Denaturation, in biology; process modifying the molecular structure of a protein. Denaturation involves the breaking of many of the weak linkages, or bonds (e.g., hydrogen bonds), within a protein molecule that are responsible for the highly ordered structure of the protein in its natural (native) state. Denatured proteins have a looser, more random structure; most are insoluble. Denaturation can be brought about in various ways—e.g., by heating, by treatment with alkali, acid, urea, or detergents, and by vigorous shaking.
source: http://www.britannica.com/science/denaturation

The preservation -tanning- of skin strongly depends on denaturation of the proteins it is build upon. With tanning the aim is to replace the hydrogen bond in the protein with a more solid linkage, which makes sure that the protein structure does not fall apart (=decay). All of this is preferably done without loosing the skin’s natural tactile, supple qualities.