Subunit interactions in pig-kidney fructose-1,6-bisphosphatase: Binding of substrate induces a second class of site with lowered affinity and catalytic activity |
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Authors: | Joel L. Asenjo,Heide C. Ludwig,Cristian A. Droppelmann,Juan G. Cá rcamo,Ilona I. Concha,Alejandro J. Yá ñ ez,Marí a L. Cá rdenas,Athel Cornish-Bowden,Juan C. Slebe |
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Affiliation: | 1. Instituto de Bioquímica y Microbiología, Facultad de Ciencias, Universidad Austral de Chile, Valdivia, Chile;2. Bioénergétique et Ingénierie des Protéines, Institut de Microbiologie de la Méditerranée, CNRS, Aix-Marseille Université, Marseilles, France |
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Abstract: | BackgroundFructose-1,6-bisphosphatase, a major enzyme of gluconeogenesis, is inhibited by AMP, Fru-2,6-P2 and by high concentrations of its substrate Fru-1,6-P2. The mechanism that produces substrate inhibition continues to be obscure.MethodsFour types of experiments were used to shed light on this: (1) kinetic measurements over a very wide range of substrate concentrations, subjected to detailed statistical analysis; (2) fluorescence studies of mutants in which phenylalanine residues were replaced by tryptophan; (3) effect of Fru-2,6-P2 and Fru-1,6-P2 on the exchange of subunits between wild-type and Glu-tagged oligomers; and (4) kinetic studies of hybrid forms of the enzyme containing subunits mutated at the active site residue tyrosine-244.ResultsThe kinetic experiments with the wild-type enzyme indicate that the binding of Fru-1,6-P2 induces the appearance of catalytic sites with lower affinity for substrate and lower catalytic activity. Binding of substrate to the high-affinity sites, but not to the low-affinity sites, enhances the fluorescence emission of the Phe219Trp mutant; the inhibitor, Fru-2,6-P2, competes with the substrate for the high-affinity sites. Binding of substrate to the low-affinity sites acts as a “stapler” that prevents dissociation of the tetramer and hence exchange of subunits, and results in substrate inhibition.ConclusionsBinding of the first substrate molecule, in one dimer of the enzyme, produces a conformational change at the other dimer, reducing the substrate affinity and catalytic activity of its subunits.General significanceMimics of the substrate inhibition of fructose-1,6-bisphosphatase may provide a future option for combatting both postprandial and fasting hyperglycemia. |
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Keywords: | FBPase, fructose-1,6-bisphosphatase (d-fructose-1,6-bisphosphate 1-phosphohydrolase, EC 3.1.3.11) PFK, phosphofructokinase (ATP:d-fructose-6-phosphate 1-phosphotransferase, EC 2.7.1.11) Fru-1,6-P2, fructose 1,6-bisphosphate Fru-2,6-P2, fructose 2,6-bisphosphate Glu-tag, C-terminal extension of nine glutamate residues |
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