protoporphyrin

n.
A metal-free porphyrin, C₃₂H₃₂N₄(COOH)₂, that combines with ferrous iron to form the heme group in hemoglobin and myoglobin, and with ferric iron to form the hemin group in catalase and some cytochromes.

(prō'tō-pôr'fə-rĭn)
n.

A metal-free porphyrin that combines with iron to form the heme of iron-containing proteins.

A porphyrin whose iron complex united with protein occurs in hemoglobin, myoglobin and certain respiratory pigments.

• p. III — combines with 4 moles of iron to form the heme moiety of hemoglobin.