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‘Multimodal’ kinetics: Cyanobacterial nitrate reductase and other enzyme, transport and binding systems
Author(s) -
Nissen Per,
MartínNieto José
Publication year - 1998
Publication title -
physiologia plantarum
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.351
H-Index - 146
eISSN - 1399-3054
pISSN - 0031-9317
DOI - 10.1034/j.1399-3054.1998.1040327.x
Subject(s) - nitrate reductase , kinetics , cyanobacteria , enzyme , nitrate , chemistry , kinetic energy , enzyme kinetics , biophysics , biochemistry , biology , active site , physics , bacteria , organic chemistry , quantum mechanics , genetics
Concentration‐dependence data for nitrate reductase (EC 1.7.99.4) from heterocystous, nitrogen‐fixing cyanobacteria (J. Martín‐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 However, data from a total of 30 experiments (published as well as unpublished) are, overall, much better ( P 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 V 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.