Along with methionine, there is an alternate version of this amino acid that replaces its most notable atom with a selenium atom.
It's not serine, but histidine activates this amino acid in the catalytic centers of interleukin-1-beta-converting enzyme and papain. In E. coli, this amino acid is synthesized using the enzymes serine acetyltransferase and O-acetylserine lyase. Two residues of this amino acid and two histidine residues coordinate a zinc atom in the most common type of zinc finger. Human (*) keratin's structural rigidity comes from the fact that it contains this amino acid in over 10% abundance. Along with methionine, there is an alternate version of this amino acid that replaces its most notable atom with a selenium atom. PDI catalyzes the formation of bonds between two residues of this amino acid inside the endoplasmic reticulum. For 10 points, name this amino acid whose thiol group allows it to form disulfide bridges.
It's not serine, but histidine activates this amino acid in the catalytic centers of interleukin-1-beta-converting enzyme and papain.
cysteine [prompt on "Cys" or "C"]