|
The carbonic anhydrases (or carbonate dehydratases) form a family of enzymes that catalyze the rapid interconversion of carbon dioxide and water to bicarbonate and protons (or vice versa), a reversible reaction that occurs relatively slowly in the absence of a catalyst. The active site of most carbonic anhydrases contains a zinc ion; they are therefore classified as metalloenzymes. One of the functions of the enzyme in animals is to interconvert carbon dioxide and bicarbonate to maintain acid-base balance in blood and other tissues, and to help transport carbon dioxide out of tissues. == Reaction == The reaction catalyzed by carbonic anhydrase is: : (in tissues - high CO2 concentration)〔Carbonic acid has a pKa of around 6.36 (the exact value depends on the medium) so at pH 7 a small percentage of the bicarbonate is protonated. See carbonic acid for details concerning the equilibria HCO3- + H+ H2CO3 and H2CO3 CO2 + H2O〕 The reaction rate of carbonic anhydrase is one of the fastest of all enzymes, and its rate is typically limited by the diffusion rate of its substrates. Typical catalytic rates of the different forms of this enzyme ranging between 104 and 106 reactions per second. The reverse reaction is relatively slow (kinetics in the 15-second range) in the absence of a catalyst. This is why a carbonated drink does not instantly degas when opening the container; however it will rapidly degas in the mouth when it comes in contact with carbonic anhydrase that is contained in saliva. An anhydrase is defined as an enzyme that catalyzes the removal of a water molecule from a compound, and so it is this "reverse" reaction that gives carbonic anhydrase its name, because it removes a water molecule from carbonic acid. : (in lungs and nephrons of the kidney - low CO2 concentration, in plant cells) 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「carbonic anhydrase」の詳細全文を読む スポンサード リンク
|