CarrierProtein

Carrier proteins, also referred to as CarrierProtein, are integral membrane proteins that bind substrates and shuttle them across the lipid bilayer by undergoing conformational changes. They differ from channel proteins, which provide open pores; carriers undergo cycles of substrate binding, conformational change, and release, allowing transmembrane movement without forming an open channel.

Carrier-mediated transport can be either passive, known as facilitated diffusion, or active, using cellular energy to move substrates against their gradient. In facilitated diffusion, transport is saturable; the rate increases with substrate concentration but plateaus once all carriers are occupied.

Active transport may be primary, driven directly by ATP hydrolysis, or secondary, driven by ion gradients (for example, sodium or proton gradients) that couple substrate movement to gradient energy.

Substrates include sugars, amino acids, nucleosides, and various ions. Transporters are organized into solute carrier (SLC) families; many are specific for one substrate or a small group of related substrates. Typical carriers show high specificity and can be competitively inhibited by structurally similar molecules.

Common examples are GLUT transporters for glucose and other hexoses, and sodium-dependent glucose transporters (SGLTs) that perform secondary active transport. Amino acid transporters and nucleoside transporters are widespread in tissues such as the intestine, kidney, brain, and muscle. Transporters also participate in neurotransmitter reuptake and vesicular loading.

Clinical relevance: mutations or dysregulation of carrier proteins can cause inherited disorders, such as GLUT1 deficiency. Transporter expression levels influence drug absorption and distribution, and transporters are targets or conduits for pharmacological therapies. In cancer, altered transporter activity can affect nutrient uptake and drug resistance.