The function of a protein can only be understood in terms of its structure. The three dimensional structures of many proteins have been determined and from these structures a few general principles can be derived.
Every protein has a unique amino acid sequence.
Tertiary Structure is the three dimensional structure of the entire polypeptide, including conformations of side chains. The two polypeptide chains are joined by two interchain disulfide bonds - the A chain also contains an intrachain disulfide bond. Determining the amino acid sequence of a protein used to be a very laborious and time consuming process involving chemical and enzymatic degradation. Today, the amino acid sequence of proteins is usually determined from the nucleotide sequence of the gene - a relatively simple and rapid process. The amino acid sequence of the same protein from many sources, e.g., cytochrome c, shows that some amino acid residues are conserved among all the proteins, whereas others are not conserved.