Chemistry Reference
In-Depth Information
The charge properties of a protein will be determined by the five amino acids with potentially charged R-
groups. At pH 7, Glu and Asp will have a negative charge, Lys and Arg a positive charge, while His residues will
be about 10% positively charged.
5
These polar, charged residues, Asp, Glu, Lys, Arg, and the protonated form of
His, will often be found at the surface of proteins, where they may not only interact with the polar layers of ordered
water molecules surrounding the protein, but may also participate in hydrogen bonds and salt bridges with other
polar/charged residues.
PRIMARY, SECONDARY, TERTIARY, AND QUATERNARY STRUCTURE OF PROTEINS
We can distinguish several levels of structural organisation in proteins. These are usually described as
primary, secondary, tertiary, and quaternary, as shown in the well-known illustration of Irving Geis
(
Figure 3.6
). The primary structure is quite simply the linear amino acid sequence of the polypeptide chain.
(a)
(b)
(c)
(d)
FIGURE 3.6
Structural organisation of proteins. (From
Voet & Voet, 2004
.
Reproduced with permission from John Wiley & Sons, Inc.)
We know, from the classic experiments of Christian B. Anfinsen (
Anfinsen, 1973
)
, that the amino acid
sequence inherently contains all the information required for the overall three-dimensional structure of the
protein (but, we still do not yet know how to predict the latter accurately from the former). The secondary
5. Assuming that the pKs are not influenced by their environment in the protein (which they often are).
