Answer:
Keratin is a protein.
Step-by-step explanation:
Keratin is a fibrous protein that has the characteristics of rigid conformation, elasticity and impermeability to water. It has animal origin and is found in the vertebrate epidermis and in some epidermal appendages, such as hair, nails, hooves, feathers and scales. This protein is produced in cells called keratinocytes, which are the most abundant in the human epidermis.
Like any protein, keratin is made up of amino acids. It has a large amount of cysteine residues, which is responsible for 7% to 20% of the total amino acids present in keratin. Note that cysteine is mainly located in the terminal region of the keratin chains.