Protein

Proteins are amino acids joined by peptide bonds. The main proteins of interest in tanning are structural proteins:

keratin The main constituent of the epidermis, a very thin outer layer of skin which is removed before tanning, along with the hair which is also largely composed of keratin.
collagen A fibrous protein which is the main constituent of skin and the part which is turned into leather by tanning. The collagen in skin forms a complex interwoven structure of fibres which give leather its strength and flexability.
elastin

A fibrous protein with high elasticity which is present in the skin in much smaller quantities than collagen. Concentrated in the grain layer of skin, where it can comprise 3% of the protein mass, elastin contributes greatly to the elasticity of skin.