ams , ▁the ▁variety ▁of ▁ce phal os por ins ▁and ▁ce ph am yc ins ▁come ▁from ▁different ▁trans am id ations , ▁as ▁is ▁the ▁case ▁for ▁the ▁pen ic ill ins . ▁ ▁While ▁the ▁ring ▁closure ▁in ▁pen ams ▁and ▁ce ph ems ▁is ▁between ▁positions ▁ 1 ▁and ▁ 4 ▁of ▁the ▁ β - la ct am ▁and ▁is ▁ox id ative , ▁the ▁cla v ams ▁and ▁car b ap en ems ▁have ▁their ▁rings ▁closed ▁by ▁two - elect ron ▁processes ▁between ▁positions ▁ 1 ▁and ▁ 2 ▁of ▁the ▁ring . ▁ β - la ct am ▁synth et ases ▁are ▁responsible ▁for ▁these ▁cycl izations , ▁and ▁the ▁car box yl ate ▁of ▁the ▁open - ring ▁substr ates ▁is ▁activ ated ▁by ▁ATP . ▁In ▁cla v ams , ▁the ▁ β - la ct am ▁is ▁formed ▁prior ▁to ▁the ▁second ▁ring ; ▁in ▁car b ap en ems , ▁the ▁ β - la ct am ▁ring ▁is ▁closed ▁second ▁in ▁sequence . ▁ ▁The ▁b ios yn thesis ▁of ▁the ▁ β - la ct am ▁ring ▁of ▁tab to x in ▁mirror s ▁that ▁of ▁the ▁cla v ams ▁and ▁car b ap en ems . ▁The ▁closure ▁of ▁the ▁la ct am ▁ring ▁in ▁the ▁other ▁mon ob act ams , ▁such ▁as ▁sul f az ec in ▁and ▁the ▁n oc ard ic ins , ▁may ▁involve ▁a ▁third ▁mechanism ▁involving ▁in version ▁of ▁configuration ▁at