TY - JOUR
T1 - Spectroscopic and computational studies on iron and manganese superoxide dismutases
T2 - Nature of the chemical events associated with active-site pKs
AU - Jackson, Timothy A.
AU - Xie, Juan
AU - Yikilmaz, Emine
AU - Miller, Anne Frances
AU - Brunold, Thomas C.
PY - 2002/9/11
Y1 - 2002/9/11
N2 - A combined spectroscopic/computational approach has been utilized to explore the chemical origins of the active-site pKs of the structurally homologous Fe- and Mn-dependent superoxide dismutases (SODs). Absorption, circular dichroism, magnetic circular dichroism, and variable-temperature, variable-field magnetic circular dichroism spectroscopic experiments have permitted us to determine electronic transition energies and polarizations, as well as ground-state spin Hamiltonian parameters. These experimental data have been used in conjunction with semiempirical intermediate neglect of differential overlap/spectroscopic parametrization configuration interaction (INDO/S-CI) computations for evaluating hypothetical active-site models for the high-pH species generated by density functional theory (DFT) geometry optimizations. Our experimental and computational data indicate that both reduced FeSOD and oxidized MnSOD do not bind hydroxide at high pH; rather, the active-site pK for these two species is attributed to deprotonation of a second-sphere tyrosine. Conversely, our data obtained on oxidized FeSOD indicate that hydroxide binding is responsible for the observed active-site pK for this species. Intriguingly, in the Fe-substituted form of MnSOD this identical chemical event occurs at a significantly lower pH. Overall, our results suggest an important role for second-sphere amino acids in tuning the active sites' interaction with small anions and bring into question the assumption that these homologous enzymes operate by the same molecular mechanism.
AB - A combined spectroscopic/computational approach has been utilized to explore the chemical origins of the active-site pKs of the structurally homologous Fe- and Mn-dependent superoxide dismutases (SODs). Absorption, circular dichroism, magnetic circular dichroism, and variable-temperature, variable-field magnetic circular dichroism spectroscopic experiments have permitted us to determine electronic transition energies and polarizations, as well as ground-state spin Hamiltonian parameters. These experimental data have been used in conjunction with semiempirical intermediate neglect of differential overlap/spectroscopic parametrization configuration interaction (INDO/S-CI) computations for evaluating hypothetical active-site models for the high-pH species generated by density functional theory (DFT) geometry optimizations. Our experimental and computational data indicate that both reduced FeSOD and oxidized MnSOD do not bind hydroxide at high pH; rather, the active-site pK for these two species is attributed to deprotonation of a second-sphere tyrosine. Conversely, our data obtained on oxidized FeSOD indicate that hydroxide binding is responsible for the observed active-site pK for this species. Intriguingly, in the Fe-substituted form of MnSOD this identical chemical event occurs at a significantly lower pH. Overall, our results suggest an important role for second-sphere amino acids in tuning the active sites' interaction with small anions and bring into question the assumption that these homologous enzymes operate by the same molecular mechanism.
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U2 - 10.1021/ja0266058
DO - 10.1021/ja0266058
M3 - Article
C2 - 12207539
AN - SCOPUS:0037063508
SN - 0002-7863
VL - 124
SP - 10833
EP - 10845
JO - Journal of the American Chemical Society
JF - Journal of the American Chemical Society
IS - 36
ER -