TY - JOUR
T1 - Spectroscopic and computational insights into second-sphere amino-acid tuning of substrate analogue/active-site interactions in iron(III) superoxide dismutase
AU - Grove, Laurie E.
AU - Xie, Juan
AU - Yikilmaz, Emine
AU - Karapetyan, Anush
AU - Miller, Anne Frances
AU - Brunold, Thomas C.
PY - 2008/5/19
Y1 - 2008/5/19
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=43649097167&partnerID=8YFLogxK
UR - http://www.scopus.com/inward/citedby.url?scp=43649097167&partnerID=8YFLogxK
U2 - 10.1021/ic702414m
DO - 10.1021/ic702414m
M3 - Article
C2 - 18433119
AN - SCOPUS:43649097167
SN - 0020-1669
VL - 47
SP - 3993
EP - 4004
JO - Inorganic Chemistry
JF - Inorganic Chemistry
IS - 10
ER -