Spectroscopic studies of the effect of ligand donor strength on the Fe-NO bond intradiol dioxygenases.
The geometric and electronic structure of NO bound to reduced protocatechuate 3,4-dioxygenase and its substrate (3,4-dihydroxybenzoate, PCA) complex have been examined by X-ray absorption (XAS), UV-vis absorption (Abs), magnetic circular dichroism (MCD), and variable temperature variable field (VTVH) MCD spectroscopies. The results are compared to those previously published on model complexes described as [FeNO]7 systems in which an S = 5/2 ferric center is antiferromagnetically coupled to an S = 1 NO-. XAS pre-edge analysis indicates that the Fe-NO units in FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] lack the greatly increased pre-edge intensity representative of most [FeNO]7 model sites. Furthermore, from extended X-ray absorption fine structure (EXAFS) analysis, the FeIIIPCD[NO-] and FeIIIPCD[PCA,NO-] active sites are shown to have an Fe-NO distance of at least 1.91 A, approximately 0.2 A greater than those found in the model complexes. The weakened Fe-NO bond is consistent with the overall lengthening of the bond lengths and the fact that VTVH MCD data show that NO(-)-->FeIII CT transitions are no longer polarized along the z-axis of the zero-field splitting tensor. The weaker Fe-NO bond derives from the strong donor interaction of the endogenous phenolate and substrate catecholate ligands, which is observed from the increased intensity in the CT region relative to that of [FeNO]7 model complexes, and from the shift in XAS edge position to lower energy. As NO is an analogue of O2, the effect of endogenous ligand donor strength on the Fe-NO bond has important implications with respect to O2 activation by non-heme iron enzymes.
Wasinger
Erik C
EC
Department of Chemistry, Stanford University, Stanford, California 94305, USA.
Davis
Mindy I
MI
Pau
Monita Y M
MY
Orville
Allen M
AM
Zaleski
Jeffrey M
JM
Hedman
Britt
B
Lipscomb
John D
JD
Hodgson
Keith O
KO
Solomon
Edward I
EI
D000465:Algorithms; D001665:Binding Sites; D002942:Circular Dichroism; D018360:Crystallography, X-Ray; D007501:Iron / Q000737:chemistry*; D008024:Ligands; D008958:Models, Molecular; D008968:Molecular Conformation; D010084:Oxidation-Reduction; D011520:Protocatechuate-3,4-Dioxygenase / Q000737:chemistry*; D017550:Spectroscopy, Fourier Transform Infrared; D013057:Spectrum Analysis
D008024:Ligands; D007501:Iron; D011520:Protocatechuate-3,4-Dioxygenase