Structural Basis for Heme Transport By Hmut in Corynebacterium Glutamicum

Heme can be used as an iron source by some pathogenic bacteria, which have the heme acquisition system(s) to uptake heme. The typical heme acquisition system comprises several heme-binding proteins and an ABC transporter. The ABC heme transporter, HmuT-HmuUV is conserved in Gram-positive and Gram-negative bacteria. In this heme transporter system, HmuT, HmuU, and HmuV act as a substrate (heme)-binding protein, a heme permease, and an ATP-binding protein, respectively.

 In this study, the crystal structure of the heme-binding protein HmuT (CgHmuT) from Gram-positive bacterium Corynebacterium glutamicum will be reported at 1.4 Å resolution. The overall structure of CgHmuT reveals a bi-lobed topology formed by an N- and C-terminal domain. A heme molecule coordinated by His141 and Tyr242 is bound in a cleft between the N- and C-terminal domains. Though the global fold of CgHmuT is similar to that of HmuT from Yersinia pestis (YpHmuT), the coordination structure of the heme is different from each other. While two heme molecules are bound to YpHmuT, CgHmuT binds a single heme molecule with His141 and Tyr242 as the axial ligands. 

 Intriguingly, CgHmuT binds a heme with two different orientations. Though the previously reported heme-binding proteins have the interaction between proteins and heme propionate groups in addition to axial ligation to iron, such interactions with heme propionates are absent in CgHmuT. The presence of two different conformations in CgHmuT might be due to a lack of specific interactions between the heme and protein except for the axial ligations.

 EPR spectroscopy reveals the heme in CgHmuT is a mixture of high-spin and low-spin states, indicating one of the axial ligand of the heme is in equilibrium between the ligated and unligated states. EPR spectra of wild-type, H141A-, and Y242A-CgHmuT suggest His141 will be reversibly dissociated/associated from/to the heme. The equilibrium between the six- and five-coordinate states in CgHmuT would be responsible for the regulation of heme-binding affinity to control the heme transport reaction between CgHmuT and HmuUV.

 The crystal structures of H141A and Y242A variants have also been determined in this study. Iwill discuss the molecular mechanism of heme recognition and heme transport by CgHmuT based on these structures.