in chlorophyll biosynthesis converts -(CH2)2-COO-CH3 side chain of Mg protoporphyrin monomethyl ester to -C=O-CH-COOCH3 ring of Mg 2,4-divinyl phaeoporphyrin a(5) monomethyl ester; requires NADPH & molecular oxygen; similar to cytochrome P-450 catalyzed systems but not inhibited by CO