'Multimodal' kinetics: Cyanobacterial nitrate reductase and other enzyme, transport and binding systems

Abstract

Concentration-dependence data for nitrate reductase (EC 1.7.99.4) from heterocystous, nitrogen-fixing cyanobacteria (J. Martin-Nieto, E. Flores and A. Herrero, 1992. Plant Physiol. 100: 157-163) have been interpreted most plausibly to reflect the operation of a single enzyme with two independent catalytic sites. However, data from a total of 30 experiments (published as well as unpublished) are, overall, much better (P < 0.0001) represented according to a 'multimodal' kinetic model rather than as due to two separate sites. This new term is introduced to refer to enzyme systems displaying multiple concentration-dependent phases separated by sharp inflections. This phenomenon is taken to reflect the operation of a single catalytic site undergoing discontinuous conformational transitions and thus able to function in distinct kinetic 'modes'. Moreover, plots of log K(m) versus log Vm(max) in these kinetic systems are perfectly linear, as also previously found for multiphasic plant uptake systems. The same multi-mode kinetic behavior is exhibited by a wide variety of enzyme, uptake and ligand-binding systems from plants, animals and microorganisms, including monomeric proteins purified to homogeneity. Multimodal kinetics thus constitute a widespread, albeit largely unrecognized, phenomenon in nature.