Unidirectional inhibition of phosphoenolpyruvate carboxykinase from Rhodospirillum rubrum by ATP

The kinetic and regulatory properties of partially purified phosphoenolpyruvate (PEP) carboxykinase (EC 4.1.1.32) from Rhodospirillum rubrum were studied. The enzyme was active with guanosine-and inosinephosphates and must thus be classified as GTP (ITP): oxaloacetate carboxylyase (transphosphorylating). In the direction of oxaloacetate-formation, the enzyme was strongly inhibited by ATP (K_i=0.03 mM). ITP, UTP, CTP and GTP were less inhibitory. The inhibition was competitive with respect to GDP or IDP, but not with respect to PEP. In the direction of PEP-synthesis, the enzyme was not inhibited, but rather activated by ATP.