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Unformatted text preview: oton donors, like
unnatural amino acids, can be added to increase catalysis. These contribute to the proton
transfer between the solution and the zinc bound solvent. To measure the specifics, sitespecific mutants were used to see if the other derivatives made a difference. vii However, it
was discovered that no matter what the internal composition of the active site of the
enzyme is the proton transfer still occurred, therefore the location of the proton donors is
irrelevant due to the flexibility of the water structures and easily formable hydronium ion.
Also the hydrogen bonded water extending from the zinc solvent towards the inward
protein structure. This order is closely related to the proton transfer. The hydrogenbonded network includes the zinc solvent from the hydrophobic pocket to the proton
shuttle and connected to the side chains of the amino acids, therefore the aqueous
solution plays a large part on the proton shuttleix.
In order to study carbonic anhydrases (CA), scientists create similar models to
learn more about the enzyme and its proton donating abilities. In the Lesnichin study,
they used zinc bound water organometallic complexes in comparison with their associated carboxylic acids to explore the role of water within the active site of the CAs’.
The enzyme increases the ability of the proton donor because is lowers the approximate
pH from 9 to between 6 and 7. Their findings through NMR consisted of stating that in
polar non-aqueous environments with higher acidity side chains the observed pKa was
2.2 therefore having more proton donating ability unlike in the aqueous solution where
the zinc bound water has a pKa of 7, which is observed in the CA His 64.viii Therefore
although we cannot conclude specifics, we can conclude that throughout the transfer
there are hydrophobic parts of the active site environment, but there are more aqueous
part that are displayed in the enzyme found by this specific study. viii This study confirms
Journal of the American Chemical within
the variability of the proton transferSociety the active site demonstrated by Figure 3.
Scheme 3. (a) Simpli ed Catalytic Cycle of CA Reactions of
eqs 1 and 2 Derived from Silverman et al.6 and (b) Schemat ic
Hydr ogen Bond Geomet ries of Model Compl exes for CAa A described in the following. In order to address this i
present a model of the catalytic mechanism of CA as c
proposed.6 ,1 3 In Scheme 3a we have depicted a s
catalytic cycle of human carbonic anhydrase II (HC
which L represents histidine residues that are direct ligan
zinc. B represents the proton-accepti ng base, i.e., H is6
wild type. Note that there are many studies using variants
II in which the proton shuttle residue His64 is repla
residue such as alanine that is incapable of proton transf
Accordi ng to this model, catalysi s occurs in two sepa
distinct stages (a ping-pong mechani sm). The rst sta
prises the hydrati on of CO2 , i.e., addi tion of CO2 to Z
hydroxide and the removal of HCO3 accompan...
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