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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.

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