Cysteine

Cysteine
is nonessential and glucogenic
– nonessential – synthesis from serine (methionine supplies the sulfur atom)
– glucogenic – cysteine is converted into pyruvate (sulfur atom is released as SO32–, HS–, or SCN–)


Oxidation of S-II to S III or SIV (sulfinate, sulfite, sulfate) is a proton-producing process, nonvolatile acids are formed from non-ionized groups. The catabolism of sulfur-containing amino acids slightly acidifies the body.

Sulfite anion is oxidized to sulfate anion, which is either excreted into the urine (approx. 20 – 30 mmol/d) or utilized for sulfations after activation:


3‘-Phosphoadenosyl-5‘-phosphosulfate (PAPS)
is the mixed anhydride of sulfuric and phosphoric acid called "active sulfate"; it serves as the sulfate donor in forming of sulfate esters (or N-sulfates).

Examples of sulfations by means of PAPS:
synthesis of proteoglycans (sulfation of glycosaminoglycans), sulfation of saccharidic components in glycolipids and glycoproteins, formation of sulfate esters in inactivation of steroid hormones, catecholamines and in the phase II of biotransformation of phenols.