Carrier proteins are proteins that transport a specific substance or group of substances through intracellular compartments or in extracellular fluids (e.g. in the blood) or else across the cell membrane. Some of the carriers are water-soluble proteins that may or may not interact with biological membranes, such as some transporters of small hydrophobic molecules, whereas others are integral transmembrane proteins.
Carrier proteins transport substances out of or into the cell by facilitated diffusion and active transport. Each carrier protein is designed to recognize only one substance or one group of very similar substances. The molecule or ion to be transported (the substrate) must first bind at a binding site at the carrier molecule, with a certain binding affinity. Following binding, and while the binding site is facing, say, outwards, the carrier will capture or occlude (take in and retain) the substrate within its molecular structure and cause an internal translocation, so that it now faces the other side of the membrane. The substrate is finally released at that site, according to its binding affinity there. All steps are reversible.
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