And that means because we have four subunits and each one of these carries one heme group we can bind four oxygen molecules per hemoglobin molecule. So once again, hemoglobin is a tetrimer that consists of four individual subunits. Each subunit is equipped with a heme group that is capable of binding oxygen molecules. Now, slight changes to the coronary structure of our hemoglobin can actually increase or decrease definitive of the hemoglobin molecule to oxygen as we'll see when we discuss the hemoglobin molecule in much more detail. So we see that there are four levels of structure in protein. So we have primary structure which is a sequence of our amino acids.