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 sulfhydryl 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 R
In proteins that contain more than one disulfide bond, proper pairing of the cysteine residues is important for normal structure and activity.
Disulfide 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.
In eukaryotic cells, disulfide bonds are formed in the lumen of the RER (rough endoplasmic reticulum) but not in the cytosol. Thus disulfide bonds are found only in secretory proteins, lysosomal proteins, and the exoplasmic domains of membrane proteins.
Hair is a biological polymer, with over 90% of its dry weight made of proteins called keratins. Under normal conditions, human hair contains around 10% water, which modifies its mechanical properties considerably. Hair proteins are held together by disulfide bonds, from the amino acid cysteine. These links are very robust: for example, virtually intact hair has been recovered from ancient Egyptian tombs. Different parts of the hair have different cysteine levels, leading to harder or softer material. Breaking and making disulfide bonds governs the phenomenon of wavy or frizzy hair.