Protein structure - Pharmaceutical Biotechnology: Concepts and Aapplications - page 36

CH 2

CH 2
CH 2

CH

CH 2 HO CH

COO -

COO -

-

COO

- -

COO COO

γ -Carboxylation
(Glu Gla)

β -hydroxylation
(Asp Hya)

CH 2

HO CH

C

C

H 2 N O H 2 N O

β -hydroxylation
(Asn Hyn)

Figure 2.11 γ -Carboxylation of glutamate residues (Glu) yields γ -carboxyglutamate (Gla), whereas β -hy-
droxylation of aspartate (Asp) yields β -hydroxyaspartate (Hya) and β -hydroxylation of asparagine (Asn)
yields β -hydroxyasparagine (Hyn)

2.5.3 Sulfation and amidation

Sulfation and amidation are two additional PTMs characteristic of a small number of biopharma-
ceuticals. Sulfation entails the enzyme-catalysed attachment of sulfate (SO 4 2 ) groups to target
polypeptides, usually via specifi c tyrosine side chains. Sulfation often plays a role in protein-pro-
tein interactions, and lack of sulfation tends to reduce a polypeptide's activity, as opposed to abol-
ishing it completely. No table therapeutic proteins that are sulfated in their natural state include the
anticoagulant hirudin (Chapter 12) and blood factors VIII and IX. The recombinant forms of these
proteins produced by genetic engineering generally have reduced levels/absence of sulfation, but
yet they remain therapeutically effective.
Amidation refers to the replacement of a protein's C-terminal carboxyl group with an amide
group (COOH

CONH 2 ). This PTM is usually characteristic of peptides (very short chains of
amino acids), as opposed to the longer polypeptides, but one therapeutic polypeptide (salmon
calcitonin, Chapter 11) is amidated, and amidation is required for full functional activity. Overall,
the function(s) of amidation is not well understood, although in some cases at least it appears to
contribute to peptide/polypeptide stability and/or activity.

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