| Bacterial catechol dioxygenases are important in the aerobic degradation of many aromatic compounds. They can be divided into two broad sub-classes, intra- and extradiol dioxygenases, based on the position of ring cleavage. The intradiol dioxygenases, such as protocatechuate 3,4-dioxygenase (3,4-PCD), contain a high-spin ferric center, whereby binding of substrate activates the substrate ring (PCA) for direct attack by dioxygen and ring cleavage between the hydroxylated carbons. In contrast extradiol dioxygenases, such as 2,3-dihydroxy 1,2-dioxygenase (1,2-DHBD), utilize a high-spin ferrous center and substrate binding activates the iron for dioxygen binding and activation and regiospecific ring cleavage on the distal or proximal side of the hydroxylated carbon atoms.; The geometric and electronic structure of 3,4-PCD and 1,2-DHBD has been examined using absorption (Abs), circular dichroism (CD), and magnetic CD (MCD) spectroscopies combined with quantum chemical calculations. Recent ferric MCD and zero-field splitting methodology was applied to a protein for the first time and showed the nature of phenolate-Fe(III) bonding in the class of iron-tyrosinate proteins. Specifically, in ferric 3,4-PCD the axial tyrosinate, with its larger Fe-O-C angle, was shown to be more weakly bound to the iron than the equatorial tyrosinate which is important since upon substrate binding this axial tyrosinate dissociates. This methodology additionally was used to show that reduction of 3,4-PCD leads to an increased coordination number and that the Fe-NO bond in 3,4-PCD-NO and 3,4-PCD-PCA-NO is much weaker than in model complexes such as [Fe(EDTA)NO]2- due to the higher total donor strength of the protein. Lastly, resting and substrate bound forms of the extradiol dioxygenase 1,2-DHBD were studied. Quantum chemical calculations show that the lack of O2 reactivity for the five coordinate ferrous active site of 1,2-DHBD is due to the high energy of forming a superoxide and an excited state iron. Upon substrate binding a second electron is available, allowing for the more favorable two electron O2 chemistry and the reaction with O2 is fast. The combination of spectroscopy and quantum chemical calculations was used to provide geometric and electronic structure descriptions of the catechol dioxygenases leading to insight into their reactivities. |
