Haemoglobin is a protein found in red blood cells that carries oxygen from the lungs to body tissues.
Haemoglobin is made up of four polypeptide chains: two alpha chains (141 amino acids each) and two beta chains (146 amino acids each). This is an example of quaternary protein structure.
Each chain contains one heme group with an iron atom at its center. These iron atoms can each bind to one oxygen molecule, allowing haemoglobin to carry four oxygen molecules in total.
When oxygen binds to one heme group, it causes a slight change in shape that makes it easier for oxygen to bind to the other heme groups. This is called cooperative binding.
Each haemoglobin molecule can carry up to four oxygen molecules. When oxygen levels are high (like in the lungs), haemoglobin binds oxygen. When oxygen levels are low (like in tissues), it releases oxygen.
Mutations in haemoglobin genes can lead to conditions like sickle cell anaemia, where abnormal haemoglobin causes red blood cells to become misshapen.