Book Description

Fe(II)- and 2-oxoglutarate (2OG)-dependent oxygenases utilize a non-heme mononuclear Fe(II) cofactor to catalyze oxidative transformations of unreactive aliphatic carbon centers in a wide variety of biological substrates. The 2OG cosubstrate allows the enzyme to access the oxidizing potential of molecular oxygen to generate a highly reactive Fe(IV)-oxo (ferryl) intermediate. This species is able to abstract an H-atom from the substrate and, in the most common outcome hydroxylation the enzyme subsequently couples the resulting OH group to a carbon-centered radical on the substrate. Excitingly, the biosynthetic capacity of this platform has expanded to include desaturation, C-O/C bond formation, halogenation, endoperoxidation, epoxidation, stereo-inversion, and even the formation of ethylene. The Fe/2OG oxygenases are considered ideal candidates for biotechnology applications owing to their catalytic diversity, simple and readily available cofactors/cosubstrates, and ability to activate inert C-H bonds. To capitalize on this promise and successfully harness this enzyme scaffold for biotechnology purposes, it is necessary to obtain detailed mechanistic and structural information, particularly for non-hydroxylation systems. The mechanism of OH installation by the Fe/2OG oxygenases is largely understood. In the non-hydroxylases reactivity likely diverges after the substrate hydrogen-atom transfer (HAT) step, resulting in alternate transformation of the carbon-centered radical. It is likely that tight spatial control of the substrate HAT target and the oxygen-derived ligands via interaction with specific active site residues and other components of the Fe coordination sphere are crucial for controlling reaction outcome. Although many Fe/2OG hydroxylases are well-characterized via x-ray crystallography, comprehensive high-resolution structural data for complete enzyme-substrate reactant complexes is lacking for non-hydroxylation systems. Here, we will explore the structural properties of non-canonical Fe/2OG oxygenases, in particular the features that dictate reactivity. A novel set of halogenase crystal structures revealed important active site features for selective catalysis. These findings subsequently allowed for the first successful demonstration of novel halogenation activity from a hydroxylating scaffold. Furthermore, crystallographic snapshots of a hydroxylating system allowed for the visualization of a previously unobserved intermediate and new structural probe for the elusive ferryl species. This work has enabled development of universal hypotheses for control of reaction outcome in these enzymes.