Sulfur in the body exists primarily as a constituent of
cysteine and methionine, although it also exists in smaller amounts in various
inorganic sulfate compounds.
Sulfate, which contains sulfur, is an important
constituent of the compound serotonin; serotonin acts as a vasoconstrictor and
aids in blood coagulation.
Disulfide bridges, or sulfur to sulfur bonding of two
cysteine residues, function to link together the various peptide constituents
of all immunoglobulin molecules. Disulfide bridges also aid in the maintenance
of structural rigidity in keratin protein. Keratin is composed of many peptide
chains, which are linked together by hydrogen and disulfide bonds. Skin is made
of a "softer" keratin which contains less disulfide bonds and therefore, less
structural rigidity. "Hard" keratin, as is found in hair and nails, has a
greater amount of structural rigidity.
Disulfide bonds are essential in maintaining the three
dimensional structures of proteins and enzymes. Enzymatic activity, in many
enzymes, depends upon the protein structure. For example, pancreatic
ribonuclease has four disulfide bonds within its protein structure and does not
function efficiently in the absence of malformation of these bonds.
Sulfur is a constituent of the molecule coenzyme A (CoA),
which is involved in a wide variety of metabolic reactions. CoA plays a role in
glycolysis, metabolism of fatty acids, degradation of various amino acids, and
the formation of the neurotransmitter acetylcholine, as well as numerous other