Biochemical and Biophysical Studies of Heme Binding Proteins from the Corynebacterium diphtheriae and Streptococcus pyogenes Heme Uptake Pathways

The Gram-positive pathogens Corynebacterium diphtheriae and Streptococcus pyogenes both require iron for survival. These bacteria have developed sophisticated heme uptake and transport protein machinery responsible for the import of iron into the cell, in the form of heme from the human host. The heme utilization pathway (hmu) of C. diphtheriae utilizes multiple proteins to bind and transport heme into the cell. One of these proteins, HmuT, delivers heme to the ABC transporter HmuUV. The axial ligation of the heme in HmuT was probed by examination of wild-type HmuT and a series of conserved heme pocket residue mutants, H136A, Y235A, R237A, Y272A, M292A, Y349A, and Y349F. Characterization by UV-visible absorption, resonance Raman, and magnetic circular dichroism spectroscopies indicated that H136 and Y235 are the axial ligands in HmuT. Electrospray ionization mass spectrometry was also utilized to assess the roles of conserved residues in contribution to heme binding. The S. pyogenes streptococcal iron acquisition (sia)/heme transport system (hts) utilizes multiple proteins to bring host heme to the intracellular space. Both the substrate binding protein SiaA and the hemoprotein surface receptor Shr were investigated. The kinetic effects on SiaA heme release were probed through chemical unfolding of axial ligand mutants M79A and H229A, as well mutants thought to contribute to heme binding, K61A and C58A, and a control mutant, C47A. The unfolding pathways showed two processes for protein denaturation. This is consistent with heme loss from protein forms differing by the orientation of the heme in the binding pocket. The ease of protein unfolding is related to the strength of interaction of the residues with the heme. Shr contains two NEAT (near-iron transporter) domains (Shr-N1 and Shr-N2) which can both bind heme. Biophysical studies of both Shr-N1 and Shr-N2 indicated a new class of NEAT domains which utilize methionine as an axial ligand, rather than a tyrosine. Thermal and chemical unfolding showed ferrous Shr-N1 and Shr-N2 to be most resistant to denaturation. Shr-N2 was prone to autoreduction. Together, sequence alignment, homology modeling, and spectral signatures are all consistent with two methionines as the heme ligands of this novel type of NEAT heme-binding domain