hydrophobic side chains at positions 2 and 4 and vicinal propionic acid residues at positions 6 and 7 of the porphyrin ring ( protoporphyrin ix , mesoporphyrin ix ) were essential for maximal enzyme activation ( ka = 7-8 nm ; vmax = 6-8 mumol of cgmp/min/mg ) .
C0032712
porphyrin
C0033733
protoporphyrin ix