Keratin builds hair, hooves, feathers and whale baleen, and sulfur makes it tough
Your fingernails, a rhino's horn, a hawk's beak and the bristly plates a whale strains krill through all rely on one family of proteins: keratin. That acrid smell when hair burns is the giveaway. Keratin is loaded with sulfur, which ties its strands together much as sulfur hardens vulcanised rubber.
Keratin is a fibrous structural protein that barely dissolves in water or organic solvents. Individual molecules coil around each other into left-handed superhelices, which bundle into tough intermediate filaments. The key ingredient is cysteine, a sulfur-bearing amino acid that forms disulfide bridges between chains; human hair is about 14 percent cysteine. Plenty of glycine, the smallest amino acid, lets the coils pack tightly. Among biological materials, only chitin, the stuff of insect shells, comes close to its toughness.
There are two broad kinds. Softer alpha-keratins occur in all vertebrates, forming mammal hair and wool, skin, nails, claws, hooves and horns, plus the slime threads of hagfish and the baleen of filter-feeding whales. Hair is flexible because it has fewer cross-links than nails and hooves. The harder beta-keratins, built from pleated sheets, belong only to reptiles and birds, making their scales, claws, beaks, feathers and some turtle shells. They differ so much genetically that some researchers now call them corneous beta proteins instead.
In skin, keratin is part of a controlled death. As cells rise through the epidermis, they fill with keratin, lose their nuclei and stop metabolising, leaving a nearly waterproof outer layer that is constantly shed. Friction thickens it into calluses, handy for guitarists. The human genome holds 54 working keratin genes, 28 of type I and 26 of type II, clustered on chromosomes 12 and 17, a sign they arose by repeated gene duplication. Israel Hanukoglu and Elaine Fuchs published the first keratin sequences in the early 1980s.
Toughness has side effects. Keratin resists stomach acid, which is why cats cough up hairballs. Fungi behind athlete's foot and ringworm feed on it. And because different tumours express characteristic keratin types, pathologists use them to trace where a cancer began.
Source: Keratin
