Spectroscopic and computational insights into second-sphere amino-acid tuning of substrate analogue/active-site interactions in iron(III) superoxide dismutase

Laurie E. Grove, Juan Xie, Emine Yikilmaz, Anush Karapetyan, Anne Frances Miller, Thomas C. Brunold

Research output: Contribution to journalArticlepeer-review

18 Scopus citations

Abstract

In this study, the mechanism by which second-sphere residues modulate the structural and electronic properties of substrate-analogue complexes of the Fe-dependent superoxide dismutase (FeSOD) has been explored. Both spectroscopic and computational methods were used to investigate the azide (N3 -) adducts of Fe3+SOD (N3-Fe3+SOD) and its Q69E mutant, as well as Fe3+-substituted MnSOD (N 3-Fe3+(Mn)SOD) and its Y34F mutant. Electronic absorption, circular dichroism, and magnetic circular dichroism spectroscopic data reveal that the energy of the dominant N3- → Fe3+ ligand-to-metal charge transfer (LMCT) transition decreases in the order N 3-Fe3+(Mn)SOD > N3-Fe3+SOD > Q69E N3-Fe3+SOD. Intriguingly, the LMCT transition energies correlate almost linearly with the Fe3+/2+ reduction potentials of the corresponding Fe3+-bound SOD species in the absence of azide, which span a range of ∼1 V (see the preceding paper). To explore the origin of this correlation, combined quantum mechanics/molecular mechanics (QM/MM) geometry optimizations were performed on complete enzyme models. The INDO/S-Cl computed electronic transition energies satisfactorily reproduce the experimental trend in LMCT transition energies, indicating that the QM/MM optimized active-site models are reasonable. Density functional theory calculations on these experimentally validated active-site models reveal that the differences in spectral and electronic properties among the four N 3- adducts arise primarily from differences in the hydrogen-bond network involving the conserved second-sphere Gln (mutated to Glu in Q69E FeSOD) and the solvent ligand. The implications of our findings with respect to the mechanism by which the second-coordination sphere modulates substrate-analogue binding as well as the catalytic properties of FeSOD are discussed.

Original languageEnglish
Pages (from-to)3993-4004
Number of pages12
JournalInorganic Chemistry
Volume47
Issue number10
DOIs
StatePublished - May 19 2008

ASJC Scopus subject areas

  • Physical and Theoretical Chemistry
  • Inorganic Chemistry

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