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Glutamine synthetase in broccoli ( Brassica oleracea ) is regulated in vivo by reciprocal cooperativity
Author(s) -
Nkoa Roger,
Desjardins Yves,
Tremblay Nicols
Publication year - 2003
Publication title -
new phytologist
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 3.742
H-Index - 244
eISSN - 1469-8137
pISSN - 0028-646X
DOI - 10.1046/j.1469-8137.2003.00717.x
Subject(s) - brassica oleracea , cooperativity , glutamine synthetase , biochemistry , biology , enzyme , isozyme , enzyme assay , cooperative binding , substrate (aquarium) , glutamine , botany , amino acid , ecology
Summary• Purified enzymes are known to exhibit either noncooperative or cooperative kinetics, with respect to their substrates. In all cases, enzyme activity increases with substrate concentration up to saturation. Here, we report a different kinetic pattern of a regulated enzyme: reciprocal cooperativity. • Broccoli plants ( Brassica oleracea var. italica ) were grown hydroponically in six nutrient solutions whose N concentrations ranged from deficient to toxic levels. Crude extracts, from leaves and roots of whole plants, were assayed for glutamine synthetase (GS) transferase activities at four developmental stages. • A negative exponential model was fitted to the mean effect of nitrogen availability on GS activity and a reciprocal activity coefficient (RAC) was defined as the reciprocal of the regulated GS activity when N was scarce. RACs were lowest in the presence of N at 25 mg l −1 and at the 30‐d developmental stage. • These observations suggest that the RAC offers great potential as the basis for selection of isoenzymes or genotypes best adapted to low N environments. We have suggested an evolutionary and physiological basis for reciprocal cooperativity and have elaborated on three likely mechanisms for regulation of GS, based on substrate inhibition, feedback inhibitors and oxidative modification by endogenous oxidases.