Thursday, October 10th, 2013
Matthew Liptak
Department of Chemistry, UVM"Heme Iron Acquisition by Pathogenic Organisms: Functional Insights from Spectroscopy and Theory"
Mycobacterium tuberculosis MhuD is a heme degrading enzyme distinct from canonical heme oxygenases, which stabilize an S = ½, (dxy)2(dxz,dyz)3 Fe(III)–hydroperoxy state, promote homolytic cleavage of the O–O bond, and utilize an ordered cluster of active site water molecules on the distal side of heme to promote nearly barrier-free attack of the α-meso carbon by the resulting hydroxyl radical. The MhuD active site does not contain an ordered cluster of active site water molecules and MhuD generates unique products from canonical heme oxygenases indicating that a different enzymatic mechanism is involved. Here, I will present variable temperature nuclear magnetic resonance and variable temperature, variable field magnetic circular dichroism evidence for the presence of a mixture of at least two S=1/2 Fe(III) states in MhuD at physiologically relevant temperatures. I will also present our spectroscopic evidence for the importance of a conserved, proximal side, tryptophan residue in determining the balance of the equilibrium between the two states. In addition to the critical catalytic role of the conserved tryptophan residue, we have also concluded that a conserved, distal side, asparagine is critical for activation of the substrate state and orientation of the distal ligand relative to the heme moiety. Finally, I will present older spectroscopic and theoretical data from my Ph.D. thesis of particular interest to the Physics Department as it regards the electronic structure of cobalt phthalocyanine.