A schematic model of system with two binding sites and a flexible linker.  (a) A system of a flexible receptor (linker, marked L) which may bind two different ligands, A and B. The binding site of the second ligand is marked X. The dashed lines indicate the possible complexes that may form. (b) A simplified subsystem in which A is constitutively bound to the linker L. This subsystem is the focus of this study.