keratin

Keratin refers to a family of fibrous structural proteins. Keratin is the key of structural material making up the outer layer of human skin. It is also the key structural component of hair and nails. Keratin monomers assemble into bundles to form intermediate filaments, which are tough and insoluble and form strong unmineralized tissues found in reptiles, birds, amphibians, and mammals. The only other biological matter known to approximate the toughness of keratinized tissue is chitin.

There are two types of the keratin:

• keratin type I - smaller, acidic ( e.x. K9–K23, Ha1–Ha8, Irs1-4)
• keratin type II - bigger, neutral or alkaline (e.x. K1–K8, Hb1–Hb6, K6irs1–4)