▁protein ▁export ▁is ▁a ▁cost ly ▁process , ▁minim izing ▁its ▁necessity ▁is ▁advantage ous ). ▁ ▁Inside ▁the ▁cell , ▁dis ulf ide ▁brid ges ▁between ▁c yst e ine ▁resid ues ▁within ▁a ▁pol ype pt ide ▁support ▁the ▁protein ' s ▁t ert i ary ▁structure . ▁ ▁Ins ul in ▁is ▁an ▁example ▁of ▁a ▁protein ▁with ▁c yst ine ▁cross link ing , ▁where in ▁two ▁separate ▁pe pt ide ▁ch ains ▁are ▁connected ▁by ▁a ▁pair ▁of ▁dis ulf ide ▁b onds . ▁ ▁Prote in ▁dis ulf ide ▁is omer ases ▁catal y ze ▁the ▁proper ▁formation ▁of ▁dis ulf ide ▁b onds ; ▁the ▁cell ▁trans fers ▁de h ydro as cor b ic ▁acid ▁to ▁the ▁end op las mic ▁ret icul um , ▁which ▁ox id izes ▁the ▁environment . ▁ ▁In ▁this ▁environment , ▁c yst e ines ▁are , ▁in ▁general , ▁ox id ized ▁to ▁c yst ine ▁and ▁are ▁no ▁longer ▁functional ▁as ▁a ▁nucle oph iles . ▁ ▁As ide ▁from ▁its ▁ox id ation ▁to ▁c yst ine , ▁c yst e ine ▁particip ates ▁in ▁numerous ▁post - trans l ational ▁modifications . ▁ ▁The ▁nucle oph il ic ▁sul f h yd ry l ▁group ▁allows ▁c yst e ine ▁to ▁conjug ate ▁to ▁other ▁groups , ▁e . g ., ▁in ▁pr en yl ation . ▁U bi quit in ▁lig ases ▁transfer ▁u bi quit in ▁to ▁its ▁pendant , ▁prote ins ,