A) Myoglobin is a single polypeptide chain folded about a heme prosthetic group. B) Hemoglobin is a tetramer, each of which binds a heme group. C) In both hemoglobin and myoglobin, iron is chelated by a tetrapyrole ring system. D) The iron in both hemoglobin and myoglobin has two coordination sites that bind to oxygen. E) Hydrogen bonding to a histidine residue assists stabilization of the Fe2+ -O2 complex in both hemoglobin and myoglobin.