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

Concentration-dependence data for nitrate reductase (EC 1.7.99.4) from hete rocys tous, nitrogen-fixing cyanobacteria (J. Martin-Nieto, E. Flores and A . Herrero, 1992. Plant Physiol. 100: 157-163) have been interpreted most pl ausibly to reflect the operation of a single enzyme with two independent ca talytic sites. However, data from a total of 30 experiments (published as w ell as unpublished) are, overall, much better (P < 0.0001) represented acco rding to a 'multimodal' kinetic model rather than as due to two separate si tes. This new term is introduced to refer to enzyme systems displaying mult iple concentration-dependent phases separated by sharp inflections. This ph enomenon is taken to reflect the operation of a single catalytic site under going discontinuous conformational transitions and thus able to function in distinct kinetic modes'. Moreover, plots of log K-m versus log V-max in th ese kinetic systems are perfectly linear, as also previously found for mult iphasic plant uptake systems. The same multi-mode kinetic behavior is exhib ited by a wide variety of enzyme, uptake and ligand-binding systems from pl ants, animals and microorganisms, including monomeric proteins purified to homogeneity. Multimodal kinetics thus constitute a widespread, albeit large ly unrecognized, phenomenon in nature.