Multiphasic modelling of ligand/acceptor interactions. The hydrophobicity-dependent binding of relatively small amphiphilic substances to acceptor proteins and the nature and facedness of acceptor sites.

The modelling of multiphasic ligand/acceptor equilibrium binding systems proceeds at three logically distinct levels: (1) A suitable response quantity, e.g. the amount of acceptor-bound ligand nEL, is expressed as a function of the ligand concentrations [Li] (L = A,B,...) in the compartment i that contains the acceptor sites. One thus obtains a response function nEL = f1([Li]). In general, the equilibrium constants KL contained in such mathematical models are physically ill-defined. (2) Each local concentration [Li] is further expressed as a function of [Laq], the corresponding concentration in the aqueous phase, leading to nEL = f2([Laq]). In this way, the constants KL are transformed into effective constants K'L which (i) can be assessed experimentally and (ii) depend on ligand hydrophobicity in a way that is characteristic of the binding site. Formulation of the functions f1 and F2 only requires knowledge of the reactions in which the acceptor sites participate directly. (3) For each ligand, the experimentally accessible total ligand concentration Lt is expressed as a function of [Laq], leading to concentration balance equations Lt = Lt([Laq]). The latter transformation takes account of any reactions, distinct from ligand/acceptor interaction, in which the ligands are involved, e.g. binding to additional protein sites. As a result of steps 2 and 3, each binding system is described by a set of simultaneous equations dependent on the auxiliary variable [Laq]: (i) the response function f2([Laq]) and (ii) a concentration balance for each ligand Lt = Lt([Laq]). The formulae are rendered more conscise and their discussion and application to data fitting are simplified by introducing, for each ligand L, a function FL characterising the distribution of unbound monomeric ligand over the various partition compartments. When the acceptor acts on unbound ligand, the formulae are further expressed in terms of a new auxiliary variable i.e. the total concentration of unbound monomeric ligand microL. In contrast to data analysis as a function of local concentrations, analysis in terms of total ligand concentrations avoids losing sight of alternate hypotheses about the nature of the binding sites. The present formulation has also permitted clarification of several consequences of the multiphasic nature of the binding systems that, as yet, have been poorly recognised.(ABSTRACT TRUNCATED AT 400 WORDS)