Phospholipid bilayers are embedded with proteins, which may be either permanently or temporarily attached to the membrane

§  Integral proteins are permanently attached to the membrane and are typically transmembrane (they span across the bilayer)

§  Peripheral proteins are temporarily attached by non-covalent interactions and associate with one surface of the membrane  

Structure of Membrane Proteins

The amino acids of a membrane protein are localised according to polarity:

§  Non-polar (hydrophobic) amino acids associate directly with the lipid bilayer 

§  Polar (hydrophilic) amino acids are located internally and face aqueous solutions

Transmembrane proteins typically adopt one of two tertiary structures: 

§  Single helices / helical bundles 

§  Beta barrels (common in channel proteins)

Membrane Protein Structures

Functions of Membrane Proteins

Membrane proteins can serve a variety of key functions:

§  Junctions – Serve to connect and join two cells together 

§  Enzymes – Fixing to membranes localises metabolic pathways 

§  Transport – Responsible for facilitated diffusion and active transport 

§  Recognition – May function as markers for cellular identification 

§  Anchorage – Attachment points for cytoskeleton and extracellular matrix 

§  Transduction – Function as receptors for peptide hormones