▁are ▁provided ▁by ▁the ▁α ▁sub unit . ▁The ▁protein ▁lig ands ▁to ▁the ▁iron ▁are ▁the ▁side ch ains ▁of ▁the ▁three ▁c yst e ine ▁( C ys ) ▁resid ues ▁and ▁two ▁main chain ▁am ide ▁nit rog ens . ▁The ▁metal ▁ion ▁is ▁oct ah ed r ally ▁coordin ated , ▁with ▁the ▁protein ▁lig ands ▁at ▁the ▁five ▁vertices ▁of ▁an ▁oct ah ed ron . ▁The ▁sixth ▁position , ▁accessible ▁to ▁the ▁active ▁site ▁c left , ▁is ▁occupied ▁either ▁by ▁NO ▁or ▁by ▁a ▁sol vent - exchange able ▁lig and ▁( h ydro x ide ▁or ▁water ). ▁The ▁two ▁C ys ▁resid ues ▁coordin ated ▁to ▁the ▁metal ▁are ▁post - trans lation ally ▁modified ▁to ▁C ys - s ul fin ic ▁( C ys - SO 2 H ) ▁and ▁- s ul fen ic ▁( C ys - SO H ) ▁ac ids . ▁ ▁Quant um ▁chemical ▁studies ▁predicted ▁that ▁the ▁C ys - SO H ▁resid ue ▁might ▁play ▁a ▁role ▁as ▁either ▁a ▁base ▁( activ ating ▁a ▁nucle oph il ic ▁water ▁mole c ule ) ▁or ▁as ▁a ▁nucle oph ile . ▁Sub sequently , ▁the ▁functional ▁role ▁of ▁the ▁SO H ▁center ▁as ▁nucle oph ile ▁has ▁obtained ▁experimental ▁support . ▁ ▁References ▁ ▁Further ▁reading ▁▁▁▁▁▁▁▁▁▁▁▁▁ ▁Category : Met allo prote ins ▁Category : C ob alt ▁en zym es ▁Category : I ron ▁en zym es ▁Category : EC ▁ 4 . 2