Cysteines, sulfhydryl-containing amino acids, which are located an appropriate distance or next to
one another within a polypeptide chain, will form a disulfide bond through their oxidisable thiol
groups. This bond will impart a fold in the chain of the protein or bend in its structure. Disulfide bond
formation and its effect on protein folding has been a subject of great interest for at least half
a century [4]. The first reported study by Anfinsen in 1973 revealed that disulfide bond formation
inside the cell is spontaneous and that the amino acid sequence is sufficient to determine correct
folding of the peptide or protein [4,5]. It was subsequently shown that there are several active
disulfide bond-promoting enzymes and cofactors functioning in the cell [6,7] meaning that disulfide
bonds are usually formed by a systematic network of intracellular enzymes [8]. These enzyme systems
form a new disulfide bond or reshuffle the existing mispaired disulfide bond in substrate peptides
(Figure 1).