Carbonic anhydrase is an enzyme found in high concentration in the cytoplasm of red blood cells. It catalyses the reversible reaction between carbon dioxide and water to form carbonic acid:CO2 + H2O ⇌ H2CO3 This reaction happens only slowly in the plasma, which contains little carbonic anhydrase. In red blood cells the enzyme speeds it up enormously, so carbon dioxide from respiring tissues is converted rapidly. The carbonic acid then dissociates into hydrogen ions and hydrogencarbonate ions:H2CO3 ⇌ H+ + HCO3− Converting carbon dioxide in this way keeps its concentration inside the red blood cell low. A steep concentration gradient is maintained, so more carbon dioxide keeps diffusing in from the tissues.The hydrogencarbonate ions diffuse out into the plasma, and chloride ions diffuse in to replace them, keeping the cell electrically balanced (the chloride shift). The hydrogen ions stay in the cell and are picked up by haemoglobin.
Because the reaction is reversible, carbonic anhydrase works in both directions. In the lung capillaries, where the partial pressure of carbon dioxide is low, it catalyses the breakdown of carbonic acid back into carbon dioxide and water. The carbon dioxide then diffuses out into the alveoli.Like all enzymes, carbonic anhydrase is a globular protein with an active site. It only increases the rate of reaction: the direction the reaction goes is set by the partial pressure of carbon dioxide.
Because the reaction is reversible, carbonic anhydrase works in both directions. In the lung capillaries, where the partial pressure of carbon dioxide is low, it catalyses the breakdown of carbonic acid back into carbon dioxide and water. The carbon dioxide then diffuses out into the alveoli.Like all enzymes, carbonic anhydrase is a globular protein with an active site. It only increases the rate of reaction: the direction the reaction goes is set by the partial pressure of carbon dioxide.