Abstract
Corynebacteria contain a heme uptake system encoded in hmuTUV genes, in which HmuT protein acts as a heme binding protein to transport heme to the cognate transporter HmuUV. The crystal structure of HmuT from Corynebacterium glutamicum (CgHmuT) reveals that heme is accommodated in the central cleft with His141 and Tyr240 as the axial ligands and that Tyr240 forms a hydrogen bond with Arg242. In this work, the crystal structures of H141A, Y240A, and R242A mutants were determined to understand the role of these residues for the heme binding of CgHmuT. Overall and heme environmental structures of these mutants were similar to those of the wild type, suggesting that there is little conformational change in the heme-binding cleft during heme transport reaction with binding and the dissociation of heme. A loss of one axial ligand or the hydrogen bonding interaction with Tyr240 resulted in an increase in the redox potential of the heme for CgHmuT to be reduced by dithionite, though the wild type was not reduced under physiological conditions. These results suggest that the heme environmental structure stabilizes the ferric heme binding in CgHmuT, which will be responsible for efficient heme uptake under aerobic conditions where Corynebacteria grow.
Highlights
Heme can be used as an iron source in some bacteria, for which a heme uptake system is required
The purified heme-bound proteins were crystallized, and the crystal structures of H141A, Y240A, and R242A-CgHmuT were determined in the holo-form at the resolution of 1.30, 1.65, and 1.30 Å, respectively
As CgHmuT is a substrate binding protein that shuttles heme to the cognate ATP-binding cassette (ABC)-type heme transporter CgHmuUV in Corynebacteria [19], the regulation of heme binding and release is very important for its physiological function
Summary
Heme can be used as an iron source in some bacteria, for which a heme uptake system is required. Gram-negative and Gram-positive bacteria utilize different heme uptake machineries [1,2,3,4,5]. In Gram-negative bacteria, heme uptake machineries consist of three components: a receptor protein at the outer membrane, a periplasmic heme-binding protein, and an ABC-type heme transporter at the inner membrane [4,5]. The receptor proteins bind heme-containing proteins or heme-bound hemophore and transport heme into the periplasm in a TonB-dependent manner via an energy-driven process [4,5]. Once the heme is transported into the periplasm, the heme is bound by a periplasmic.
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