Sunday, October 18, 2009
permeases-celltransport-4th chapter-cellbiology-2-1-btechbiotechnology-jntu syllabus
The permeases are membrane transport proteins, a class of multipass transmembrane protein that facilitate the diffusion of a specific molecule in or out of the cell.
A membrane transport protein (or simply transporter) is a protein involved in the movement of ions, small molecules, or macromolecules, such as another protein across a biological membrane. Transport proteins are integral membrane proteins; that is they exist within and span the membrane across which they transport substances. The proteins may assist in the movement of substances by facilitated diffusion or active transport.
The mechanism of action of these proteins is known as carrier-mediated transport. There are two forms of carrier-mediated transport, active transport and facilitated diffusion.
Active Transport - cotransporters
Transport of glucose through the apical membrane of intestinal and kidney epithelial cells depends on the presence of secondary active Na+/glucose symporters, SGLT-1 and SGLT-2, which concentrate glucose inside the cells, using the energy provided by cotransport of Na+ ions down their electrochemical gradient.
Passive transport - GLUTs
Facilitated diffusion of glucose through the cellular membrane is otherwise catalyzed by glucose carriers (protein symbol GLUT, gene symbol SLC2 for Solute Carrier Family 2) that belong to a superfamily of transport facilitators (major facilitator superfamily) including organic anion and cation transporters, yeast hexose transporter, plant hexose/proton symporters, and bacterial sugar/proton symporters. Molecule movement by such transporter proteins occurs by facilitated diffusion.  This makes them energy independent, unlike active transporters which often require the presence of ATP to drive their translocation mechanism, and stall if the ATP/ADP ratio drops too low.
Lactose permease has a structure that consists of two halves, each of which comprises six membrane-spanning alpha helixces. The two halves are well separated and are joined by a single stretch of polypeptide. The sugar lies in a pocket in the center of the protein and is accessible from a path that leads from the internior of the cell.
Lactose permease can be classified as a symporter, that uses the gradient of H+ towards to cell to transfer lactose along into the cell.
β-galactoside permease is a membrane-bound transport protein that pumps lactose into the cell.