MEMBRANE PROTEINS

By- Lovnish ThakurASU20140101000993RD SEM, Integrated BiotechBSBT-213(Bioenergetic;Biophysics)

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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.

Membrane Structure

Nelson & Cox, Lehninger Principles of Biochemistry, 4th ed., Fig. 11-3

Membrane protein functionBiological 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.

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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

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.

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.

Examples of Transmembrane Proteins

Glycophorin AGlycophorin 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

Berg et al., Fig. 12-27a

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.

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• 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

Conclusion

Thus we can say membrane protein perform a wide variety of function & important for

proper functioning of cell.

References

• cavanaghlab.bch.ncsu.edu/bch701/Membrane_proteins.pdf

• casegroup.rutgers.edu/lnotes/memprotb.pdf• cbc.arizona.edu/classes/bioc460/.../

LEC19_MembraneProteins_08-ppt.p..• www.mhhe.com/biosci/genbio/

maderinquiry9/etext/chapt04.pdf• arxiv.org/pdf/0902.3132