Primary structure

Introduction: By peptide bonds the linear sequence of amino acids joined together which is termed the primary structure of the protein. Position of covalent disul?de bonds between cysteine residues is included in the primary structure also.

In a protein Primary level of structure is the linear sequence of amino acids as by peptide   bonds joined together.  By the sequence of nucleotide bases this sequence   is determined   in the gene encoding the protein .Also  included  under  primary  structure  is  the  location  of  any  other  covalent bonds. These are chiefly  di-sulfide  bonds between  cysteine residues  that can be adjacent  in  space  but  not  in  the  linear  amino  acid  sequence.  These  covalent cross-links  between  different  parts  of the  same  chain or between  separate  polypeptide  chains  are  composed on  cysteine residues by  the  oxidation  of  the  SH  groups  that are juxtaposed  in space .The resulting  di-sulfide  is known a cystine residue.  Disul?de bonds are frequently present in extracellular proteins, but found rarely in intracellular proteins. Some proteins, like collagen, contain covalent cross-links composed between the side-chains of Lys residues .

                             Figure:Creation of a disul?de bond between 2 cysteine residues, producing a cystine residue.