When the first oxygen molecule binds to one of the hemoglobin subunits, the affinity of the other three subunits to oxygen increases greatly. And this behavior of our enzyme is known as positive cooperativity. Now, hemoglobin not only displays positive cooperativity, it also displays something called negative cooperativity, which is basically the opposite of positive. So a molecule known as two three biphosphoglycerate, or simply two three BPG can bind to hemoglobin at a specific allosteric site at low oxygen concentrations. And by binding to our hemoglobin, what the two three BPG essentially does is it decreases definitive to our oxygen molecules by basically changing the three dimensional shape of hemoglobin. Now, why in a world when our two three BPG want to actually lower the activity of hemoglobin for oxygen?