Question

Recognize the structure of the heme group and know its purpose in hemoglobin or myoglobin

Answer 1

The heme is a small but important non-protein molecule, or prosthetic group, that is associated with oxygen binding proteins such as hemoglobin and myoglobin. The heme is comprised of protoporphyrin IX (an organic ring structure) which has a chelated (bound) iron atom.

At the center of the heme is an Fe(II) atom. Four of the six coordination sites around this atom are occupied by nitrogen atoms from a planar porphyrin ring. The fifth coordination site is occupied by a nitrogen atom from a histidine side chain on one of the amino acids in the protein. The last coordination site is available to bind an O₂ molecule. The heme is therefore the oxygen-carrying portion of the hemoglobin and myoglobin molecules. The heme resides in a small hydrophobic cleft within each polypeptide. There is one heme associated with myoglobin. There are four hemes found in hemoglobin, one in each of the subunits.