hemoglobinbound

Hemoglobin-bound refers to the state in which a molecule is attached to hemoglobin, the oxygen-transport protein in red blood cells. Hemoglobin contains four heme groups, each capable of reversibly binding one molecule of oxygen to the iron ion in the ferrous (Fe2+) state. When oxygen is bound, the molecule is called oxyhemoglobin; when not bound, deoxyhemoglobin. The binding of O2 is cooperative, described by the sigmoidal oxygen-hemoglobin dissociation curve. Affinity is modulated by factors such as pH (the Bohr effect), carbon dioxide, temperature, and 2,3-bisphosphoglycerate, shifting the curve and altering P50, the partial pressure at which hemoglobin is half-saturated.

Beyond oxygen, other ligands can bind to hemoglobin. Carbon monoxide binds with very high affinity to the heme iron, forming carboxyhemoglobin and markedly reducing O2 transport. Carbon dioxide can bind covalently to the amino termini of the globin chains, forming carbaminohemoglobin, and also exists as dissolved CO2 or bicarbonate, contributing to CO2 transport from tissues to lungs. Methemoglobin, where iron is in the ferric (Fe3+) state, cannot bind oxygen, and elevated levels impair oxygen delivery.

Clinically, measuring hemoglobin-bound ligands helps assess gas exchange and poisoning. Pulse oximetry estimates O2 saturation but is affected by carboxyhemoglobin; co-oximetry can quantify oxyhemoglobin, deoxyhemoglobin, carboxyhemoglobin, and methemoglobin. Understanding hemoglobin-bound states clarifies oxygen transport, respiratory physiology, and related pathophysiology.