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Regulators of iron balance in humans.
The dissertation outlines the progress made since its initiation in understanding the mechanism(s) that regulate Abcb6 expression and the significance of Abcb6 expression to cellular heme homeostasis.
Because the availability of heme A is a key determinant in the assembly of CcO, the reactions catalyzed by HOS and HAS are likely targets for regulation. Because of its high reduction potential (), free heme A is potentially even more toxic than free heme B. Therefore, cells presumably must have a controlled mechanism to regulate the synthesis, transport, and insertion of heme A to ensure that there is sufficient but not excess heme A available in cells. Although the presence of heme A is strongly connected to copper availability under most circumstances, neither the transcription, translation, stability, nor the activity of either HOS or HAS is affected by intracellular copper levels ().
Regulation of the heme a biosynthetic pathway …
The assembly and activity of cytochrome c oxidase is dependent on the availability of heme A, one of its essential cofactors. In eukaryotes, two inner mitochondrial membrane proteins, heme O synthase (Cox10) and heme A synthase (Cox15), are required for heme A biosynthesis. In this report, we demonstrate that in Saccharomyces cerevisiae the transcription of COX15 is regulated by Hap1, a transcription factor whose activity is positively controlled by intracellular heme concentration. Conversely, COX10, the physiological partner of COX15, does not share the same regulatory mechanism with COX15. Interestingly, protein quantification identified an 8:1 protein ratio between Cox15 and Cox10. Together, these results suggest that heme A synthase and/or heme O synthase might play a new, unidentified role in addition to heme A biosynthesis.
N2 - The assembly and activity of cytochrome c oxidase is dependent on the availability of heme A, one of its essential cofactors. In eukaryotes, two inner mitochondrial membrane proteins, heme O synthase (Cox10) and heme A synthase (Cox15), are required for heme A biosynthesis. In this report, we demonstrate that in Saccharomyces cerevisiae the transcription of COX15 is regulated by Hap1, a transcription factor whose activity is positively controlled by intracellular heme concentration. Conversely, COX10, the physiological partner of COX15, does not share the same regulatory mechanism with COX15. Interestingly, protein quantification identified an 8:1 protein ratio between Cox15 and Cox10. Together, these results suggest that heme A synthase and/or heme O synthase might play a new, unidentified role in addition to heme A biosynthesis.
Heme biosynthesis and regulation in the filamentous …
Although, 5-aminolevulinic acid synthase (ALAS) mediated regulation of heme synthesis is considered the key step in heme biosynthesis, recent reports have identified a second regulatory step in heme biosynthesis mediated by the mitochondrial ATP binding cassette transporter b6 (Abcb6).
Handschin C, Lin J, Rhee J, et al. (2005) Nutritional regulation of hepatic heme biosynthesis and porphyria through PGC‐1alpha. Cell 122 (4): 505–515.
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Heme biosynthesis and its regulation - NARCIS
To identify whether the stability of Cox10 is altered by the presence of its physiological partner Cox15 or by CcO, we determined the Cox10 protein level in cox15 Δ and mss51Δ strains. The comparison between mss51Δ cells and wild-type cells demonstrates that the protein level of Cox10 is affected neither by Mss51 nor by the presence/absence of CcO. Similarly, the stability of Cox10 is not affected by the absence of its physiological partner Cox15 (data not shown). Thus, HOS appears to be regulated completely independently of HAS.
Regulation of Heme Biosynthesis
The Stability of Cox15 or Cox10 Is Not Decreased in the Absence of Cytochrome c Oxidase—It has been reported previously that heme A levels decreased dramatically in cells deficient in CcO (). It is therefore reasonable to hypothesize that the stability of Cox15 might be down-regulated by a key subassembly complex of CcO, especially the one containing Cox1, the final heme A acceptor. To ascertain if Cox15 stability is decreased in the absence of CcO, we genomically inserted a 13Myc epitope tag to the C terminus of Cox15 in an mss51Δ strain (W303a, mss51::HIS3; cox15::cox15-Myc-HIS3). MSS51 is a nuclear gene encoding for a translational activator of Cox1, subunit I of CcO (). In the absence of Mss51, the translation of Cox1 is completely blocked, resulting in total loss of CcO activity. We utilized quantitative Western blot analysis to compare the Cox15 levels in the wild-type strain versus the mss51Δ strain (). The data demonstrated that there is not a decrease in Cox15 levels in an mss51Δ strain, suggesting that the stability of Cox15 is not affected by Mss51, Cox1, or a Cox1-associated subassembly complex of CcO.
179, 1997 REGULATION OF HEME BIOSYNTHESIS IN S
COX10 Is Not a Heme B-regulated Gene—Because Cox10 and Cox15 work in concert to synthesize heme A, we hypothesized that the expression of Cox10 and Cox15 might be coordinated and regulated by a similar mechanism to form a 1:1 stoichiometry. To probe the regulation of COX10, we transformed pCOX10/lacZ into MHY100 and MHY200 strains grown under various ALA concentrations. The β-galactosidase activity assay () demonstrated that the expression of COX10/lacZ is independent of intracellular heme B levels and Hap1, indicating that COX10 is not utilizing the same regulatory mechanism as COX15. We also searched the promoter region of COX10, and no potential Hap1 binding site was identified. Together, our data indicate that COX10 is regulated neither by intracellular heme B levels nor by Hap1.
Regulation of the Heme A Biosynthetic Pathway - …
Regulation of COX15 under Anaerobic Conditions—Because of the central role that heme A plays in the assembly and function of CcO, it is reasonable to ask whether COX15 might also be regulated by O2. Clearly O2 will indirectly affect COX15 transcription because intracellular heme levels in yeast are dependent on O2 concentrations, and heme obviously regulates COX15 transcription via Hap1. However, might O2 also play a more direct role in regulating COX15?
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