Abstract
The half-site reactivity of trypsin and chymotrypsin binding to two double-headed black-eyed pea protease inhibitors a trypsin-chymotrypsin inhibitor (BEPCI) had a trypsin inhibitor (BEPTI), is explained in terms of the energetics of these inhibitor-protease interactions. Free energy diagrams are constructed to facilitate interpretation of the energetics. Coupling-free energies are calculated to reflect the magnitude of the interdependence of protease-binding (alloassociation) and inhibitor subunit interactions (isoassociation). The experiment observation of the predominance of liganded monomer complexes for the lima bean inhibitor and the Bowman-Birk soybean inhibitor and the predominance of half-site-liganded complexes for BEPCI and BEPTI is the direct result of the magnitudes and signs of the coupling free energies which result from these protease-inhibitor interactions.
Highlights
University, New York, The half-site reactivity of trypsin and chymotrypsin binding to two double-headed black-eyed pea protease inhibitors a trypsin-chymotrypsin inhibitor (BEPCI) had a trypsin inhibitor (BEPTI), is explained in terms of the energetics of these inhibitor-protease interactions
The experiment observation of the predominance of liganded monomer complexes for the lima bean inhibitor and the Bowman-Birk soybean inhibitor and the predominance of half-site-liganded complexes for BEPCI and BEPTI is the direct result of the magnitudes and signs of the coupling free energies which result from these protease-inhibitor interactions
Interpretation of the ligand interactions with oligomeric proteins must take into account three equilibria: binding between protein and ligand, conformational changes of the protein, and oligomeric equilibria among protein subunits
Summary
Weber’s description of allostery (4) will be summarized in a nomenclature appropriate for protease-inhibitor interactions. AG:,, + AC:,, = AG," t AG:,,;I + IP + P = I,P, The necessary condition for the independence of isoassociation and alloassociation is that AG:,, = AGP,, = AG," which leads to coupling free energies of zero. This reflects equal affinities of protease for inhibitor regardless of its state of association and the equal strengths of dimerization of the inhibitor, regardless of its ligands. For the case of AGo,,, > AG::,, > AG,“, there is negative cooperativity and alloassociation promotes isoassociation: AGE, and AGE, < 0. When AGg,, > AG,” > AG;,,, there is negative cooperativity, and alloassociation promotes isoassociation of the half-site-liganded dimer
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