Regulatory Mechanisms Involving Nicotinamide Adenine Nucleotides as Allosteric Effectors

Phosphoenolpyruvate carboxykinase of Escherichia coli is inhibited in an allosteric manner by DPNH. This inhibition is relatively quite specific. Other coenzymes such as TPN+ and DPN+ are completely ineffective and TPNH is only 15% as effective as DPNH in the inhibition of the enzyme. In cells grown on glucose DPNH concentration is 1.5 to 2 times higher than in those grown on succinate. From this it is concluded that DPNH levels serve as indicators of the state of glycolysis, and DPNH inhibition of Penolpyruvate carboxykinase is seen as a mechanism whereby gluconeogenetic channels can be prevented from functioning when active glycolysis is progressing. A kinetic study of the enzyme in the presence and absence of DPNH is presented. Initial velocity and product inhibition patterns show that the free enzyme form possibly binds ADP (or ATP) first. The inhibitor and the remaining substrates possibly bind to the E-ADP or E-ATP complexes only. The sigmoidality of oxalacetate, P-enolpyruvate, and bicarbonate plots is postulated to arise as a result of an isomerization of the enzyme in the presence of DPNH.