Introduction to Enzyme and Coenzyme Chemistry - E-Library Home

All Enzymes are Proteins 25
Figure 2.23 Structure of bacteriorhodopsin (PDB Wle 1C3W), a seven-transmembrane helix membrane
protein, solved in the presence of phospholipid. Protein structure shown in red; phospholipid
and water molecules shown in monochrome (spaceWll).
the cytoplasmic membrane of the cell, in which the sugar residues attached to
the protein are located on the exterior of the cell membrane. Since these
glycoproteins are exposed to the external environment of the cell, they are
often important for cell–cell recognition processes. In this respect they act as
a kind of ‘bar-code’ for the type of cell on which they are residing. This function
has been exploited in a sinister fashion, as a means of recognition and entry into
mammalian cells, by viruses such as inXuenza virus and human immunodeWciency
virus (HIV).
The carbohydrate residues are attached in one of two ways shown in Figure
2.24: either to the hydroxyl group of a serine or threonine residue (O-linked
glycosylation); or to the primary amide nitrogen of an asparagine residue
(N-linked glycosylation).
The level of glycosylation can be very substantial: in some cases up to 50%
of the molecular weight of a glycoprotein can be made up of the attached
carbohydrate residues. The pattern of glycosylation can also be highly complex,
for example highly branched mannose-containing oligosaccharides are often
found. The sugar attachments are generally not involved in the active site
catalysis, but are usually required for full activity of the protein.