Regulatory Mechanisms Involving Nicotinamide Adenine Nucleotides as Allosteric Effectors

The malate dehydrogenase of Escherichia coli is controlled by DPNH in an allosteric manner. This is concluded from kinetic studies in which oxalacetate (in the forward direction) and malate (in the reverse direction) are, individually, the variable substrates and DPNH is the inhibitor. With oxalacetate as the variable substrate the initial velocity plots at very low (0.013 mm) and very high (0.65 mm) concentrations of DPNH are hyperbolic but become sigmoidal at intermediate ranges. Similarly, when malate is the variable substrate, the hyperbolic rate concentration plots in the absence of DPNH become sigmoidal in its presence. It has shown that ATP, ADP, and AMP also inhibit the enzyme in an allosteric manner, possibly by binding at the DPNH site. From a consideration of the kinetic data it is proposed that there is an active and an allosteric site for DPNH on the enzyme surface with only the active site releasing a product. The cooperativity of the rate concentration plots is explained on the basis of the development of alternate pathways for substrate binding and product release in the presence of DPNH. The physiological importance of inhibition of malate dehydrogenase (and other enzymes) by DPNH is considered and the conclusion is reached that this inhibition is necessitated in bacteria because of the absence of rigid, compartmentation controls and the desirability of preventing gluconeogenetic channels from functioning when growth is occurring on glucose.