Online

Conger, Matthew A.

Chemistry

2018

Ph. D.

IsdG and IsdI are non-canonical heme oxygenases (HO) from Staphylococcus aureus that catalyze the oxidative cleavage of heme to give novel organic products (staphylobilins) and iron as a nutrient for the pathogen. Comparison of the reported equilibrium dissociation constant (Kd) values for heme from IsdG and IsdI compared to the reported concentration of the labile heme pool called into question whether these enzymes are competent HOs in vivo. We took advantage of a second-sphere Trp whose fluorescence is quenched upon heme binding, which led to Kd values 2-3 orders of magnitude smaller than reported in the literature. Importantly, these Kd values were on the same order of magnitude as human HO, precluding design of a competitive inhibitor as an effective therapeutic. Based upon the kinetic and equilibrium data, and the finding that the half-life of IsdG is increased 2.5-fold by the presence of heme, we proposed IsdG is the main HO involved in iron acquisition which motivated further characterization of IsdG. IsdG-catalyzed heme catabolism proceeds through ferric-peroxoheme and meso-hydroxyheme intermediates en route to staphylobilin. A second-sphere Asn is known to be critical for enzymatic function, but its role in heme cleavage was unknown. Site-directed mutagenesis was employed to probe the role of Asn using ferric-azidoheme and ferric-cyanoheme as models of the putative ferric-peroxoheme intermediate. An optical spectroscopic study established that a hydrogen-bond between Asn and the iron-ligating ([alpha]) atom of the distal ligand perturbs the heme electronic structure. Density functional theory (DFT) suggested this hydrogen-bond triggers rotation of the distal ligand, which was corroborated by circular dichroism (CD), and delocalizes spin density onto the meso carbons. Electron paramagnetic resonance (EPR) revealed the Asn hydrogen-bond increases the Fe 3dxy character in the singly occupied molecular orbital (SOMO), a mechanism that can increase spin density on the meso carbons. Finally, the Asn hydrogen-bond moves the meso carbon resonances downfield in the 13C nuclear magnetic resonance (NMR) spectrum, consistent with excess spin density, confirming a DFT-predicted, Asn-induced spin delocalization. These results suggest IsdG funnels the reactivity of ferric-peroxoheme toward heme hydroxylation through an Asn-dependent bridged transition state, circumventing production of reactive, uncontrolled intermediates.