Keratin refers not to one particular substance but to a family of tough, fibrous proteins found in the animal kingdom. Keratin is present in hair, horn, hooves, beaks, shells, fingernails, toenails, and claws. All proteins contain carbon (C), hydrogen (H), nitrogen (N), and oxygen (O). One of the proteins found in keratin is cysteine. Cysteine contains the fifth element, sulfur (S).
The nitrogen in keratin, along with hydrogen, forms alkaline amino groups (-NH2). The oxygen in keratin, along with hydrogen and carbon, forms carboxylic acid groups (-COOH). Acids and bases interact, forming strong chemical bonds.
The keratins are natural polymers made of proteins. Linking strands together imparts strength. The types of links in keratin are peptide linkage (forms strands), and the sulfur-sulfur (or disulfide) linkage (bridge strands).
The simple peptide linkage is illustrated below, using glycine as the example:
(NH?)CH?-COOH + (NH?)CH?-COOH ? (NH?)CH?-CO-(NH)CH?-COOH +H?O
Thus the link itself, the peptide linkage, is: -C-C(O)-NH-C-
Disulfide or Sulfur-Sulfur Linkage
Three-dimensionality is provided by sulfur-sulfur linkage. The chemical structure of cysteine is:
If two cysteine-containing peptides in different, parallel strands of proteins are bonded together through the sulfurs, we get:
The basic linkage is: -C-S-S-C-
View the proteins, joined by peptide linkages, as being two long horizontal chains, separated just slightly. Between the two lengthy horizontal chains, and connecting them together, are the vertical disulfide linkages.
- Photo Credit Image by Flickr.com, courtesy of Jurij Skoblenko
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