The role of ligand-binding proteins in increasing the transcellular flux of metabolites, in particular that of calcium, bilirubin, and fatty acids

Both for transcellular transport and intracellular transport, low intracellular concentrations of a diffusing molecule or ion will result in low fluxes and hence in a low rate of delivery of substrate across the cell or within the cell, respectively. For both situations, the presence of ligand-binding proteins will speed up transport, and hence metabolism, by co-diffusion, provided that the binding-protein is present at a reasonably high concentration, has a small size and hence a relatively high diffusion coefficient, and has a high affinity for the ligand. These conditions seem to be met both for the calcium-binding proteins involved in transport of calcium across the intestinal and kidney epithelia and for the proteins that bind hydrophobic substrates such as fatty acids and bilirubin in the liver cell.