The reduced nicotinamide adenine nucleotide-activated phosphoenolpyruvate carboxylase from Pseudomonas MA. Further studies on regulatory properties.

Phosphoenolpyruvate carboxylase from Pseudomonas MA grown on methylamine as a sole carbon source is an allosteric enzyme activated by NADH. Activation is accompanied by a change in the sedimentation value of the enzyme from 12 S to 9 S. In this paper ADP is shown to be an inhibitor of the enzyme. ADP has its most potent effect on the NADH-activated enzyme. Kinetics of ADP inhibition in the presence of NADH and of NADH activation in the presence of ADP are allosteric. The presence of ADP prevents the decrease in sedimentation value in the presence of NADH. Cross-linking studies indicate that the 12 S form of the enzyme is a tetramer of identically sized subunits and that the 9 S form corresponds to a dimer. The cross-linked enzyme is active and is activated by NADH and inhibited by ADP. It is proposed that NADH and ADP are a regulatory pair for this enzyme and reflect the energy status of the organism, allowing the carboxylase to control the flow of carbon into anabolic or catabolic pathways.