Heme group in haemoglobin. Artwork showing the heme group (centre) in the haemoglobin molecule (partly shown), the chemical unit that is responsible for the transport of oxygen in the blood. The heme consists of an iron ion (red) at the centre of a ring-structure called a porphyrin. Molecular oxygen (blue) is shown bound to the heme group. Above and below are histidine groups (orange and purple). The histidine group opposite to the oxygen-binding site is called the proximal histidine, while the other one is the distal histidine. The partially seen surroundings parts of the haemoglobin moilecule are green.