Allosteric Modulation of Membrane Proteins by Low-Affinity Ligands

Under an applied external stimulus, membrane proteins may respond to a variety of ligands, including small low-affinity molecules accounting for functional effects in the mM range. How such ligands modulate protein function must build on understanding their atomic-level interactions under dilution, thus currently challenging the resolution of theoretical and experimental routines. Part of the problem derives from the fact that small low-affinity ligands may interact with multiple sites of a membrane protein in a highly degenerate manner to a degree that it’s better conceived as a partition phenomenon, hard to be tracked at the molecular interface of the protein. Looking for new developments in the field, we rely on the classic two-state Boltzmann model to devise a novel theoretical description of the allosteric mechanism of membrane proteins in presence of small low-affinity ligands and external stimulus. Free-energy stability of the partition process and its energetic influence on the protein coupling with the external stimulus are quantified and the outcome is a simple formulation that allows equilibrium shifts of the protein to be described in terms of the grand-canonical partition function of the ligand at dilute concentrations. Model’s predictions of the spatial distribution and response probability shifts across a variety of ligand concentrations and thermodynamic conjugates can be directly compared to macroscopic measurements, making it especially useful to interpret experimental data at the atomic level.