User:Antemas/Carbonic anhydrase

Structure

In mammalian CA II, the active site comprises of the following: a hard Lewis acid Zn+2 metal atom coordinated to His -94, -96, and -119 residues 109˚ apart from one another and a hydroxide ion (pKa=6.8; 120° in Td  configuration, a hydrophobic pocket adjacent to Zinc-bound hydroxide consisting of by Val-143 at its base and Val-121, Trp-209, and Leu-198 at its neck, a Proton Shuttling Residue (PSR) His-64 H+ shuttles H+ in and out of active site via conformational switching, and a hydrogen bonding network consisting of Thr-199 hydroxyl group and Glu-106 the carboxyl group which stabilizes the Zinc-bound hydroxide by facilitating the orientation of water molecules in the active side to a specific geometric configuration. CA II has a turnover frequency of 106 s-1 which is 107 times faster than the uncatalyzed reaction.

CO$2$ Transport
Carbon dioxide is transported in the blood in three forms:

1.    Dissolved as gas in plasma – 7-10%

2.    Bound to hemoglobin as carbaminohemoglobin in red blood cells – 20%

3.    Present as bicarbonate ion in plasma and transported a bicarbonate - 70%

Mechanism of catalysis in CA II
Kinetic studies performed determine the following mechanism for the enzyme: In Step 1 & 2, the nucleophile O- on the hydroxide ion coordinated to Zn2+ performs a nucleophilic attack on the partially electrophilic carbon on the CO2 molecule. Here the Zn2+ acts as a Lewis acid that lowers the pKa of the coordinated OH2 ligand from ~7-8 down to 6.8  as Td, which drives the deprotonation of water to a hydroxide ion and the free proton is neutralized by the surrounding buffer. In step 3), a proton transfer (H+) occurs from the OH-1 to the non-coordinated O- in CO3-2 coordinated to the Zn+2 atom in the active site. Next, a bicarbonate ion is released and the catalytic site is regenerated through the binding of another water molecule in exchange for the bicarbonate ion . In step 4), the coordinated water ligand is deprotonated facilitated by the Zn+2 to generate another hydroxide ion to start the cycle over again.