The HupDCG protein complex is a putative ABC-transporter for heme in the pathogenic Gram-positive bacterium Listeria monocytogenes, where HupD functions as a heme-binding protein. UV-vis absorption, EPR, and resonance Raman spectroscopy have revealed that HupD binds a heme with two histidine residues as the axial ligands. His 105 and His259 are identified as the axial ligands by site-directed mutagenesis. HupD is the first example of a heme-binding protein having a bis-histidine coordination environment among the heme-binding proteins working in bacterial heme acquisition systems. While mutation of His259 to Ala resulted in a loss in the heme-binding ability of HupD, the H105A variant of HupD retained its heme-binding ability with lower heme-binding affinity compared with the wild type. These results suggest that His259 is an essential ligand for heme acquisition by HupD and that His105 might be responsible for regulation of the heme-binding affinity of HupD during the heme-transport process.
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