BLBS102-c07 BLBS102-Simpson March 21, 2012 11:12 Trim: 276mm X 219mm Printer Name: Yet to Come
7 Biocatalysis, Enzyme Engineering and Biotechnology 129(A)(B)Figure 7.2.Representation ofα-helix andβ-sheet. Hydrogen bonds are depicted by doted lines.network of water molecules. Apart from the peptide bond, the
only other type of covalent bond involved in linking amino-
acids is the disulphide (–S–S–) bond, which can be formed
between two cysteine side-chains under oxidising conditions.
The disulphide bond contributes significantly to the structural
stability of an enzyme and more precisely in tertiary structural
stabilisation (see below for details; Matthews 1993, Estape et al.
1998).Detailed studies have shown that certain combinations ofα-
helices orβ-sheet together with turns occur in many proteins.
These often-occurring structures have been termedmotifs,or
super-secondary structure (Kabsch and Sander 1983, Adamson
et al. 1993). Examples of motifs found in several enzymes are
shown in Figure 7.3. These protein folds represent highly stable
structural units, and it is believed that they may form nucleating
centres in protein folding (Richardson 1981).