part to there being more bound mobile anions (or cations) than bound cations (anions)
at this pH. This could mask an imbalance of charges on the actual protein.
In practice, protein molecules are always studied in buffered solutions, so it is the
isoelectric point that is important. It is the pH at which, for example, the protein has
minimum solubility, since it is the point at which there is the greatest opportunity for
attraction between oppositely charged groups of neighbouring molecules and conse-
quent aggregation and easy precipitation.
8.2 Protein structure
Proteins are formed by condensing thea-amino group of one amino acid or the imino
group of proline with thea-carboxyl group of another, with the concomitant loss of a
molecule of water and the formation of a peptide bond.
NH 3 CH + CH CH
RR′
R′
R
COO– COO– CO
Peptide
bond
+
NH 3
+
NH 3
–H 2 O
NH CH COO–
+
Table 8.2Ionisable groups found in proteins
Amino acid group pH-dependent ionisation Approx. pKa
N-terminala-amino NH 3 ÐNH 2 þHþ 8.0
C-terminala-carboxyl COOHÐCOOþHþ 3.0
Asp-b-carboxyl CH 2 COOHÐCH 2 COOþHþ 3.9
Glu-g-carboxyl ðCH 2 Þ 2 COOHÐðCH 2 Þ 2 COOþHþ 4.1
His-imidazolyl CH 2
HN+ NH NH
+H+
N
CH 2 6.0
Cys-sulphydryl CH 2 SHÐCH 2 SþHþ 8.4
Tyr-phenolic
OH O–+ H+
10.1
Lys-e-amino ðÞCH 24 NH 3
þ
ÐðÞCH 24 NH 2 þHþ 10.3
Arg-guanidino NHCNH 2 ÐNHCNH 2 þHþ
jj jj
þNH 2 NH
12.5
304 Protein structure, purification, characterisation and function analysis
