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Most of molecules enter or leave cells mainly via membrane transport proteins, which play important roles in several cellular functions, including cell metabolism, ion homeostasis, signal transduction, binding with small molecules in extracellular space, the recognition process in the immune system, energy transduction, osmoregulation, and physiological and developmental processes. There are three major types of transport proteins, ATP-powered pumps, channel proteins and transporters.
ATP-powered pumps are ATPases that use the energy of ATP hydrolysis to move ions or small molecules across a membrane against a chemical concentration gradient or electric potential. Channel proteins transport water or specific types of ions down their concentration or electric potential gradients. Many other types of channel proteins are usually closed, and open only in response to specific signals. Because these types of ion channels play a fundamental role in the functioning of nerve cells. Transporters, a third class of membrane transport proteins, move a wide variety of ions and molecules across cell membranes. Membrane transporters either enhance or restrict drug distribution to the target organs. Depending on their main function, these membrane transporters are divided into two categories: the efflux (export) and the influx (uptake) transporters.
Transport proteins such as channels and transporters play important roles in the maintenance of intracellular homeostasis, and mutations in these transport protein genes have been identified in the pathogenesis of a number of hereditary diseases. In the central nervous system ion channels have been linked to many diseases such, but not limited to, ataxias, paralyses, epilepsies, and deafness indicative of the roles of ion channels in the initiation and coordination of movement, sensory perception, and encoding and processing of information. Furthermore, drug transporters can serve as drug targets or as a mechanism to facilitate drug delivery to cells and tissues.
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