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Membrane Proteins


  • Broadly of two types:

    • Integral membrane proteins.

    • Peripherally associated proteins.

Integral Membrane Proteins

  • Embedded within the lipid bilayer ⟶ Very difficult to remove from the membrane.

  • Embed in only one leaflet of the lipid bilayer.

  • Have only one segment outside the bilayer ⟶ Called monotonic proteins.

  • E.g. prostaglandin E synthase.

Transmembrane proteins
  • Span the lipid bilayer.


  • Span the membrane only once ⟶ Called single-pass or single-span transmembrane proteins.

  • Have two segments outside the membrane ⟶ Called bitopic proteins.

  • E.g. Insulin receptor.


  • Span the membrane multiple times ⟶ Called multi-span transmembrane proteins.

  • Have more than two segments outside the membrane ⟶ Called polytopic proteins.

  • E.g. G protein-coupled receptors.

Lipid Anchored Proteins
  • Not truly embedded in the membrane.

  • Covalently bound to the membrane lipid ⟶ Difficult to remove.

  • E.g. Proteins attached to glycosyl phosphatidylinositol in the outer leaflet.

Peripherally Associated Proteins

  • Neither embedded in the lipid bilayer nor attached to it covalently.

  • Associated with:

    • Polar headgroup of membrane lipid or

    • Integral proteins

  • Easy to remove from mebrane.

  • E.g. Cytochrome C.



Segment of protein inside the membrane:
  • Largely made up of nonpolar amino acids ⟶ hydrophobic ⟶ comfortable in the hydrophobic environment inside the lipid bilayer.

  • Usually α helix.

Segment of protein outside the membrane:
  • Largely made up of polar amino acids ⟶ hydrophilic ⟶ comfortable in contact with water on both sides of the membrane.


  • When the protein is not attached to anything, it can diffuse freely along the entire surface of the membrane.

  • Protein molecules are bulky and sometimes attached to cytoskeletal ⟶ movement tends to be slow and some proteins can't move at all.


As Receptors

Lipid insoluble signal molecule

Cannot pass through the cell membrane

Relays the signal to membrane receptors

  • E.g. insulin receptor.

As Adhesion Molecules

  • Make physical contact with:

    • Surrounding extracellular matrix e.g. integrins.

    • Neighboring cells e.g. cadherins.

As Transport Proteins

  • For movement of water and water-soluble substances across the membrane.

  • E.g. Sodium channels.

As Enzymes

  • E.g. enzymes on the luminal side of intestinal cells.

As Second Messenger

  • Participate in intracellular signaling.

  • E.g. G proteins.


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