This enzyme hydrolyses the ring A acetate ¦Ä-lactone of cobalt-precorrin-5A resulting in the loss of the C-20 carbon and its attached methyl group in the form of acetaldehyde. This is a key reaction in the contraction of the porphyrin-type tetrapyrrole ring and its conversion to a corrin ring in the anaerobic (early cobalt insertion) adenosylcobalamin biosynthesis pathway.
