The Regulatory Function of Heme Oxygenase in Modulating Mammalian Heme Bioavailability and Homeostasis
Creators: name: Liu, Liu
Publisher: name: My University
Year: 2022
Subjects: subject: Heme oxygenase and heme homeostasis
subject: Biological Chemistry
subject: Health Sciences

description: Heme is an essential cofactor that participates in a myriad of biological processes. The vast number and variety of heme-associated processes necessitates the tightly controlled distribution and bioavailability of heme to ensure its proper amount and function. Heme oxygenase (HO) is the only known enzyme that degrades heme to biliverdin, Fe2+, and carbon monoxide, forming an essential link in the maintenance of mammalian heme homeostasis and bioavailability. HO1 and HO2 are two HO isoforms that possess similar structures of the catalytic core and exhibit similar enzymatic activities. A significant difference between HO1 and HO2 exists in the intrinsically disordered region between their catalytic cores and C-terminal membrane spanning regions. In this region, HO1 possesses a PEST domain, which directs it to proteasomal degradation after its overexpression upon heme overload stimulus; whereas, in HO2, this region harbors two heme regulatory motifs (HRMs), a conserved Cys-Pro motif that has been proposed to mediate proteasomal degradation upon heme binding in other HRM-containing proteins. However, by using biochemical and cell biological approaches, my work recently demonstrated that the degradation of HO2 is processed by the lysosome through chaperone-mediated autophagy (CMA) and this event is post-translationally regulated by heme occupancy at the HO2 catalytic site, not the HRMs, differentiating HO2 from other HRM-containing proteins. Therefore, we propose that HO2 responds to heme deficiency and helps restore heme homeostasis through decreasing its own activity. Apart from regulating heme homeostasis through tuning enzymatic activity, HOs also play a role in systemic heme trafficking and bioavailability through binding and buffering heme at different heme binding sites. The fluorescence and UV-visible spectroscopic data demonstrate that both HO1 and HO2 interact with and acquire their heme from a recently discovered heme chaperone, glyceraldehyde phosphate dehydrogenase (GAPDH). However, the amount of heme transferred to HO1 is significantly less than that to HO2, indicating that GAPDH-HO1 heme transfer may be facilitated by extracellular components. By using our toolbox of FlAsH-labeled HO2 variants with heme pulse-chase assays in HEK293 cells, we found that GAPDH delivers heme to both the catalytic core and the HRMs of HO2. Interestingly, kinetic data indicates that the HRMs receive their heme from GAPDH faster than the catalytic core of HO2; this observation is consistent with a model in which the HRMs bind heme and actively transfer them to the catalytic core for degradation through intramolecular domain-domain interaction. I am currently developing a novel FRET-based assay to probe the heme-related conformational changes of HO2 as well as the ensemble of HO2 dynamics during the intramolecular heme transfer process. Collectively, my work has provided molecular insights into HO’s cytoprotective function and how it fits into the broader regulatory network of cellular heme trafficking, bioavailability, and homeostasis.