▁hom oc yst e ine ▁and ▁ser ine ▁to ▁form ▁the ▁asym met rical ▁th io ether ▁c yst ath ion ine . ▁ ▁The ▁en zym e ▁c yst ath ion ine ▁g amma - ly ase ▁converts ▁the ▁c yst ath ion ine ▁into ▁c yst e ine ▁and ▁alpha - ket ob ut yr ate . ▁ ▁In ▁plants ▁and ▁b acter ia , ▁c yst e ine ▁b ios yn thesis ▁also ▁starts ▁from ▁ser ine , ▁which ▁is ▁converted ▁ ▁to ▁O - ac et yl ser ine ▁by ▁the ▁en zym e ▁ser ine ▁trans ac et yl ase . ▁The ▁en zym e ▁c yst e ine ▁synth ase , ▁using ▁sul f ide ▁sources , ▁converts ▁this ▁est er ▁into ▁c yst e ine , ▁re le asing ▁ac et ate . ▁ ▁Bi ological ▁functions ▁The ▁c yst e ine ▁sul f h yd ry l ▁group ▁is ▁nucle oph il ic ▁and ▁easily ▁ox id ized . ▁The ▁react ivity ▁is ▁en hanced ▁when ▁the ▁th i ol ▁is ▁ion ized , ▁and ▁c yst e ine ▁resid ues ▁in ▁prote ins ▁have ▁p K a ▁values ▁close ▁to ▁neutral ity , ▁so ▁are ▁often ▁in ▁their ▁re active ▁th i olate ▁form ▁in ▁the ▁cell . ▁ ▁Because ▁of ▁its ▁high ▁react ivity , ▁the ▁sul f h yd ry l ▁group ▁of ▁c yst e ine ▁has ▁numerous ▁bi ological ▁functions , ▁and ▁c yst e ine ▁may ▁have ▁played ▁an