Membrane Proteins
TYPES
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.
Monotopic
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.
Bitopic
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.
Polytopic
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.
COMMON PROPETIES
Structure
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.
Movement
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.
FUNCTIONS
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.