Structure-Function Relationship study of HupZ

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Abstract A heme utilization protein operon hupYZ encoding periplasmic proteins has recently been identified from Streptococcus pyogenes, a significant pathogen involved in a wide range of diseases. One of the encoded proteins, HupZ, was initially assigned as a heme oxygenase. However, our study revealed that this weak activity was due to a poly‐His‐tag induced structural similarity with the active site of genuine heme oxygenases. The tag‐free HupZ does not degrade heme; however, we recently discovered that tag‐free HupZ binds FMN. The FMN‐bound HupZ crystal structure superimposes with the biliverdin reductase (BVR) recently found in Mycobacterium tuberculosis (Mtb) that utilizes an F420H2 cofactor. The flavin moiety of FMN in HupZ aligns well with the deazaflavin moiety of the F420H2 cofactor. Therefore, we propose that HupZ is a flavin/deazaflavin oxidoreductase (FDOR) dependent BVR in the heme utilization pathway (Hup) of Streptococcus. Structural and functional characterizations are proposed to further the molecular understanding of this Hup protein using biochemical, spectroscopic, and structural approaches. We will interrogate how tag‐free HupZ interacts with its cofactor and substrate, and we will determine the critical residues involved in biliverdin reduction. We will attempt to characterize catalytic intermediates and elucidate the catalytic mechanism. Completing the proposed biochemical and structural studies will fill the gaps in our current understanding of the heme utilization pathway in a significant pathogen. Only a few FDOR‐dependent enzymes have been described, and the F420H2‐dependent BVR mechanism remains to be elucidated; therefore, this work is also highly significant to mechanistic enzymology.