Heme regulatory motifs (HRMs) are present in many proteins that are involved in diverse biological functions. The C-terminal tail region of human heme oxygenase-2 (HO2) contains two HRMs whose cysteine residues form a disulfide bond; when reduced, these cysteines are available to bind Fe3+-heme. Heme binding to the HRMs occurs independently of the HO2 active site in the core of the protein, where heme binds with high affinity and is degraded to biliverdin. Members of the Ragsdale lab and their collaborators characterized the protein-mediated transfer of heme between the HRMs and the HO2 core and used their results to propose a model in which HRM-bound Fe3+-heme is readily transferred to the catalytic site for degradation to facilitate turnover, but can also equilibrate between the sites to maintain heme homeostasis.
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