I heard that chlorophyll is of a similar structure to the human haemoglobin, only that the iron is replaced by magnesium instead.
Heme is made of four pyrrole groups united to form a porphyrin ring.
In the centre of the prophyrin ring is an atom of iron linked covalently to the nitrogens of the four pyrrole groups. A molecule of chlorophyll has four pyrrole groups united to form a porphyrin ring as in haemoglobin. The prophyrin ring contains a phytol ester and a cyclopentanone ring is added to one pyrolle residue. Instead of the atom of iron in the molecule of haemoglobin, chlorophyll contains magnesium. So it cannot substitute for heme.