Histidine is H. Spell "hello" in amino acids molecular beads. Did you know that this amino acid is used as a precursor to the biosynthesis of histamine? Histamine is the molecule that makes us itchy. Also, in a histidine proton shuttle, it is used to quickly shuttle protons. Histidine can do this by abstracting a proton with its basic nitrogen to make a positively-charged intermediate and then use another molecule, a buffer, to extract the proton from its acidic nitrogen.