It is the specific sequence of amino acids that determines the structure and properties of a protein. Each protein has a unique amino acid sequence. The 20-odd amino acids are arranged in a definite order in a protein, which may have hundreds of amino acid residues, just as the 26 letters of the alphabet can form many words and sentences. Proteins with similar structures may have entirely different physiological action if they differ in just one or a few amino acid residues. Most long polypeptide chains are folded or coiled in a number of ways. This brings about a second level of organisation called the secondary structure. The most common form of coiling is the right handed alpha helix. Only the right-handed alpha helix exists in nature.