Understanding Heme Enzyme Catalysis Starts with Their Active Site Coordination Structure: Identifying Heme Iron Axial Ligands Using Magnetic Circular Dichroism Spectroscopy

Magnetic circular dichroism (MCD) spectroscopy provides diagnostic spectral data sensitive to the identity of the axial ligands and to the spin and oxidation states of heme iron centers in proteins.  In this effort, we have found the proximal ligand His93Gly myoglobin cavity mutant to be a remarkably versatile scaffold for preparation of model heme complexes of defined ligation.  In particular, the difference in accessibility of the two sides of the heme iron center offers the advantage of forming ambient-temperature mixed-ligand heme model complexes, which are very difficult to prepare with model systems in organic solvents.  Moreover, in the H93G Mb system, the protective environment provided by the protein allows for the formation of relatively stable oxyferrous and ferryl [Fe(IV)=O] complexes with variable ligands trans to the normally reactive dioxygen and oxo substituents.  Ferrous, ferric and ferryl His93Gly Mb derivatives with various exogenous ligands have been prepared as models for native heme iron active sites ligated by proximal Lys (amines), Asp or Glu (carboxylates), Tyr (phenols), seleno-Cys (selenols), Cys (thiols) and Met (thioethers).    Building upon this foundation, we have focused our attention on the use of the H93G Mb cavity mutant system to aid our investigation of the coordination structure of novel heme binding and transport proteins and heme-containing oxidative enzymes.  (Funding NIH GM 26730)