2,3-bisphosphoglycerate (also called glycerate-2,3-bisphosphate) is a three-carbon molecule with two negatively charged (2-) phosphate groups attached to the central carbon, forming a tetrahedral structure. This highly anionic molecule is most commonly found in the red blood cells at about the same concentration as that of hemoglobin (around 2 mM); this is responsible for the great efficiency of oxygen transport that takes place in hemoglobin molecules. 2,3-bisphosphoglycerate, abbreviated as 2,3-BPG and also referred to as 2,3-diphosphoglycerate (2,3-DPG), binds with greater affinity to deoxygenated hemoglobin typically found in tissues than to oxygenated hemoglobin found in the lungs. It is for this reason that hemoglobin, and not myoglobin, is more used in transferring oxygen between tissues and the lungs. There are high concentrations of 2,3-BPG found in erythrocytes.
2,3-bisphosphoglycerate compound has two important biological activities.
1. 2,3-bisphosphoglycerate is formed both by phosphoglycerate mutase (transiently) and bisphosphopglycerate mutase. The mechanism of the reaction catalyzed by phosphoglycerate mutase involves formation of 2,3-bisphosphoglycerate via transient phosphorylation of a histidine residue of the enzyme. 2,3BPG can diffuse from phosphoglycerate mutase, however, leaving the enzyme trapped in an unusable state. Cells make excess 2,3BPG (using the enzyme bisphosphoglycerate mutase) in order to drive 2,3BPG back to phosphoglycerate mutase, so the reaction can go to completion.
2. 2,3-BPG binds in the cavity between the beta-chains of hemoglobin, making electrostatic interactions with positively charged groups surrounding this opening. Comparison of the two hemoglobin conformations shows that this opening is much narrower in oxyhemoglobin than in deoxyhemoglobin. In fact, 2,3-BPG cannot be accommodated in the oxy form. The higher the 2,3-BPG content in red blood cells, the more stable the deoxy structure will be. The decrease in O2 affinity by hemoglobin is explained by stabilization of the deoxy structure. Increased 2,3-BPG levels are also found in the blood of smokers, who because of the carbon monoxide in smoke also suffer from limitation in oxygen supply.