Protein structure - Pharmaceutical Biotechnology: Concepts and Aapplications - page 32

Table 2.7 Types of PTM that polypeptides may undergo. Refer to text for additional details

Modifi cation Example

Proteolytic processing Various proteins become biologically active only

upon their proteolytic cleavage (e.g. some blood
factors)

Glycosylation For some proteins glycosylation can increase

solubility, infl uence biological half-life and/or
biological activity

Phosphorylation Infl uences/regulates biological activity of various

polypeptide hormones

Acetylation Function unclear

Acylation May help some polypeptides interact with/anchor in

biological membranes

Amidation Infl uences biological activity/stability of some

polypeptides

Sulfation Infl uences biological activity of some neuropeptides

and the proteolytic processing of some polypeptides

Hydroxylation Important to the structural assembly of certain

proteins

γ-Carboxyglutamate formation Important in allowing some blood proteins to bind

calcium

ADP-ribosylation Regulates biological activity of various proteins

Disulfi de bond formation Helps stabilize conformation of some proteins

or by enzymatic degradation of the glycocomponent of preformed glycoproteins using glucosidase
enzymes). However, in other cases the sugar component plays a direct role in the biological activ-
ity of the glycoprotein ( Table 2.8). Native hCG (Chapter 11), for example, is a heavily glycosylated
gonadotrophic hormone. Removal of its sugar components usually abolishes its ability to induce a
biological response, although the hormone's binding affi nity for its receptor remains unaltered, or
is sometimes actually increased. Therapeutic proteins now approved for general medical use that
are glycosylated are listed in Table 2.9.
Carbohydrate side chains are synthesized by a family of enzymes known as glycosyltransferases,
located mainly in the endoplasmic reticulum. Two types of glycosylation can occur: N-linked and
O-linked. In the case of N-linked glycosylation, the sugar chain (the oligosaccharide) is attached
to the protein via the nitrogen atom of an asparagine (Asn) residue, whereas in O-linked systems
the sugar chain is attached to the oxygen atom of hydroxyl groups, usually those of serine or thero-
nine residues ( Figure 2.9). Monosaccharides most commonly found in the sugar side chain(s) in-
clude mannose, galactose, glucose, fucose, N -acetylgalactosamine, N -acetylglucosamine, xylose
and sialic acid. These can be joined together in various sequences and by a variety of glycosidic
linkages. The carbohydrate chemistry of glycoproteins, therefore, is quite complex. The structure
of two such example oligosaccharide chains is presented in Figure 2.10.
N-linked glycosylation is sequence specifi c, involving the transfer of a pre-synthesized oli-
gosaccharide chain to an asn residue found in a characteristic sequence Asn-X-Ser, or Asn-X-
Thr or Asn-X-Cys, where X represents any amino acid residue, with the exception of proline.
An additional glycosylation determinant must also apply, as not all potential N-linked sites are
glycosylated in some proteins. The pre-synthesized oligosaccharide side chain then undergoes