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Unformatted text preview: onstant. The tight binding of zinc is associated with second shell ligands. ELG M AD !G @C !A Q C @ @B!= FIL? = K ' D ##j N Ei ##j N = ##j N E? !' Li ##j Z D E? !' ( >!
@! H B I @I F G E > E
E ! i !7
!$ i !C
ML The removal of the metal is done with no catalytic activity with certain chelating agents <C @ @E!A Q A@O <@ @G @A !D !FD A = C = E!C D BB @ @C !J C = >!K @B5N A G !
J !M @ @ D B ? !& !K D B5 H D ? E BD M L ! < !G B I A@ A C I !HD DaC @A I!H A7 Ei ##j !C ? !$ = ##j !B !B= theI!additionI!of !'zinc.!S iiB !D I5!' D ##j Zother
FB < ? can be reversed with K EC MQ A G = C H A Li ##j Nv =< here are
=5 C ? D
nd C D !
>LMBB B ? !' ( !A B = = KB @ @A <@ A I!FD A = C = E!C D BB @ @C !i ' D !@!C O : : : M!
> =L @
@C E E ! < !B BC ? C D ? E BD M L ! < !G B I
T B I !9
N ! metal replacements, such as Co(II), Mn(II), Ni (II), Cd(II). Although preserving functionality, these divalent ion
substitutions are inferior with a lower catalytic activity than the natural form with Zn (II).
Delving more deeply into the catalytic process, there is more than one accepted
theory on the initiation of the catalytic reaction. The one displayed above in Figure 1 is
the one with the most compelling evidence. The other pathway is said to follow the zinc
bound water, rather than the hydroxide, that performs the nucleophilic attack on the
CO2.vii Although this mechanism is possible, the evidence against this possibility
outweighs its actual chance of occurrence. However it has been proven through two
mechanism: the Lindskog mechanism, which involves a proton transfer from a proton on
the zinc bound oxygen to the oxygen on the bicarbonate and the Lipscomb mechanism,
which is an inner rotation of the bicarbonate rather than a proton transfer. The zinc
hydroxide mechanism is much more prevalent due to the likelihood of the zinc hydroxide
and carbon dioxide complex formation because of the rigidity of the building center, the
formation without change during the proton transfer, and also the kinetic energy barrier is
limited by the rearrangement of the zinc active site. vii
The proton transfer between the solution and the zinc bound solvent, specifically
His 64, has been numerically described and captured due to Silverman and Lindskog.
Silverman and Lindskog also confirm that although the rate of proton transfer for mutants
is decreased compared the wild type enzyme that the same amount of introversion
between carbon dioxide and the bicarbonate ion is very similar. They also describe that
this pathway is the most energetically favorable path of the proton in human carbonic
anhydrase (HCA II).vii The proton shuttle in His 64 has its side chains in two different
orientations that were determined by crystallography shown in Figure 2. One chain is orientated with the imidazole ring pointed towards the zinc and the second is pointing out
of the active site cavity. vii To enhance the occurrence of the proton shuttle transfer, extra pr...
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