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Concept
Phosphoenolpyruvate carboxykinase
Summary
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.
Since the eclipsed formation is one that is high in energy, phosphoryl group transfer has a decreased energy of activation, meaning that the groups will transfer more readily. This transfer likely happens via a mechanism similar to SN2 displacement.
PEPCK gene transcription occurs in many species, and the amino acid sequence of PEPCK is distinct for each species.
For example, its structure and its specificity differ in humans, Escherichia coli (E. coli), and the parasiteTrypanosoma cruzi.
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