ic7b01945_si_001.pdf (1.32 MB)
Distinguishing Nitro vs Nitrito Coordination in Cytochrome c′ Using Vibrational Spectroscopy and Density Functional Theory
journal contribution
posted on 2017-10-20, 19:44 authored by Zach N. Nilsson, Brian L. Mandella, Kakali Sen, Demet Kekilli, Michael A. Hough, Pierre Moënne-Loccoz, Richard W. Strange, Colin R. AndrewNitrite coordination
to heme cofactors is a key step in the anaerobic production of the
signaling molecule nitric oxide (NO). An ambidentate ligand, nitrite
has the potential to coordinate via the N- (nitro) or O- (nitrito)
atoms in a manner that can direct its reactivity. Distinguishing nitro
vs nitrito coordination, along with the influence of the surrounding
protein, is therefore of particular interest. In this study, we probed
Fe(III) heme-nitrite coordination in Alcaligenes xylosoxidans cytochrome c′ (AXCP), an NO carrier that
excludes anions in its native state but that readily binds nitrite
(Kd ∼ 0.5 mM) following a distal
Leu16 → Gly mutation to remove distal steric constraints. Room-temperature
resonance Raman spectra (407 nm excitation) identify ν(Fe–NO2), δ(ONO), and νs(NO2) nitrite
ligand vibrations in solution. Illumination with 351 nm UV light results
in photoconversion to {FeNO}6 and {FeNO}7 states,
enabling FTIR measurements to distinguish νs(NO2) and νas(NO2) vibrations from
differential spectra. Density functional theory calculations highlight
the connections between heme environment, nitrite coordination mode,
and vibrational properties and confirm that nitrite binds to L16G
AXCP exclusively through the N atom. Efforts to obtain the nitrite
complex crystal structure were hampered by photochemistry in the X-ray
beam. Although low dose crystal structures could be modeled with a
mixed nitrite (nitro)/H2O distal population, their photosensitivity
and partial occupancy underscores the value of the vibrational approach.
Overall, this study sheds light on steric determinants of heme-nitrite
binding and provides vibrational benchmarks for future studies of
heme protein nitrite reactions.
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Keywords
351 nm UV light resultsheme environmentheme protein nitrite reactionsVibrational SpectroscopyDensity Functional Theory Nitrite coordinationFTIR measurementsfuture studiesnitrite coordination modeDistinguishing nitro vs nitrito coordinationambidentate ligandvibrational benchmarksheme-nitrite binding7 statesX-ray beamDistinguishing Nitro vs Nitrito Coordinationheme cofactorsvibrational approachsteric constraintsvibrational propertiestheory calculationssteric determinantscrystal structurenitrite ligand vibrationsmolecule nitric oxideL 16G AXCPdose crystal structuresONON atom
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