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Figure 6.
Fig. 6. Possible mechanisms for the formation of the
covalent bond found in the V169C variant of HPII between the
mutated residue Cys169 and the essential histidine His128.
Scheme A shows the proposed nucleophilic mechanism based on the
acid catalyzed nucleophilic addition of the thiol group of
Cys169 onto the imidazole ring of His128. Scheme B shows an
alternate mechanism where protonation of His128, in the initial
step of compound I formation, triggers the nucleophilic attack
of the thiol group of Cys169 on the imidazole ring. Scheme C
depicts the proposed free radical mechanism based on the
oxidation of the thiol group of Cys169 to yield a thiyl radical
which can attack the imidazolic system.
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