Wikibooks: Structural Biochemistry/Enzyme Catalytic Mechanism/Carbonic Anhydrase

• Format: Book
• Language: English
• Subjects: Carbonic acid
• Online Access: https://en.wikibooks.org/wiki/Structural_Biochemistry/Enzyme_Catalytic_Mechanism/Carbonic_Anhydrase

= Overview = A carbonic anhydrase or carbonate dehydratase is a type of enzyme that rapidly catalyzes the conversion of carbon dioxide into a proton and the bicarbonate ion (HCO3 ). This reaction is rather slow in the absence of the anhydrase catalyst as the reaction with the enzyme takes place typically ten thousand to one million (10^4 10^6) times per second. The active site by which the enzyme binds contains a zinc ion (Zn2+) by which the pKa is lowered and allows for nucleophilic attack on the carbon dioxide group. In humans this reaction mechanism is vital in maintaining pH balance and in transporting carbon dioxide out of the tissues and into the lungs. Carbon dioxide hydration needs a buffer because a buffer as we mentioned before can work as an acid or a base and in this case the buffer helps enzyme to reach its highest catalytic rate. In some cases the active site of carbonic anhydrase is inaccessible to bulky buffers interfering with efficient proton transfer. In response carbonic anhydrase II developed a proton shuttle made up of a histidine residue that removes an H+ from the bound water molecule activating its nucleophilicity and then transfers the proton to the edge of the protein (allowing the buffer to easily remove it). Therefore the reaction uses both [[ acid base catalysis ]] and [[ metal ion catalysis ]] strategies. =Structure= In carbon anhydrase as well as all biological systems the zinc atom is in the +2 state. The zinc is bound to four ligands three of its coordination sites are occupied by the imidazole rings of three histidine residues and a fourth is occupied by a water molecule. This active site is located in a cleft near the center of the enzyme. =Function= Carbonic anhydrase is a catalytic enzyme specific to accelerating the formation of carbonic acid from carbon dioxide (CO 2 ) and water (H 2 O) H 2 O + CO 2 ⇌ H 2 CO 3 It is important to note that the carbonic anhydrase does not shift the equilibrium of the reaction but rather helps the equilibrium be reached much quicker ...