Which five statements about hemoglobin and myoglobin structure are true? Each hemoglobin molecule can bind four oxygen molecules, and each myoglobin can bind only one oxygen molecule. Molecular oxygen binds irreversibly to Fe2+ in heme. By itself, heme is not a good oxygen carrier. It must be part of a larger protein to prevent a change in the oxidation state of the iron ion. Both hemoglobin and myoglobin contain a prosthetic group called heme, which contains a central iron ion. Heme is composed of an organic protoporphyrin component and a metal ion. Each iron ion can form six coordination bonds: One of these bonds is formed between iron and oxygen. Hemoglobin and myoglobin are heterotetramers.