Membrane protein

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Membrane Protein.png
Membrane proteins are proteins situated at the lipid bilayer or membrane of the cell. In animal cells, these proteins make up around 50% of the mass of the plasma membrane. Different types of membrane proteins are present at the membrane and each has different structure. They also carry out different functions. These functions are vital to the cell. Membrane protein acts as transporters, anchors, receptors and lastly enzymes. Moreover, different types of membrane proteins associate with the lipid bilayer in different ways: transmembrane, monolayer-associated, lipid-linked and protein-attached.

Contents

 Functions 

Transporters

Transporters proteins transports ions or molecules in or out of the cell. It transports ions and some hydrophilic molecules across the membrane. It regulates the intracellular composition of the cell. 

Examples of transporters are Na+ pump, ligand-gated ion channels and Ca2+ ATPase.

Anchors 

Anchor links the membrane with intracellular molecules or extracellular molecules at either side of the plasma membrane.

Receptors 

Receptors are important proteins that regulate cell signalling. It helps the cell to communicate with the surrounding environment. Receptors receive chemical signal from the extracellular environment and transmit the message into the cell. The cell then decides what process to carry out via these messages.

Example of receptors is nicotinic acetylcholine receptor

Enzymes

Enzymes serve as catalysts. It catalyse specific reactions, producing certain molecules that are responded by the cell.

Example of enzyme situated at the bilayer is protein kinase

Association with Lipid Bilayer 

Transmembrane proteins

Transmembrane proteins are proteins that extend across the lipid bilayer. Transmembrane proteins have two specific regions: hydrophobic region and hydrophilic region. The proteins are positioned in a way that the hydrophobic region is located within the bilayer where the hydrophobic tails region of the lipid molecules are. The hydrophilic region in the other hand are exposed to the intracellular or extracellular environment of the cell; interacting with molecules. 

Monolayer-associated proteins

Monolayer-associated proteins are proteins that situated within the cell; or the cytosol. It interact with the inner layer of the lipid bilayer. 

Lipid-linked proteins

Lipid-linked proteins interact with a lipid molecule of the bilayer through covalent bond. It can be at the either side of the cell.

Protein-attached proteins

Protein-attached proteins are indirectly associated with the bilayer through other proteins which are situated within the bilayer. They are attached to the proteins through weak, noncovalent interactions. They can also be at the either side of the cell. 

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