Other side groups occur when the protein isconjugated, implying that
other groups become covalently bonded to some amino acid residues. This
may concern phosphorylation (mostly of Ser or Thr); glycosylation, which
takes many forms; hydroxylation (mostly of Pro or Lys); as well as
attachment of some other groups. One thus commonly speaks of the
conjugated phosphoproteins and glycoproteins; the latter may contain a
wide variety of glucide groups, from one per molecule to several times 10%
by mass. Metalloproteins contain one or more, bivalent or trivalent, cations,
that are tightly bound, but not by covalent bonds; in other words, they are
not part of the protein, though the biological function of the protein
generally depends on their presence. A still weaker association is involved in
the formation of lipoproteins. Most of these are not proteins but complexes
of several protein molecules and several lipid molecules, held together by
weak forces (van der Waals, hydrophobic, etc.).
The side groups determine chemical reactivity, i.e., the possibility of
forming covalent bonds. Also much of the physicochemical behavior follows
from the nature of the side groups. Some can be involved in hydrogen
bonding, either as a hydrogen donor ( 22 OH and 55 NH) or as an acceptor
( 55 O, 22 O 22 , 55 N 22 and 22 S 22 ). The bulkiness greatly varies, group molar
mass ranging from 1 (Gly) to 130 (Trp).
Some groups can be ionized, i.e., carry anelectric charge. A proton can
dissociate from a carboxyl group above a certain pH, giving a negative
charge; and several other groups become protonated below a certain pH,
giving positive charges. Table 7.1 gives pK values (pK¼pH of 50%
dissociation; see Section 2.3.1). Some ionizable groups can be added to those
in Table 7.1. Several proteins are glycosylated, and some of these glucides
contain carboxyl groups. Other proteins are phosphorylated, especially the
caseins, which contain phosphoserine residues (R is 22 CH 2 O 22 PO 3 H 2 ).
Two protons can dissociate from a phospho group, giving pKvalues of
about 1.5 and 6.5.
Proteins are thus polyelectrolytes; see Section 6.3.1 for a general
discussion, especially on titration curves (Figure 6.7). It may be clear from
that discussion that a titration curve cannot be obtained by merely adding
the titrations of the separate ionizable groups. This is the more so for most
proteins, where the ionizable groups often are quite close to each other, and
where adjacent groups can be of the same or of opposite charge; the pKof a
group can readily be shifted by a full unit. This also means that the
isoelectricpH can vary with conditions, e.g., ionic strength. Many proteins
used in the food industry have an isoelectric pH not far from 5, implying
that they are negatively charged at neutral pH.
Another important property of side groups is their solvation by water,
because this greatly affects protein conformation (see below) and solubility
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