Keratin in the nail is exceptionally stronger than others because of

Mammals contin alpha keratin. It is classified into soft and hard keratin depending on the suphur content. The cysteine residues are resposible for disulphide bridge formation which confers characteristic texture for each type of protein. Soft keratin having low sulphur content is present in skin.Hard keratin is present in hair, horn and nails and has high sulphur content.The disulphide bridges resist the forces that try to deform them. REFERENCE : DM VASUDEVAN TEXTBOOK SEVENTH EDITION ; PAGE NO: 39