4. Amino acid serine and sulfur fixation.

Serine family amino acids include 3 amino acids: serine, glycine and cysteine.
The biosynthesis of cysteine involve sulfur fixation in the biological world. 
  1. Direct sulfhydrylation: addition of sulfur group to an acetylayted (activated form of) serine. 
  2. Trans-sulfuration route: Homocysteine is the intermediate which then serves as substrate for trans-sulfuration reaction.
Sulfate (SO42 -) was reduced to sulfide (H2S) before incorporation into amino acids. The reduction reaction uses 3'-phosphoadenosine-5' phosphosulfate (PAPS) as intermediate.

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