Catalyzes the formation of the isocyclic ring in chlorophyll biosynthesis. 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.