When two amino acids bond to each other through their side chains, they normally do so through a disulfide bond. The particular side chain involved is the thiol group (-SH). Oxidation of the thiol group yields a disulfide (S-S) bond. The presence of S-S then helps to maintain the tertiary structure of the protein. An amino acid that commonly forms S-S bonds in proteins in cysteine. When two cysteines are bonded by an S-S bond, the resulting molecule between the two protein chains is called cystine. The figure below shows the formation of a disulfide bond. The R on each side group represents the remainder of the amino acid.
R R | | SH S oxidation-> | + 2H SH S | | R RDisulfide bonds play an imporant protective role for bacteria as a reversible switch that turns a protein on or off when bacterial cells are exposed to oxidation reactions. Hydrogen peroxide (H2O2) in particular can severely damage DNA and kill the bacterium at low concentrations if it weren't for the protective action of the SS-bond.