Spectroscopic and Electronic Structure Studies of Protocatechuate 3,4-Dioxygenase: Nature of Tyrosinate−Fe(III) Bonds and Their Contribution to Reactivity

The geometric and electronic structure of the high-spin ferric active site of protocatechuate 3,4-dioxygenase (3,4-PCD) has been examined by absorption (Abs), circular dichroism (CD), magnetic CD (MCD), and variable-temperature−variable-field (VTVH) MCD spectroscopies. Density functional (DFT) and INDO/S-CI molecular orbital calculations provide complementary insight into the electronic structure of 3,4-PCD and allow an experimentally calibrated bonding scheme to be developed. Abs, CD, and MCD indicate that there are at least seven transitions below 35 000 cm-1 which arise from tyrosinate ligand-to-metal-charge transfer (LMCT) transitions. VTVH MCD spectroscopy gives the polarizations of these LMCT bands in the principal axis system of the D-tensor, which is oriented relative to the molecular structure from the INDO/S-CI calculations. Three transitions are associated with the equatorial tyrosinate and four with the axial tyrosinate. This large number of transitions per tyrosinate is due to the π and importantly the σ overlap of the two tyrosinate valence orbitals with the metal d orbitals and is governed by the Fe−O−C angle and the Fe−O−C−C dihedral angles. The previously reported crystal structure indicates that the Fe−O−C angles are 133° and 148° for the equatorial and axial tyrosinate, respectively. Each tyrosinate has transitions at different energies with different intensities, which correlate with differences in geometry that reflect pseudo-σ bonding to the Fe(III) and relate to reactivity. These factors reflect the metal−ligand bond strength and indicate that the axial tyrosinate−Fe(III) bond is weaker than the equatorial tyrosinate−Fe(III) bond. Furthermore, it is found that the differences in geometry, and hence electronic structure, are imposed by the protein. The consequences to catalysis are significant because the axial tyrosinate has been shown to dissociate upon substrate binding and the equatorial tyrosinate in the enzyme−substrate complex is thought to influence asymmetric binding of the chelated substrate moiety via a strong trans influence which activates the substrate for reaction with O2.