Glycophorin

Since erythrocytes  red blood cells do not hold any subcellular  organelles they are essentially  a membranous  sac for carrying  hemoglobin their plasma membrane  is a convenient  model system for studies of membrane structure  as it can readily  be isolated  from other membranes  and intracellular elements.   One of the main   glycoproteins   in the plasma   membrane   of erythrocytes is glycophorin A. amino acid protein a 131 that was the first integral protein to be sequenced. This revealed in which the polypeptide chain of glycophorin consists with the three domains:

1.  An N-terminal region on the extracellular side of the membrane which contains all the N- and O-linked glycosylation sites;

2.  A hydrophobic central region which is buried in the hydrophobic core of the bilayer; and

3.  A C-terminal region rich in polar and charged residues which is exposed on the cytosolic side of the membrane.

As with the high majority of transmembrane  proteins the hydrophobic membrane- spanning region consists mostly of amino acid residues with hydrophobic  side- chains  which  are  folded  in  an  α-helical conformation.  As  every amino  acid  residue  to  the  length adds  0.15  nm of  an  α-helix and  a  helix  of  25 residues would have a length of 3.75 nm such as enough to span the hydrophobic core  of  the  bilayer.  Hydrophobic the side-chains  of  the  residues  in  the  helix protrude  outwards  from the helix axis to interact  through hydrophobic  bonds with the  fatty  acid  chains.  Whichever  side  of  this  hydrophobic  α-helix  are  clusters  of amino acids with charged side-chains  that interact noncovalently  with opposite charges on the polar headgroups of the membrane lipids.