1.3 Protein structures have conventionally been understood at four different levels 1.3.1 The primary structure is the amino acid sequence(including the locations of disulfide bonds). 1.3.2 The secondary structure refers to the regular recurring arrangements of adjacent residues resulting mainly from hydrogen bonding between backbone groups, with a-helices and b-pleated sheets as the two most common ones 1.3.3 The tertiary structure refers to the spatial relationship among all amino acid residues in a polypeptide chain, that is, the complete three dimensional structure 1.3.4 The quaternary structure refers to the spatial arrangements of each subunit in a multisubunit protein, including nature of their contact.1.3 Protein structures have conventionally been understood at four different levels. 1.3.1 The primary structure is the amino acid sequence (including the locations of disulfide bonds). 1.3.2 The secondary structure refers to the regular, recurring arrangements of adjacent residues resulting mainly from hydrogen bonding between backbone groups, with a-helices and b-pleated sheets as the two most common ones. 1.3.3 The tertiary structure refers to the spatial relationship among all amino acid residues in a polypeptide chain, that is, the complete three￾dimensional structure. 1.3.4 The quaternary structure refers to the spatial arrangements of each subunit in a multisubunit protein, including nature of their contact