Catalyzes the formation of the isocyclic ring in chlorophyll biosynthesis under oxygen- and copper-deficient 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). 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. Induced in absence of copper and oxygen. Regulated by CRR1 protein, which activates its transcription in absence of copper. Belongs to the AcsF family.