In the above image, you can see how gaseous oxygen can reversibly bind to the heme complex. Heme is named for the Greek derivative for blood, which is where it was first identified. Hemoglobin is made up of four symmetrical subunits and four heme groups.
The iron itself is bonded to nitrogen on histidine group. How does the structure of the proteins in haemoglobin hold the heme group. The red color of blood is produced by the heme and iron ion interacting to absorb other colors and only reflect red. Please explaining the binding capacity of oxygen to heme in the lun.