1AQ2

Phosphoenolpyruvate carboxykinase (PEPCK) catalyses first committed (rate-limiting) step in hepatic gluconeogenesis, namely the reversible decarboxylation of oxaloacetate to phosphoenolpyruvate (PEP) and carbon dioxide. This reaction can occur using either ATP or GTP as a source of the phosphate. The ATP-utilising (EC:4.1.1.49, represented here) and GTP-utilising (EC:4.1.1.32) enzymes form two divergent subfamilies, which have little sequence similarity but which retain conserved active site residues.

The reaction requires two divalent cations for activity, usually magnesium and manganese. One cation interacts with the enzyme at metal binding site 1 to elicit activation, while the second cation interacts at metal binding site 2 to serve as a metal-nucleotide substrate. In bacteria, fungi and plants, PEPCK is involved in the glyoxylate bypass, an alternative to the tricarboxylic acid cycle.

PEPCK consists of an N-terminal and a catalytic C-terminal domain, with the active site and metal ions located in a cleft between them. Both domains have an alpha/beta topology that is partly similar to one another [PMID:15023367, PMID:8609605]. Substrate binding causes PEPCK to undergo a conformational change, which accelerates catalysis by forcing bulk solvent molecules out of the active site [PMID:15890557]. PCK uses an alpha/beta/alpha motif for nucleotide binding, this motif differing from other kinase domains.