Andrew Fielding's doctoral research focused on understanding the mechanism of O2 activation and catalysis by two similar catechol dioxygenases: Fe(II)-homoprotocatechuate 2,3-dioxygenase (Fe-HPCD) and Mn(II)-MndD. He prepared and characterized the cobalt-substituted variant of HPCD, which showed higher activity than Mn- or Fe-HPCD despite Co(II) being a poorer reducing agent. Using electron-poor substrate analogs, he was able to trap and characterize three O2 intermediates by EPR, providing new insights into dioxygenase mechanisms. Comparing the properties of different metal-substituted enzymes allowed full characterization