Thermodynamic cycle for ligand binding to a periplasmic-BP. Thermodynamic cycle for ligand binding to a periplasmic-BP. Munliganded and Mliganded are the equilibrium allosteric constants for the interconversion of the open and closed forms of the BP in the absence and presence of ligand (filled circles). Kopen and Kclosed are the theoretical equilibrium association constants for the binding of ligand to open and closed forms of the BP. (Reprinted from reference 94.)‏ Amy L. Davidson et al. Microbiol. Mol. Biol. Rev. 2008; doi:10.1128/MMBR.00031-07