1.3.1.33 Fe2+ contains two 4Fe-4S clusters 686758 1.3.1.33 Iron enzyme component L-protein contains an oxygen-sensitive 4Fe-4S cluster similar to nitrogenase Fe protein. Activity quickly disappears upon exposure to air with a half-life of 20 s 673673 1.3.1.33 Mg2+ removal of Mg2+ from the protochlorophyllide leads to inactivity of the compound as a substrate, activity can be restored by replacing the magnesium with zinc, whereas nickel, copper or cobalt fail to restore substrate activity 393834, 393859 1.3.1.33 Zn2+ removal of Mg2+ from the protochlorophyllide leads to inactivity of the compound as a substrate, activity can be restored by replacing the magnesium with zinc, whereas nickel, copper or cobalt fail to restore substrate activity 393834, 393859 1.3.1.33 Mg2+ required for activity 686758 1.3.1.33 Ca2+ specifically stimulated by Ca2+ 393849