A membrane protein is a protein molecule that is attached to, or associated with the membrane of a cell or an organelle.
More than half of all proteins interact with membranes.
2. Membrane Proteins
A membrane protein is a protein molecule that
is attached to, or associated with the
membrane of a cell or an organelle.
More than half of all proteins interact with
membranes.
4. Membrane protein function
Biological membranes consist of a phospholipid bilayer and a variety of
proteins that accomplish vital biological functions.
• Structural proteins are attached to microfilaments in the cytoskeleton
which ensures stability of the cell.
• Cell adhesion molecules allow cells to identify each other and interact.
• Membrane enzymes produce a variety of substances essential for cell
function.
• Membrane receptor proteins serve as connection between the cell's
internal and external environments.
• Transport proteins play an important role in the maintenance of
concentrations of ions.
6. Main categories
• Integral membrane proteins which are
permanently bound to the lipid bilayer
• Peripheral membrane proteins that are
temporarily associated with lipid bilayer or with
integral membrane proteins
• Lipid-anchored proteins bound to lipid bilayer
bound through lipidated amino acid residues
7. Integral membrane proteins
Integral membrane proteins are permanently attached to the
membrane. They can be defined as those proteins which require a
detergent (such as SDS or Triton X-100) or some other a polar solvent
to be displaced.
They can be classified according to their relationship with the bilayer:
Integral polytopic proteins, also known as transmembrane proteins,
are protein that are permanently attached to the lipid membrane
and span across the membrane
Integral monotopic proteins are proteins that are permanently
attached to the lipid membrane from only one side and do not span
across the membrane.
8. Peripheral membrane proteins
Peripheral membrane proteins are temporarily attached
either to the lipid bilayer or to integral proteins by a
combination of hydrophobic, electrostatic, and other
non-covalent interactions.
Peripheral proteins dissociate following treatment with a
polar reagent, such as a solution with an elevated pH or
high salt concentrations.
• Integral and peripheral proteins may be post-
translationally modified, with added fatty acid chains
which may be anchored in the lipid bilayer.
10. Glycophorin A
Glycophorin A is a glycoprotein(integral membrane protein in
erythrocytes) by mass ~60% carbohydrate, ~40% protein.
• has a single transmembrane α-helix
• Has only 131 aa residues
• These are rich in sialic acid, which gives the red blood cells a very
hydrophilic-charged coat. This enables them to circulate without
adhering to other cells or vessel walls.
Most of protein (N-terminal portion) on outside of cell, exposed to
water; mainly hydrophilic residues
Extracellular part of protein also receptor for influenza virus binding
to cells
C-terminal portion on cytosolic side of membrane, interacts with
cytoskeletal proteins
12. Porins
Porins are beta proteins that cross a cellular membrane and
act as a pore through which molecules can diffuse.
Porins are large enough to allow passive diffusion, i.e., they
act as channels that are specific to different types of
molecules.
• They are present in the outer membrane of Gram-negative
bacteria and some Gram-positive bacteria.
• Porins typically control the diffusion of small metabolites
like sugars, ions, and amino acids.
14. • The amino acid composition of the porin beta
sheets is unique in that polar and non-polar
residues alternate along them.
The non-polar residues face outward so as to
interact with the non-polar lipid membrane,
whereas the polar residues face inwards into
the center of the beta barrel to interact with
the aqueous channel
15. Conclusion
Thus we can say membrane protein perform a
wide variety of function & important for
proper functioning of cell.
