, ▁c yst e ine ▁is ▁now ▁often ▁grouped ▁among ▁the ▁hydro ph ob ic ▁am ino ▁ac ids , ▁though ▁it ▁is ▁sometimes ▁also ▁class ified ▁as ▁slightly ▁polar , ▁or ▁polar . ▁ ▁While ▁free ▁c yst e ine ▁resid ues ▁do ▁occur ▁in ▁prote ins , ▁most ▁are ▁c oval ently ▁bond ed ▁to ▁other ▁c yst e ine ▁resid ues ▁to ▁form ▁dis ulf ide ▁b onds , ▁which ▁play ▁an ▁important ▁role ▁in ▁the ▁fol ding ▁and ▁stability ▁of ▁some ▁prote ins , ▁usually ▁prote ins ▁secret ed ▁to ▁the ▁ext rac ell ular ▁medium . ▁Since ▁most ▁cell ular ▁compart ments ▁are ▁reducing ▁environments , ▁dis ulf ide ▁b onds ▁are ▁generally ▁un stable ▁in ▁the ▁c yt os ol ▁with ▁some ▁exceptions ▁as ▁noted ▁below . ▁ ▁Dis ulf ide ▁b onds ▁in ▁prote ins ▁are ▁formed ▁by ▁ox id ation ▁of ▁the ▁sul f h yd ry l ▁group ▁of ▁c yst e ine ▁resid ues . ▁The ▁other ▁sul fur - cont aining ▁am ino ▁acid , ▁m eth ion ine , ▁cannot ▁form ▁dis ulf ide ▁b onds . ▁ ▁More ▁ag gress ive ▁ox id ants ▁convert ▁c yst e ine ▁to ▁the ▁corresponding ▁sul fin ic ▁acid ▁and ▁sul f onic ▁acid . ▁C yst e ine ▁resid ues ▁play ▁a ▁valuable ▁role ▁by ▁cross link ing ▁prote ins , ▁which ▁increases ▁the ▁rig id ity ▁of ▁prote ins ▁and ▁also ▁functions ▁to ▁confer ▁prote ol yt ic ▁resistance ▁( since