Metallic elements are essential components of many important enzymes operating within our bodies. Carbonic anhydrase, which contains Zn2+in its active site, is responsible for rapidly interconverting dissolved CO2 and bicarbonate ion, HCO3-.The zinc in carbonic anhydrase is tetrahedrally coordinated by three neutral nitrogen-containing groups and a water molecule. The coordinated water molecule has a pKa of 7.5, which is crucial for the enzyme's activity.
(a) Draw the active site geometry for the Zn(II) center in carbonic anhydrase, just writing "N" for the three neutral nitrogen ligands from the protein.
(b) Compare the pKa of carbonic anhydrase's active site with that of pure water; which species is more acidic?
(c) When the coordinated water to the Zn(II) center in carbonic anhydrase is deprotonated, what ligands are bound to the Zn(II) center? Assume the three nitrogen ligands are unaffected.
(d) The pKa of [Zn(H2O)6]2+ is 10. Suggest an explanation for the difference between this pKa and that of carbonic anhydrase.
(e) Would you expect carbonic anhydrase to have a deep color, like hemoglobin and other metal-ion containing proteins do? Explain.