Catalyzes the formation of the isocyclic ring in chlorophyll biosynthesis in aerobic conditions. Mediates the cyclase reaction, which results in the formation of divinylprotochlorophyllide (Pchlide) characteristic of all chlorophylls from magnesium-protoporphyrin IX 13-monomethyl ester (MgPMME) (By similarity). 2 H(+) + Mg-protoporphyrin IX 13-monomethyl ester + 3 NADPH + 3 O2 = 3,8-divinyl protochlorophyllide a + 5 H2O + 3 NADP(+) Porphyrin-containing compound metabolism; chlorophyll biosynthesis. Belongs to the AcsF family.