12,18-didecarboxysiroheme + 2 AH2 + 2 S-adenosyl-L-methionine = Fe-coproporphyrin III + 2 5'-deoxyadenosine + 2 L-methionine + 2 acetate + 2 A + 2 H(+)
12,18-didecarboxysiroheme + 2 ah2 + 2 s-adenosyl-l-methionine = fe-coproporphyrin iii + 2 5'-deoxyadenosine + 2 l-methionine + 2 acetate + 2 a + 2 h(+)