The basic building blocks of fish bodies are proteins.
Proteins have structure at several levels.
The primary structure is determined by a sequence of the component amino acids, themselves with a structure determined by their sequence of atoms of carbon, hydrogen, etc. The secondary structure of most protein is a primary coil, similar to a braid. A third-level structure can emerge when the braids fold onto themselves, with various loops weakly connected by hydrogen bonds. It is this tertiary structure which determines the external shape of a protein, e.g. of an enzyme and hence how it will lock into receptors , often other molecules on the surface of cells. As a result, animals must break down such denatured molecules into their constituent parts and re-synthesize them.