▁converted ▁to ▁al an ine ▁in ▁the ▁process . ▁ ▁Metal ▁ion ▁binding ▁Bey ond ▁the ▁iron - s ulf ur ▁prote ins , ▁many ▁other ▁metal ▁c of act ors ▁in ▁en zym es ▁are ▁bound ▁to ▁the ▁th i olate ▁subst itu ent ▁of ▁c yst ein yl ▁resid ues . ▁ ▁Ex amples ▁include ▁z inc ▁in ▁z inc ▁fingers ▁and ▁alco hol ▁de h ydro gen ase , ▁co pper ▁in ▁the ▁blue ▁co pper ▁prote ins , ▁iron ▁in ▁cy to chrome ▁P 4 5 0 , ▁and ▁nick el ▁in ▁the ▁[ N i Fe ] - h ydro gen ases . ▁The ▁sul f h yd ry l ▁group ▁also ▁has ▁a ▁high ▁aff inity ▁for ▁heavy ▁met als , ▁so ▁that ▁prote ins ▁ ▁containing ▁c yst e ine , ▁such ▁as ▁met all oth ione in , ▁will ▁bind ▁met als ▁such ▁as ▁mer cur y , ▁lead , ▁and ▁cad m ium ▁tight ly . ▁ ▁R oles ▁in ▁protein ▁structure ▁In ▁the ▁translation ▁of ▁mess enger ▁R NA ▁mole cules ▁to ▁produce ▁pol ype pt ides , ▁c yst e ine ▁is ▁c oded ▁for ▁by ▁the ▁U G U ▁and ▁U GC ▁cod ons . ▁ ▁C yst e ine ▁has ▁tradition ally ▁been ▁considered ▁to ▁be ▁a ▁hydro ph il ic ▁am ino ▁acid , ▁based ▁largely ▁on ▁the ▁chemical ▁parallel ▁between ▁its ▁sul f h yd ry l ▁group ▁and ▁the ▁hydro x yl ▁groups ▁in ▁the ▁side ▁ch