Serine is S. String a serine bead in a necklace to spell your name or initials! Serine plays an important role in the catalytic function of many enzymes. It has been shown to occur in the active sites of chymotrypsin, trypsin, and many other enzymes. The so-called nerve gases and many substances used in insecticides have been shown to act by combining with a residue of serine in the active site of acetylcholine esterase, inhibiting the enzyme completely. The unmetabolized acetylcholine cannot be recycled into the nerve for signaling. This results in depletion of acetylcholine at the neuromuscular junction, resulting in the inability to control muscles, which results in asphyxiation, and death.