Escherichia coliphosphoenolpyruvate (PEP) carboxykinase catalyzes thedecarboxylationof oxaloacetateand transfer of the c-phosphoryl group of ATP to yield PEP,ADP,and CO2 . The interaction of the enzyme with the substrates ori-ginates important domainmovements in the protein. In this work,the interaction of several substrates and ligands with E. coliPEP carboxykinase has been studied in the phos-phopyridoxyl (P-pyridoxyl)-enzyme adduct.