But in hemoglobin, because we have four different polypeptide chains, we have four different heme groups. Now, one heme group can bind one oxygen. And what that means is a single hemoglobin can bind four times as many oxygen molecules as myoglobin can. Now, if hemoglobin can bind more of these oxygen molecules, why do we need myoglobin molecules in the first place? And in general, why does our body need to use two different proteins to carry out the same function of bringing the oxygen to the cell? Well, as it turns out, because these two proteins have slightly different structures, they have slightly different properties and therefore slightly different functions.