Transmembrane proteins

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Transmembrane proteins are proteins which are situated in the lipid membrane of cells. They have transmembrane spanning regions which pass through the lipid bilayer of the cell membranes any number of times depending on the protein in question. There are multiple families of transmembrane proteins and each protein has a specific role. The two main forms of transmembrane proteins are channels and carriers. Channels form a constant pore across the plasma membrane and allow for fast diffusion of molecules. Carriers (or transporters/pumps) bind to the solute that is going to cross the membrane and undergo a conformational change so that the binding site closes on the side of the membrane the molecule bound to and opens on the opposite side releasing the substance[1]. Carrier proteins can be divided into three different groups:

  1. uniporters
  2. symporters
  3. antiporters.

Uniporters transport only one molecule across the plasma membrane at a time, unlike symporters and antiporters which transport two different molecules at once (cotransport). However, symporters transport two molecules in the same direction across the membrane and antiporters transport the molecules in opposite directions[2].

A common antiporter example is the sodium-potassium ATPase pump. 3 sodium ions start by attaching themselves to the binding site of the carrier protein. ATPase breaks down adenosine triphosphate which results in an inorganic phosphate molecule binding to the transmembrane protein. A conformational change in the protein is triggered in which the 3 sodium ions are released outside of the membrane. The 2 potassium ions are now enabled to bind to the same protein which stimulates the dephosphorylation of the attached phosphate. Conformational change again occurs causing the protein to return to its original shape. By doing this, the 2 potassium ions are released in the membrane's interior. This is very important in sustaining life[3].

These proteins are normal highly structured as the amino acid primary protein sequence allows for the formation of alpha-helices or beta-sheets.

For extraction, transmembrane protein require detergent or any non-polar solvent. Some of them (beta-sheets) however can be extracted using a denaturing agent.

G Protein Linked Receptors:

This is a form of transmembrane protein consisting of 7 membrane domains and 1 ligand binding domain, for example, the Glycolipids, the most complex form of these been the gangliosides. Associated to these domains via a binding domain is a trimeric G protein, consisting of 3 heterologous subunits (α, β, γ). The α-subunit is the catalytically active region, more specifically GTPase[4].

The binding of a signal ligand induces a conformation change within the α-subunit causing GDP to be released, consequently binding GTP. It then dissociates from the connected β and γ units and is now said to be activated.

This goes on to induce many secondary messenger pathways inside cells (cAMP & IP3 cycles).

References:

  1. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Wakter P, 2015, Molecular Biology of the Cell 6th Edition
  2. Alberts B, Johnson A, Lewis J, Morgan D, Raff M, Roberts K, Walter P. Molecular Biology of the Cell, 6th ed. New York: Garland Science; 2015. Pages 601-602.
  3. Boundless. Primary active transport. 2016 [cited04/12/16]. Available from: www.boundless.com/biology/textbooks/boundless-biology-textbook/structure-and-function-of-plasma-membranes-5/active-transport-66/primary-active-transport-337-11474
  4. Alberts, B. Johnson A, Lewis J, Taff M, Roberts K, Walter P, 2007. Molecular Biology of the Cell, 5th edition, Taylor and Francis
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