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. tsca definition 2008: natural, fibrous proteins occurring in vertebrates. they contain all of the common amino acids and are characterized by high cystine content.
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