The two main categories for membrane proteins are integral (transmembrane) proteins and peripheral proteins. Integral membrane proteins are comprised of non-polar amino acids with alpha helix coiling (hydrophobic) and hydrophilic regions at the extra cellular surface and cytoplasm surface. Exterior peripheral proteins are always attached to the surface of the plasma membrane, by ionic interactions, and exterior peripheral proteins are often held in place by the cytoskeleton of protein filaments.
Transport proteins are a sub-category of integral proteins. At each polar region, the protein has a binding site which attracts only specific molecules. When a molecule binds to the carrier protein, the transport protein alters its shape (moving the binding site to the opposite polar region) to allow the substance to travel freely through the membrane. This process can require ATP (energy), but the ATP complex will be within the transport protein. When ATP is involved it is used to move substances against the concentration gradient; active transport. If ATP is not involved facilitated diffusion occurs. Many of the cells essential ions, sugars, amino acids and other nutrients are transported across the membrane by transport proteins. There are three types of transport proteins uniporter (binds to one molecule of solute at a time and transports it along the solute gradient), / (binds to two molecules at a time and uses the gradient of one solutes concentration to force the other molecule against its gradient ) and antiporter (binding to one of solute (S1) outside the membrane, and one molecule (S1) on the inside. By using S2's gradient, we are able to transport S1 against its gradient).
The above protein assists the movement of molecules across the partially permeable membrane by the following processes; facilitated diffusion, active transport and exocytosis. Facilitated diffusion moves molecules down the concentration, but needs to be assisted by a carrier protein. Active transport also requires a carrier transport protein, however this process goes against the concentration gradient and therefore requires ATP (energy) This is achieved by either altering the inclination of the binding site or altering the rate at which the protein changes shape to induce movement.
Enzymatic proteins are peripheral proteins. They are one of the most specialised proteins due to their catalytic ability. The enzymes needed for metabolic pathway reactions can be aligned along the membrane surface to allow controlled reactions to take place. The active site will be easily found by the substrate and will react to form the product. This product may then become the substrate for another enzymatic protein bound to the membrane surface.
Signal transduction proteins are a form of integral proteins. They allow signal molecules such as hormones to attach themselves to a polar region . The signal molecule then induces a change in the protein (ligand interaction), which then informs the cell of the signal molecules presence, mainly by movement of ions, and action can then be taken. This membrane protein can be essential in cell defence. Also, the more complex ligand interactions can also effect cell activity. For example, protein phosphorylations change enzyme activities and protein structures, which in turn alters the biochemical pathways of a cell.
Attachment membranes are used structurally. They can be peripheral (collagen) or integral (integrins). They could also be attached to the cytoskeleton and connect to a transmembrane linker protein (cell adhesion protein). These proteins attach themselves to the extra cellular matrix or/and the cytoskeleton. These proteins can also be one of any other protein mentioned in this essay.
Recognition proteins use glycoproteins as surface receptors to identify approaching cells as foreign or ‘one’s own cell’. The immune system would not function without these glycoproteins as there would be little ability to identify foreign cells. They play a unique role in cellular communication and signal transduction. Cell adhesion proteins are used to form cell junctions. This allows for tissues to be formed. Cell recognition and adhesion is used for growth and development. Homotypic adhesion proteins allow growth and regrowth to occur by attaching themselves to identical homotypic proteins (ligands) to form tissues, but need glycoproteins to first recognise an identical cell. Heterotypic recognition proteins allow sexual reproduction to occur with gamete cells. Without cell adhesion white blood cells could not function as they would be unable to attach themselves to alien cells. Alzheimer’s disease is caused by poor adhesion of sypnases (cell junctions in the nervous system), which results in poor signalling between cells.
Sources
Bellevue community life science faculty. 2004. /lectures/pdfs/membranes201.pdf.
Wikipedia. 2004. .
Saunders, Dr. N. 2004. .
Simpkins, J. Williams, J.I. 1992. Advanced Biology - 3rd Ed. Scot Print Ltd.
Fullick, A. 2000. Heinemann ADVANCED Science BIOLOGY - 2nd Ed. Heinemann.
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