seen only in specific cell types, including the areas of the phloem. It has also been discovered that, in cucumber (Cucumis sativus L.), PEPCK levels are increased by multiple effects that are known to decrease the cellular pH of plants, although these effects are specific to the part of the plant. PEPCK of Mycobacterium tuberculosis has been shown to trigger the immune system in mice by increasing cytokine activity. As a result, it has been found that PEPCK may be an appropriate ingredient in the development of an effective subunit vaccination for tuberculosis. Therefore, PEPCK M could have a role in cancer cells, especially under nutrient limitation or other stress conditions.

PEPCK C is enhanced, both in terms of its production and activation, by many factors. Transcription of the PEPCK C gene is stimulated by glucagon, glucocorticoids, retinoic acid, and adenosine 3',5' monophosphate (cAMP), while it is inhibited by insulin. Of these factors, insulin, a hormone that is deficient in the case of type 1 diabetes mellitus, is considered dominant, as it inhibits the transcription of many of the stimulatory elements. In prolonged acidosis, PEPCK C is upregulated in renal proximal tubule brush border cells, in order to secrete more NH3 and thus to produce more HCO3−. The GTP specific activity of PEPCK is highest when Mn2+ and Mg2+ are available. In addition, hyper reactive cysteine (C307) is involved in the binding of Mn2+ to the active site.

As discussed previously, PEPCK abundance increased when plants were watered with low pH ammonium chloride, though high pH did not have this effect.

It is classified under EC number 4.1.1. There are three main types, distinguished by the source of the energy to drive the reaction:
4.1.1.32 – GTP (PCK1, PCK2)
4.1.1.38 – diphosphate
4.1.1.49 – ATP