Anion regulation of lupin asparagine synthetase: Chloride activation of the glutamine-utilizing reactions |
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Authors: | Sven Erik Rognes |
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Institution: | Botanical Laboratory, University of Oslo, P.O. Box 1045, Blindern, Oslo 3, Norway |
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Abstract: | Small monovalent anions strongly activate glutamine-dependent asparagine synthesis and glutamine hydrolysis catalysed by highly purified asparagine synthetase (EC 6.3.5.4) from cotyledons of Lupinus luteus seedlings. Cl? and Br? are most effective, but F?, I?, NO3? and CN? also stimulate both reactions. The synthetase reactions with NH3, or NH2OH are only slightly stimulated by Cl? and Br?, indicating that the anions selectively accelerate the reactions involving glutamine cleavage. In asparagine synthesis Cl? is a competitive activator vs glutamine and a noncompetitive activator vs MGATP and aspartate. Addition of Cl? changes the substrate saturation kinetics of glutamine from negatively cooperative to normal hyperbolic and causes a 50-fold increase in the affinity for glutamine. The inherent glutaminase activity of the enzyme is enhanced up to 30-fold by addition of Cl?, MgATP and aspartate. Thus, ligands of the synthetase reaction act as allosteric activators of the glutaminase step in the enzyme mechanism. |
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Keywords: | Leguminosae yellow lupincotyledons asparagine synthetase glutaminase anion activation chloride kinetics |
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