Carbonic Anhydrase
Which metal is vital for catalytic activity by carbonic anhydrases?
Zinc
If pka = 7, what must K equal to?
10⁻⁷
What is the pka of ordinary water?
15.7
Which oxidation state does Zinc exist in, in Biological Systems?
2+
What is the zinc bound to in carbonic anhydrase?
3 imidazole rings of 3 histidines and a water molecule(or hydroxide depending on pH)
What is the pka of zinc bound water?
7
How many different types of carbonic anhydrases have we currently identified in humans?
7. (each with its own gene)
What is the other advantage of possessing this hydroxyl rather than a water in terms of the kinetics of the reaction?
A zinc-bound hydroxide ion is sufficiently nucleophilic to attack carbon dioxide much more readily than water does.
Which group is actually responsible for the decrease in efficiency of Carbonic Anhydrase II as pH decreases?
Although some amino acids, notably histidine, have pKa values near 7, a variety of evidence suggests that the group responsible for this transition is not an amino acid but is the zinc-bound water molecule.
What is an Anhydrase?
An enzyme that catalyses the removal of water
the rate of proton diffusion limits the rate of proton release to less than 10⁴ s-1 for a group with pKa= 7. However, carbon dioxide is hydrated at a rate of 10⁶ s⁻¹. How can you explain this?
Buffer concentrations can be higher
Why are carbonic anhydrases required?
CO₂ hydration and HCO₃⁻ dehydration are often coupled to rapid processes, particularly transport processes. For example, HCO₃⁻ in the blood must be dehydrated to form CO₂ for exhalation as the blood passes through the lungs. Conversely, CO₂ must be converted into HCO₃⁻ for the generation of the aqueous humor of the eye and other secretions. Furthermore, both CO2 and HCO3- are substrates and products for a variety of enzymes, and the rapid interconversion of these species may be necessary to ensure appropriate substrate levels.
Which carbonic anhydrase hydrates CO₂ at rates as high as kcat = 10⁶ s⁻¹
Carbonic anhydrase II
How does a water molecule binding to the zinc change the pka of water?
It makes the pka smaller because, positively charged zinc is more stabilised by a negatively charged hydroxyl, rather than neutral water. (The strong positive charge is dampened).
Why do 1/3 of all catalytic enzymes contain bound metal ions?
Metal ions are reactive due to their strong positive charge Metal ions can form relatively strong bonds, but these bonds are also kinetically labile. Some metal ions are stable in more than 1 oxidation state.
Where is carbonic anhydrase II found in relatively high concentrations?
Red Blood Cells
Why will a carbonated (fizzy) drink rapidly degas in the mouth?
Saliva contains carbonic anhydrase.
How many ligands is zinc often bound to?
four or more
Carbonic anhydrases are associated with which abnormalities?
mutations in some carbonic anhydrases have been found to cause osteopetrosis (excessive formation of dense bones accompanied by anemia) and mental retardation.
At what pH does carbonic anhydrase II catalyse at its maximum rate?
pH 8
Midpoint of transition from maximum efficiency to minimum efficiency occurs at pH 7. What does this suggest? To see graph, flip card. Answer is below graph so be careful.
suggests that a group with pKa = 7 plays an important role in the activity of carbonic anhydrase and that the deprotonated (high pH) form of this group participates more effectively in catalysis.