Structure and Composition of Biomembranes

Dr. Abhishek Roy

Junior Resident (2nd Year)

Dept. of Biochemistry

Grant Govt. Medical College

& Sir J.J. Group of Hospitals, Mumbai

Email: mail@abhishek.ro

Date: 24/06/2014

• Viewed in cross section, all cell membranes share a characteristic trilaminar appearance. This erythrocyte was stained with osmium tetroxide & viewed with Electron Microscope.

• The plasma membrane appears as a three-layer structure, 5 to 8 nm (50 to 80 Å) thick. The trilaminar image consists of two electron-dense layers (the osmium, bound to the inner and outer surfaces of the membrane) separated by a less dense central region.

• The functional specialization of each membrane type is reflected in its unique lipid composition.

• Cholesterol is prominent in plasma membranes but barely detectable in mitochondrial membranes.

• Cardiolipin is a major component of the inner mitochondrial membrane but not of the plasma membrane.

• Phosphatidylserine, phosphatidylinositol, and phosphatidylglycerol are relatively minor components (yellow) of most membranes but serve critical functions; phosphatidylinositol and its derivatives, for example, are important in signal transductions triggered by hormones.

Lipid composition of the plasma membrane and organelle membranes of a rat hepatocyte.

Fluid Mosaic Model for membrane structure

Peripheral, integral, and amphitropic proteins

• Most peripheral proteins are released by changes in pH or ionic strength, removal of Ca2+ by a chelating agent, or addition of urea or carbonate.

• Integral proteins are extractable with detergents, which disrupt the hydrophobic interactions with the lipid bilayer and form micelle-like clusters around individual protein molecules.

• Integral proteins covalently attached to a membrane lipid, such as a glycosyl phosphatidylinositol (GPI), can be released by treatment with phospholipase C.

Transbilayer disposition of glycophorin in an erythrocyte.

• A segment of 19 hydrophobic residues (residues 75 to 93) forms an α-helix that traverses the membrane bilayer.

• The segment from residues 64 to 74 has some hydrophobic residues and probably penetrates the outer face of the lipid bilayer

Integral membrane proteins.

• Types I and II have a single transmembrane helix; the amino-terminal domain is outside the cell in type I proteins and inside in type II.

• Type III proteins have multiple transmembrane helices in a single polypeptide.

• In type IV proteins, transmembrane domains of several different polypeptides assemble to form a channel through the membrane.

• Type V proteins are held to the bilayer primarily by covalently linked lipids

• Type VI proteins have both transmembrane helices and lipid (GPI) anchors.

Bacteriorhodopsin, a membrane-spanning protein.

• The single polypeptide chain folds into seven hydrophobic α-helices, each of which traverses the lipid bilayer roughlyperpendicular to the plane of the membrane.

• The seven transmembrane helices are clustered, and the space around and between them is filled with the acyl chains of membrane lipids.

Lipid annuli associated with two integral membraneproteins.

a) The crystal structure of sheep aquaporin (PDB ID 2B6O),a transmembrane water channel, includes a shell of phospholipids positioned with their head groups (blue) at the expected positions on the inner and outer membrane surfaces and their hydrophobic acyl chains (gold) intimately associated with the surface of the protein exposed to the bilayer. The lipid forms a “grease seal” around the protein, which is depicted as a green surface representation.

b) The crystal structure of the Fo integral protein complex of the V-type Na+ -ATPase from Enterococcus hirae (PDB ID 2BL2) has 10 identical subunits, each with four transmembrane helices, surrounding a central cavity filled with phosphatidyl-glycerol(PG). Here five of the subunits have been cut away to reveal the PG molecules associated with each subunit around the interior of this structure.

Tyr and Trp residues of membrane proteins clustering at the water-lipid interface.

• Residues of Tyr (orange) and Trp (red) are found predominantly where the nonpolar region of acyl chains meets the polar head group region.

• Charged residues (Lys, Arg, Glu, Asp; shown in blue) are found almost exclusively in the aqueous phases.

Thank You