Thermodynamics of protein-cation interaction: Ca+2 and Mg+2 binding to the fifth binding module of the LDL receptor

The ligand binding domain of the LDL receptor (LDLR) contains seven structurally homologous repeats. The fifth repeat (LR5) is considered to be the main module responsible for the binding of lipoproteins LDL and beta-VLDL. LR5, like the other homologous repeats, is around 40-residue long and contains three disulfide bonds and a conserved cluster of negatively charged residues surrounding a hexa-coordinated calcium ion. The calcium coordinating cage is formed by the backbone oxygens of W193 and D198, and side-chain atoms of D196, D200, D206, and E207. The functionality of LDLR is closely associated with the presence of calcium. Magnesium ions are to some extent similar to calcium ions. However, they appear to be involved in different physiological events and their concentrations in extracellular and intracellular compartments are regulated by different mechanisms. Whether magnesium ions can play a role in the complex cycle of LDLR internalization and recycling is not known. We report here a detailed study of the interaction between LR5 and these two cations combining ITC, emission fluorescence, high resolution NMR, and MD simulations, at extracellular and endosomal pHs. Our results indicate that the conformational stability and internal dynamics of LR5 are strongly modulated by the specific bound cation. It appears that the difference in binding affinity for these cations is somewhat compensated by their different concentrations in late LDL-associated endosomes. While the mildly acidic and calcium-depleted environment in late endosomes has been proposed to contribute significantly to LDL release, the presence of magnesium might assist in efficient LDLR recycling.