heme group

Meaning: A heme group is a porphyrin ring (four pyrrole rings joined by methine bridges) with an iron ion at its center. In hemoglobin and myoglobin the iron is Fe²⁺ and binds O₂ reversibly. Iron has six coordination positions: four bind the ring nitrogens, one binds a histidine of the protein, and one is open for O₂. Heme is a prosthetic group, a non-amino-acid part tightly bound to a protein. Each hemoglobin subunit has one heme, so hemoglobin carries up to four O₂. Cytochromes in the electron transport chain also contain heme, with the iron cycling between Fe²⁺ and Fe³⁺ to carry electrons.

On the MCAT: Passages ask why hemoglobin has four O₂ binding sites, how O₂ binding at one heme raises affinity at the others (cooperativity, shift from the T state to the R state), or how CO competes for the heme iron and binds more tightly than O₂. Myoglobin has one heme and a hyperbolic binding curve; hemoglobin gives a sigmoidal curve.

Common confusion: Heme is not the whole protein; the protein part is globin. Also, O₂ binding is oxygenation, not oxidation: the iron stays Fe²⁺ while O₂ is bound. Iron oxidized to Fe³⁺ (methemoglobin) binds O₂ poorly.