6 7 ▁people ▁were ▁liter ate . ▁ ▁References ▁ ▁Category : V ill ages ▁in ▁P ud uk k otta i ▁district <0x0A> </s> ▁C yst e ine ▁( symbol ▁C ys ▁or ▁C ; ▁) ▁is ▁a ▁semi ess ential ▁protein ogen ic ▁am ino ▁acid ▁with ▁the ▁formula ▁HO 2 C CH ( N H 2 ) CH 2 SH . ▁The ▁th i ol ▁side ▁chain ▁in ▁c yst e ine ▁often ▁particip ates ▁in ▁en zym atic ▁re actions , ▁as ▁a ▁nucle oph ile . ▁ ▁The ▁th i ol ▁is ▁sus cept ible ▁to ▁ox id ation ▁to ▁give ▁the ▁dis ulf ide ▁derivative ▁c yst ine , ▁which ▁serves ▁an ▁important ▁struct ural ▁role ▁in ▁many ▁prote ins . ▁When ▁used ▁as ▁a ▁food ▁add itive , ▁it ▁has ▁the ▁E ▁number ▁E 9 2 0 . ▁ ▁It ▁is ▁encoded ▁by ▁the ▁cod ons ▁U G U ▁and ▁U GC . ▁ ▁C yst e ine ▁has ▁the ▁same ▁structure ▁as ▁ser ine , ▁but ▁with ▁one ▁of ▁its ▁o xygen ▁atoms ▁replaced ▁by ▁sul fur ; ▁replacing ▁it ▁with ▁selenium ▁gives ▁s elen oc yst e ine . ▁Like ▁other ▁natural ▁protein ogen ic ▁am ino ▁ac ids , ▁c yst e ine ▁has ▁ ▁ch ir ality ▁in ▁the ▁older ▁/ ▁notation ▁based ▁on ▁hom ology ▁to ▁- ▁and ▁- gly cer al de hy de . ▁ ▁In ▁the ▁newer ▁R / S ▁system ▁of ▁design ating ▁ch ir ality ,