This molecule is iron protoporphyrin IX, from a crystal structure of myoglobin. Porphyrins are common prosthetic groups in many important biological molecules, including hemoglobin, which carries oxygen in the red blood cells of your body; myoglobin, which stores oxygen in muscles; cytochromes, which are involved in electron transport; and peroxidases and catalases, which accomplish a lot of important oxygen chemistry in biological systems.

This graphic was made with Linus!Lite, a freeware molecular graphics program which is the predecessor of MacMolecule2. The porphyrin shown was taken from a structure of myoglobin distributed by the folks who produce MacMolecule2 at the University of Arizona.

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