Which Amino acid has a double sulfide bond is?

ANSWER: (D) CystineREF: Lippincottsbiochemistry 4th Ed page 19A disulfide bond is a covalent linkage formed from the sulfhydryl group (-SH) of each of two cysteine residues, to produce a cystine residue.The two cysteines may be separated from each other by many amino acids in the primary sequence of a polypeptide, or may even be located on two different polypeptide chains; the folding of the polypeptide chain (s) brings the cysteine residues into proximity, and permits covalent bonding of their side chains. A disulfide bond contributes to the stability of thethree-dimensional shape of the protein molecule, and prevents it from becoming denatured in the extracellular environment. Many disulfide bonds are found in proteins such as immunoglobulins that are secreted by cells.