oxyhemoglobin

Oxyhemoglobin is the form of hemoglobin in which the protein is bound to molecular oxygen. Hemoglobin, a tetrameric protein in red blood cells, contains four heme groups, each capable of binding one O2 molecule to iron in the ferrous (Fe2+) state. Oxyhemoglobin forms predominantly in the lungs, where alveolar oxygen partial pressure is high, and releases oxygen in tissues where partial pressure is low.

Binding is cooperative: binding of O2 to one heme increases affinity at the remaining sites, producing a sigmoidal oxygen dissociation curve. The overall reaction can be represented as Hb + 4 O2 ⇌ Hb(O2)4, with stepwise binding occurring at the four heme centers.

Affinity is modulated by pH, carbon dioxide, temperature, and 2,3-bisphosphoglycerate (2,3-BPG). The Bohr effect explains that a lower pH and higher CO2 reduce Hb's oxygen affinity, promoting oxygen release in tissues. Higher temperature or 2,3-BPG also lowers affinity.

Color and oxygen content: oxygenated blood appears bright red, while deoxygenated blood is darker. In healthy individuals, arterial blood typically has oxygen saturation around 95–100%, whereas venous blood is about 70–75% saturated.

Clinical relevance: carbon monoxide binds with high affinity to hemoglobin, forming carboxyhemoglobin and markedly reducing oxygen delivery. Methemoglobinemia and other disorders alter oxygen binding and release characteristics, illustrating the importance of oxyhemoglobin in tissue respiration.