how oxygen binds to heme
The Heme Group. In hemoglobin, each subunit contains a heme group, which is displayed using the ball-and-stick representation in Figure 2. Each heme group contains an iron atom that is able to bind to one oxygen (O 2) molecule. Therefore, each hemoglobin protein can bind four oxygen molecules. One of the most important classes of chelating agents in nature are the porphyrins.