We have studied lipids, proteins, and carbohydrates. Although phospholipids can spontaneously form bilayers, the actual structure of biological membranes is made much more complicated through addition of protein and carbohydrate substituents to the membrane.
Normally, soluble proteins can be made to insert into bilayers by addition of nonpolar attachments. Several examples of such attachments include:
- N-myristoylation (attached myristic acid - 14:0 - through an amide link)
- S-palmitoylation (attach palmitic acid - 16:0 - through a thioester link with a Cys)
- farnesyl or geranylgeranyl additon to a CAAX carboxy-terminal sequence in a target protein, where C is Cys, A is aliphatic, and X is any amno acid
- addition of a protein to a glycosyl phophatidylinositol (GPI), through a complex which usually contains a conserved tetrasaccharide core of 3 Man and 1 GlcNAc residues linked to a protein. The GPI can be further modified with extra Gal's and Man, as well as additions to the PI group, which secures the protein in the membrane. GPIs are found in eukaryotic cells, and link many surface antigens, adhesion molecules, and hydrolases to the membrane. GPIs from Plasmoidium falciparum, the malarial parasite which kills about two million people each year, appears to act as a toxin and is the most common CHO modification of the parasite protein. Mice immunized against the GPI sequence, NH2-CH2-CH2-PO4-Man (a1-2) 6Man (a1-2) Man (a1-6) Man (a1-4) GlcNH2 (a1-6) myo-inositol-1,2-cyclic-phosphate, were substantially protected from malarial symptoms and death after they were exposed to the actual parasite.
Figure: Biological Membranes: Simple to Complex