sequence that contained spans of 20–28 amino acid residues, each of which contained
predominantly hydrophobic residues flanked on eitherside by hydrophilic residues.
These regions represent the seven membrane-spanning helical regions of the protein.
For membrane proteins defining aqueous channels, hydrophilic residues are also
present in the transmembrane section. Pores comprise amphipathica-helices, the polar
sides of which line the channel, whereas the hydrophobic sides interact with the
membrane lipids. More advanced algorithms are used to detect these sequences, since
such helices would not necessarily be revealed by simple hydrophobicity analysis.8.4.4 Glycoproteins
Glycoproteins result from the covalent attachment of carbohydrate chains (glycans),
both linear and branched in structure, to various sites on the polypeptide backbone of
a protein. These post-translational modifications are carried out by cytoplasmic
enzymes within the endoplasmic reticulum and Golgi apparatus. The amount of
polysaccharide attached to a given glycoprotein can vary enormously, from as little
as a few per cent to more than 60% by weight. Glycoproteins tend to be found in the
serum and in cell membranes. The precise role played by the carbohydrate moiety of
glycoproteins includes stabilisation of the protein structure, protection of the protein
from degradation by proteases, control of protein half-life in blood, the physical
maintenance of tissue structure and integrity, a role in cellular adhesion and cell–cell
interaction, and as an important determinant in receptor–ligand binding.
The major types of protein glycoconjugates are:- N-linked;
- O-linked;
- glycosylphosphatidylinositol (GPI)-linked.
N-linked glycans are always linked to an asparagine residue side-chain (Fig. 8.3) at a
consensus sequence Asn-X-Ser/Thr where X is any amino acid except proline.
O-linked glycosylation occurs where carbohydrate is attached to the hydroxyl group
of a serine or threonine residue (Fig. 8.3). However, there is no consensus sequence
similar to that found for N-linked oligosaccharides. GPI membrane anchors are a more
recently discovered modification of proteins. They are complex glycophospholipids
that are covalently attached to a variety of externally expressed plasma membrane
proteins. The role of this anchor is to provide a stable association of protein with the
membrane lipid bilayer, and will not be discussed further here.
There is considerable interest in the determination of the structure of O- and
N-linked oligosaccharides, since glycosylation can affect both the half-life and func-
tion of a protein. This is particularly important of course when producing therapeutic
glycoproteins by recombinant methods as it is necessary to ensure that the correct
carbohydrate structure is produced. It should be noted that prokaryotic cells do not
produce glycoproteins, so cloned genes for glycoproteins need to be expressed in
eukaryotic cells. The glycosylation of proteins is a complex subject. From one glyco-
protein to another there are variations in the sites of glycosylation (e.g. only about334 Protein structure, purification, characterisation and function analysis