el at ases ▁are ▁en zym es ▁that ▁catal y ze ▁the ▁insert ion ▁(" met al ation ") ▁of ▁naturally ▁occurr ing ▁t etra py r ro les . ▁ ▁Many ▁t etra py r role - based ▁c of act ors ▁exist ▁in ▁nature ▁including ▁hem es , ▁ch lor oph yll s , ▁and ▁vit amin ▁B 1 2 . ▁ ▁These ▁metal lo ▁c of act ors ▁are ▁derived ▁by ▁the ▁reaction ▁of ▁metal ▁c ations ▁with ▁t etra py r ro les , ▁which ▁are ▁not ▁lig ands ▁per ▁se , ▁but ▁the ▁conjug ate ▁ac ids ▁there of . ▁ ▁In ▁the ▁case ▁of ▁ferro ch el at ases , ▁the ▁reaction ▁that ▁ch el at ases ▁catal y ze ▁is : ▁Fe 2 + ▁ ▁+ ▁ ▁H 2 P ▁▁ ▁→ ▁▁ ▁Fe P ▁ ▁+ ▁▁ 2 ▁H + ▁In ▁the ▁above ▁equation ▁H 2 P ▁represents ▁a ▁si ro h ydro ch lor in ▁or ▁a ▁por ph yr in , ▁such ▁as ▁pro top orph yr in ▁IX . ▁ ▁Ch el at ases ▁are ▁required ▁because ▁por ph yr ins ▁and ▁related ▁macro cyc lic ▁lig ands ▁are ▁extremely ▁slow ▁to ▁metal ate , ▁despite ▁favor able ▁therm od ynam ics . ▁ ▁These ▁low ▁rates ▁are ▁attributed ▁to ▁the ▁tight ▁fit ▁of ▁the ▁metal ▁into ▁the ▁rig id ▁ 1 8 - ▁or ▁ 1 7 - member ed ▁t etra py r role ▁macro cycle . ▁Several ▁families ▁of