Sulfur occurs principally in the body as a constituent of the cysteine and methionine. As a constituent of these amino acids it plays an important role in the structural conformation of almost all proteins and enzymes found in the body. It is also important in the structure of antibodies, and the sulfide group of cysteine is important in the activity of various enzymes. The disulfide linkage, which is formed between two cysteine residues, tends to bind up adjacent parallel peptide chains, thereby adding to the structural rigidity of many proteins. It is a significant constituent of thiamine (vitamin B-1) and biotin, and is found in insulin, keratin, and various glycoproteins. It is essential in the molecule chondroitin sulfate in bone and cartilage.