Membrane Proteins

Biological membranes consist of a continuous double layer of lipid molecules in which membrane proteins are embedded.
Although the lipid bilayer provides the basic structure of biological membranes, the membrane proteins perform most of the membrane’s specific tasks and therefore give each type of cell membrane its characteristic functional properties.
The amounts and types of proteins in a membrane are highly variable.
In the myelin membrane, which serves mainly as electrical insulation for nerve-cell axons, less than 25% of the membrane mass is protein. By contrast, in the membranes involved in ATP production (such as the internal membranes of mitochondria and chloroplasts), approximately 75% is protein.
A typical plasma membrane is somewhere in between, with protein accounting for about half of its mass.
Because lipid molecules are small compared with protein molecules, however, there are always many more lipid molecules than protein molecules in cell membranes—about 50 lipid molecules for each protein molecule in cell membranes that are 50% protein by mass.
Membrane proteins vary widely in structure and in the way they associate with the lipid bilayer, which reflects their diverse functions.
Like the lipids, membrane proteins are amphiphilic, having hydrophobic and hydrophilic regions.

Image Source: Bio Ninja

Table of Contents

Location of Membrane Proteins

Many membrane proteins extend across the lipid bilayer.
In some of these transmembrane proteins, the polypeptide chain crosses the bilayer as a single α helix (single-pass proteins).
In others, including those responsible for the transmembrane transport of ions and other small water-soluble molecules, the polypeptide chain crosses the bilayer multiple times—either as a series of α helices or as a β sheet in the form of a closed barrel (multipass proteins).
Other membrane-associated proteins do not span the bilayer but instead are attached to either side of the membrane.
Many of these are bound by noncovalent interactions with transmembrane proteins, but others are bound via covalently attached lipid groups.
Like the lipid molecules in the bilayer, many membrane proteins are able to diffuse rapidly in the plane of the membrane.
However, cells have ways of immobilizing specific membrane proteins and of confining both membrane protein and lipid molecules to particular domains in a continuous lipid bilayer.

Integral proteins are permanently anchored and embedded within the lipid bilayer.
They cannot easily be removed from the cell membrane without the use of harsh detergents that destroy the lipid bilayer.
Integral proteins float rather freely within the bilayer, much like oceans in the sea.
In addition, integral proteins are usually transmembrane proteins, extending through the lipid bilayer so that one end contacts the interior of the cell and the other touches the exterior.
The stretch of the integral protein within the hydrophobic interior of the bilayer is also hydrophobic, made up of non-polar amino acids. Like the lipid bilayer, the exposed ends of the integral protein are hydrophilic.

They can be classified according to their relationship with the bilayer:

Integral polytopic proteins are transmembrane proteins that span across the membrane more than once. These proteins may have different transmembrane topology. These proteins have one of two structural architectures:
Helix bundle proteins, which are present in all types of biological membranes;
Beta barrel proteins, which are found only in outer membranes of Gram-negative bacteria, and outer membranes of mitochondria and chloroplasts.
Bitopic proteins are transmembrane proteins that span across the membrane only once. Transmembrane helices from these proteins have significantly different amino acid distributions to transmembrane helices from polytopic proteins.
Integral monotopic proteins are integral membrane proteins that are attached to only one side of the membrane and do not span the whole way across.

They are only temporarily attached to the lipid bilayer or to other integral proteins.
They are easily separable from the lipid bilayer, able to be removed without harming the bilayer in any way.
Peripheral proteins are less mobile within the lipid bilayer.
They are attached by a combination of hydrophobic, electrostatic, and other non-covalent interactions.

Lipid-Bound Proteins

Lipid-bound proteins are located entirely within the boundaries of the lipid bilayer.

Membrane proteins perform a variety of functions vital to the survival of organisms:

Membrane receptor proteins relay signals between the cell’s internal and external environments.
Transport proteins move molecules and ions across the membrane. They can be categorized according to the Transporter Classification Database.
Membrane enzymes may have many activities, such as oxidoreductase, transferase or hydrolase.
Cell adhesion molecules allow cells to identify each other and interact. For example, proteins involved in the immune response.

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