A class of fibrous proteins or scleroproteins that represents the principal constituent of EPIDERMIS; HAIR; NAILS; horny tissues, and the organic matrix of tooth ENAMEL. Two major conformational groups have been characterized, alpha-keratin, whose peptide backbone forms a coiled-coil alpha helical structure consisting of TYPE I KERATIN and a TYPE II KERATIN, and beta-keratin, whose backbone forms a zigzag or pleated sheet structure. alpha-Keratins have been classified into at least 20 subtypes. In addition multiple isoforms of subtypes have been found which may be due to GENE DUPLICATION.

Synonyms: Keratins, Cytokeratin, Protein, Keratin Associated, Keratin, Glycosylated, 0, Keratin, Keratin-Associated Proteins, Associated Protein, Keratin, 68238-35-7, Glycosylated Keratin, Keratin Associated Protein, Keratin Associated Proteins, alpha-Keratin, alpha Keratin