Involved in the tetrapyrrole biosynthetic pathways leading to chlorophyll and bacteriochlorophyll (BChl). Catalyzes the anaerobic formation of the isocyclic ring (E-ring) in Mg-protoporphyrin monomethyl ester (MPE) to yield protochlorophyllide a (PChlide a) via a six-electron oxidation and the formation of an oxo group at position C13 using oxygen from a water molecule. H2O + Mg-protoporphyrin IX 13-monomethyl ester + 3 S-adenosyl-L-methionine = 3,8-divinyl protochlorophyllide a + 3 5'-deoxyadenosine + 4 H(+) + 3 L-methionine Binds 1 [4Fe-4S] cluster. Binds 1 adenosylcobalamin. Porphyrin-containing compound metabolism; bacteriochlorophyll biosynthesis (light-independent). Cells do not exhibit a phenotype under high-aeration respiratory growth conditions. In contrast, under low-aerated respiratory growth conditions, the absence of BchE leads to an accumulation of MgPMME. Belongs to the BchE family.