Regulates cataplerosis and anaplerosis, the processes that control the levels of metabolic intermediates in the citric acid cycle. At low glucose levels, it catalyzes the cataplerotic conversion of oxaloacetate (OAA) to phosphoenolpyruvate (PEP), the rate-limiting step in the metabolic pathway that produces glucose from lactate and other precursors derived from the citric acid cycle. At high glucose levels, it catalyzes the anaplerotic conversion of phosphoenolpyruvate to oxaloacetate. In addition to the phosphoenolpyruvate carboxykinase activity, also acts as a protein kinase when phosphorylated at Ser-90: phosphorylation at Ser-90 reduces the binding affinity to oxaloacetate and promotes an atypical serine protein kinase activity using GTP as donor. GTP + oxaloacetate = CO2 + GDP + phosphoenolpyruvate GTP + L-seryl-[protein] = GDP + H(+) + O-phospho-L-seryl-[protein] Binds 1 Mn(2+) ion per subunit. Phosphoenolpyruvate carboxykinase activity is regulated by glucose levels (By similarity). Phosphorylation at Ser-90 reduces the binding affinity to oxaloacetate and converts the enzyme into an atypical protein kinase using GTP as donor (By similarity). Carbohydrate biosynthesis; gluconeogenesis. Monomer. Phosphorylation at Ser-90 reduces the binding affinity to oxaloacetate and promotes the protein kinase activity. In eukaryotes there are two isozymes: a cytoplasmic one and a mitochondrial one. Belongs to the phosphoenolpyruvate carboxykinase [GTP] family.