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8 Protein structure, purification,

characterisation and function

analysis

J. WALKER

8.1 Ionic properties of amino acids and proteins

8.2 Protein structure

8.3 Protein purification

8.4 Protein structure determination

8.5 Proteomics and protein function

8.6 Suggestions for further reading

8.1 IONIC PROPERTIES OF AMINO ACIDS AND PROTEINS

Twenty amino acids varying in size, shape, charge and chemical reactivity are found

in proteins and each has at least one codon in the genetic code (Section 5.3.5).

Nineteen of the amino acids area-amino acids (i.e. the amino and carboxyl groups

are attached to the carbon atom that is adjacent to the carboxyl group) with the

general formula RCH(NH 2 )COOH, where R is an aliphatic, aromatic or heterocyclic

group. The only exception to this general formula is proline, which is an imino acid in

which the -NH 2 group is incorporated into a five-membered ring. With the exception

of the simplest amino acid glycine (R¼H), all the amino acids found in proteins

contain one asymmetric carbon atom and hence are optically active and have been

found to have theLconfiguration.

For convenience, each amino acid found in proteins is designated by either a three-

letter abbreviation, generally based on the first three letters of their name, or a

one-letter symbol, some of which are the first letter of the name. Details are given

in Table 8.1.

Since they possess both an amino group and a carboxyl group, amino acids are

ionised at all pH values, i.e. a neutral species represented by the general formula does

not exist in solution irrespective of the pH. This can be seen as follows:

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