3 research outputs found
Role of the Proximal Cysteine Hydrogen Bonding Interaction in Cytochrome P450 2B4 Studied by Cryoreduction, Electron Paramagnetic Resonance, and ElectronāNuclear Double Resonance Spectroscopy
Crystallographic studies have shown
that the F429H mutation of
cytochrome P450 2B4 introduces an H-bond between His429 and the proximal
thiolate ligand, Cys436, without altering the protein fold but sharply
decreases the enzymatic activity and stabilizes the oxyferrous P450
2B4 complex. To characterize the influence of this hydrogen bond on
the states of the catalytic cycle, we have used radiolytic cryoreduction
combined with electron paramagnetic resonance (EPR) and (electronānuclear
double resonance (ENDOR) spectroscopy to study and compare their characteristics
for wild-type (WT) P450 2B4 and the F429H mutant. (i) The addition
of an H-bond to the axial Cys436 thiolate significantly changes the
EPR signals of both low-spin and high-spin heme-ironĀ(III) and the
hyperfine couplings of the heme-pyrrole <sup>14</sup>N but has relatively
little effect on the <sup>1</sup>H ENDOR spectra of the water ligand
in the six-coordinate low-spin ferriheme state. These changes indicate
that the H-bond introduced between His and the proximal cysteine decreases
the extent of S ā Fe electron donation and weakens the FeĀ(III)āS
bond. (ii) The added H-bond changes the primary product of cryoreduction
of the FeĀ(II) enzyme, which is trapped in the conformation of the
parent FeĀ(II) state. In the wild-type enzyme, the added electron localizes
on the porphyrin, generating an <i>S</i> = <sup>3</sup>/<sub>2</sub> state with the anion radical exchange-coupled to the FeĀ(II).
In the mutant, it localizes on the iron, generating an <i>S</i> = <sup>1</sup>/<sub>2</sub> FeĀ(I) state. (iii) The additional H-bond
has little effect on <i>g</i> values and <sup>1</sup>Hā<sup>14</sup>N hyperfine couplings of the cryogenerated, ferric hydroperoxo
intermediate but noticeably slows its decay during cryoannealing.
(iv) In both the WT and the mutant enzyme, this decay shows a significant
solvent kinetic isotope effect, indicating that the decay reflects
a proton-assisted conversion to Compound I (Cpd I). (v) We confirm
that Cpd I formed during the annealing of the cryogenerated hydroperoxy
intermediate and that it is the active hydroxylating species in both
WT P450 2B4 and the F429H mutant. (vi) Our data also indicate that
the added H-bond of the mutation diminishes the reactivity of Cpd
I
Binding of Yeast Cytochrome <i>c</i> to Forty-Four Charge-Reversal Mutants of Yeast Cytochrome <i>c</i> Peroxidase: Isothermal Titration Calorimetry
Previously,
we constructed, expressed, and purified 46 charge-reversal
mutants of yeast cytochrome <i>c</i> peroxidase (CcP) and
determined their electronic absorption spectra, their reaction with
H<sub>2</sub>O<sub>2</sub>, and their steady-state catalytic properties
[Pearl, N. M. et al. (2008) Biochemistry 47, 2766ā2775]. Forty-four of the mutants involve the conversion of
either an aspartate or glutamate residue to a lysine residue, while
two are positive-to-negative mutations, R31E and K149D. In this paper,
we report on a calorimetric study of the interaction of each charge-reversal
mutant (excluding the internal mutants D76K and D235K) with recombinant
yeast iso-1 ferricytochrome <i>c</i>(C102T) (yCc) under
conditions where only one-to-one yCc/CcP complex formation is observed.
Thirteen of the 44 surface-site charge-reversal mutants decrease the
binding affinity for yCc by a factor of 2 or more. Eight of the 13
mutations (E32K, D33K, D34K, E35K, E118K, E201K, E290K, E291K) occur
within, or on the immediate periphery, of the crystallographically
defined yCc binding site [Pelletier, H. and Kraut, J. (1992) Science 258, 1748ā1755], three of the mutations (D37K, E98K, E209K)
are slightly removed from the crystallographic site, and two of the
mutations (D165K, D241K) occur on the āback-sideā of
CcP. The current study is consistent with a model for yCc binding
to CcP in which yCc binds predominantly near the region defined by
crystallographic structure of the 1:1 yCcāCcP complex, whether
as a stable electron-transfer active complex or as part of a dynamic
encounter complex
Structural and Functional Characterization of a Cytochrome P450 2B4 F429H Mutant with an Axial ThiolateāHistidine Hydrogen Bond
The
structural basis of the regulation of microsomal cytochrome
P450 (P450) activity was investigated by mutating the highly conserved
heme binding motif residue, Phe429, on the proximal side of cytochrome
P450 2B4 to a histidine. Spectroscopic, pre-steady-state and steady-state
kinetic, thermodynamic, theoretical, and structural studies of the
mutant demonstrate that formation of an H-bond between His429 and
the unbonded electron pair of the Cys436 axial thiolate significantly
alters the properties of the enzyme. The mutant lost >90% of its
activity;
its redox potential was increased by 87 mV, and the half-life of the
oxyferrous mutant was increased ā¼37-fold. Single-crystal electronic
absorption and resonance Raman spectroscopy demonstrated that the
mutant was reduced by a small dose of X-ray photons. The structure
revealed that the Ī“N atom of His429 forms an H-bond with the
axial Cys436 thiolate whereas the ĪµN atom forms an H-bond with
the solvent and the side chain of Gln357. The amide of Gly438 forms
the only other H-bond to the tetrahedral thiolate. Theoretical quantification
of the histidineāthiolate interaction demonstrates a significant
electron withdrawing effect on the heme iron. Comparisons of structures
of class IāIV P450s demonstrate that either a phenylalanine
or tryptophan is often found at the location corresponding to Phe429.
Depending on the structure of the distal pocket heme, the residue
at this location may or may not regulate the thermodynamic properties
of the P450. Regardless, this residue appears to protect the thiolate
from solvent, oxidation, protonations, and other deleterious reactions