2 research outputs found
Spectroscopic Investigations of Catalase Compound II: Characterization of an Iron(IV) Hydroxide Intermediate in a Non-thiolate-Ligated Heme Enzyme
We
report on the protonation state of <i>Helicobacter pylori</i> catalase compound II. UV/visible, Mössbauer, and X-ray absorption
spectroscopies have been used to examine the intermediate from pH
5 to 14. We have determined that HPC-II exists in an iron(IV) hydroxide
state up to pH 11. Above this pH, the iron(IV) hydroxide complex transitions
to a new species (p<i>K</i><sub>a</sub> = 13.1) with Mössbauer
parameters that are indicative of an iron(IV)-oxo intermediate. Recently,
we discussed a role for an elevated compound II p<i>K</i><sub>a</sub> in diminishing the compound I reduction potential. This
has the effect of shifting the thermodynamic landscape toward the
two-electron chemistry that is critical for catalase function. In
catalase, a diminished potential would increase the selectivity for
peroxide disproportionation over off-pathway one-electron chemistry,
reducing the buildup of the inactive compound II state and reducing
the need for energetically expensive electron donor molecules
Setting an Upper Limit on the Myoglobin Iron(IV)Hydroxide p<i>K</i><sub>a</sub>: Insight into Axial Ligand Tuning in Heme Protein Catalysis
To provide insight into the iron(IV)hydroxide
p<i>K</i><sub>a</sub> of histidine ligated heme proteins,
we have probed the
active site of myoglobin compound II over the pH range of 3.9–9.5,
using EXAFS, Mössbauer, and resonance Raman spectroscopies.
We find no indication of ferryl protonation over this pH range, allowing
us to set an upper limit of 2.7 on the iron(IV)hydroxide p<i>K</i><sub>a</sub> in myoglobin. Together with the recent determination
of an iron(IV)hydroxide p<i>K</i><sub>a</sub> ∼ 12
in the thiolate-ligated heme enzyme cytochrome P450, this result provides
insight into Nature’s ability to tune catalytic function through
its choice of axial ligand