These proteins are bi-directional membrane channels which transport water, but they are not ion channels because the H 2O is transported as an uncharged molecule and not as an ion. Īquaporins are a unique class of transporter. The SLC superfamily comprises 55 gene families, having at least 362 putatively functional protein-coding genes. Many of these transporters belong to the solute-carrier (SLC) gene superfamily, and include passive transporters, sym-porters and antiporters, as well as mitochondrial and vesicular transporters. Transporters move molecules at only about 10 2-10 4 sec -1, a rate much slower than that associated with channel proteins. Transporters facilitate the movement of a specific substrate - either with or against its concentration gradient - and the conformational change in the transporter protein is important in this transfer process. More than 400 genes are known to encode ion channel subunits. Cationic and anionic substrates are transferred down their electrochemical gradients at extremely high efficiencies (as much as 10 8 sec -1). Ion channels usually occur in the closed state. As such, they regulate the cell's electric potential by allowing the flow of ions down their electrochemical gradient. Ion channels are pore-forming membrane proteins that help to establish and maintain small-voltage gradients across plasma membrane surfaces of all living cells. If the intracellular net charge following transport becomes more negative, the process is termed electronegative if the intracellular net charge becomes more positive, the process is called electropositive if the resulting intracellular net charge remains unchanged, the process is termed electroneutral. When the transported molecule and cotransported ion move in the same direction across a membrane, the transporter is called a symporter when they move in opposite directions, the transporter is called an antiporter (or exchanger). Like ATP pumps, cotransporters mediate coupled reactions in which an energetically unfavourable reaction is coupled to an energetically favourable reaction. By contrast, active transporters (or cotransporters) couple the movement of one type of ion or molecule against its concentration gradient, to the movement of another ion or molecule down its concentration gradient. Passive transporters (also called uni-porters or facilitative transporters) transport substrates down a concentration gradient. Membrane transport proteins can be either passive or active. Movement of ions or other molecules into the cell is called influx movement of ions or other molecules out of the cell is termed efflux. The portion of the cell exposed to the lumen is called the apical surface the rest of the cell (ie sides and base) makes up the basolateral surface. Transporters in eukaryotic cells move ions, sugars, amino acids and other molecules across all cellular and organelle membranes (cell surface, mitochondrial, endoplasmic reticulum, Golgi apparatus and other vesicles) - with the possible exception of nuclear membranes (which have pores). Genes from all four categories are ancient - with members present in most, if not all, prokaryotes, as well as in virtually all cell types of all eukaryotes. Membrane transport proteins can be divided into four types: ion channels transporters aquaporins and ATP-powered pumps. ABC transporters also participate in the movement of most drugs and their metabolites across cell surface and cellular organelle membranes thus, defects in these genes can be important in terms of cancer therapy, pharmacokinetics and innumerable pharmacogenetic disorders. That ABC genes are important is underscored by the fact that mutations in at least I I of these genes are already known to cause severe inherited diseases (eg cystic fibrosis and X-linked adrenoleukodystrophy ). The human genome contains 49 ABC genes, arranged in eight subfamilies and named via divergent evolution. ABC proteins transport a number of endogenous substrates, including inorganic anions, metal ions, peptides, amino acids, sugars and a large number of hydrophobic compounds and metabolites across the plasma membrane, and also across intracellular membranes. In mammals, ABC transporters are expressed predominantly in the liver, intestine, blood-brain barrier, blood-testis barrier, placenta and kidney. These pumps can move substrates in (influx) or out (efflux) of cells. ABC transporters are ubiquitous membrane-bound proteins, present in all prokaryotes, as well as plants, fungi, yeast and animals. ATP-binding cassette (ABC) transporters are an example of ATP-dependent pumps. There exist four fundamentally different classes of membrane-bound transport proteins: ion channels transporters aquaporins and ATP-powered pumps.
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