Journal
PROCEEDINGS OF THE NATIONAL ACADEMY OF SCIENCES OF THE UNITED STATES OF AMERICA
Volume 114, Issue 32, Pages 8556-8561Publisher
NATL ACAD SCIENCES
DOI: 10.1073/pnas.1706527114
Keywords
heme cofactor; X-ray spectroscopy; quantum chemistry; O-2 binding; spin state
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Funding
- Bundesministerium fur Bildung und Forschung [05K14KE1]
- Unicat (Cluster of Excellence Berlin)
- Deutsche Forschungsgemeinschaft [He5790/3-1]
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Proteins carrying an iron-porphyrin (heme) cofactor are essential for biological O-2 management. The nature of Fe-O-2 bonding in hemoproteins is debated for decades. We used energy-sampling and rapid-scan X-ray K beta emission and K-edge absorption spectroscopy as well as quantum chemistry to determine molecular and electronic structures of unligated (deoxy), CO-inhibited (carboxy), and O-2-bound (oxy) hemes in myoglobin (MB) and hemoglobin (HB) solutions and in porphyrin compounds at 20-260 K. Similar metrical and spectral features revealed analogous heme sites in MB and HB and the absence of low-spin (LS) to high-spin (HS) conversion. Amplitudes of K beta main-line emission spectra were directly related to the formal unpaired Fe(d) spin count, indicating HS Fe(II) in deoxy and LS Fe(II) in carboxy. For oxy, two unpaired Fe(d) spins and, thus by definition, an intermediate-spin iron center, were revealed by our static and kinetic X-ray data, as supported by (time-dependent) density functional theory and complete-active-space self-consistent-field calculations. The emerging Fe-O-2 bonding situation includes in essence a ferrous iron center, minor superoxide character of the noninnocent ligand, significant double-bond properties of the interaction, and three-center electron delocalization as in ozone. It resolves the apparently contradictory classical models of Pauling, Weiss, and McClure/Goddard into a unifying view of O-2 bonding, tuned toward reversible oxygen transport.
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