Discovery and Characterization of HemQ

Abstract

Here we identify a previously undescribed protein, HemQ, that is required for heme synthesis in Gram-positive bacteria. We have characterized HemQ from Bacillus subtilis and a number of Actinobacteria. HemQ is a multimeric heme-binding protein. Spectroscopic studies indicate that this heme is high spin ferric iron and is ligated by a conserved histidine with the sixth coordination site available for binding a small molecule. The presence of HemQ along with the terminal two pathway enzymes, protoporphyrinogen oxidase (HemY) and ferrochelatase, is required to synthesize heme in vivo and in vitro. Although the exact role played by HemQ remains to be characterized, to be fully functional in vitro it requires the presence of a bound heme. HemQ possesses minimal peroxidase activity, but as a catalase it has a turnover of over 10 4 min ؊1 . We propose that this activity may be required to eliminate hydrogen peroxide that is generated by each turnover of HemY. Given the essential nature of heme synthesis and the restricted distribution of HemQ, this protein is a potential antimicrobial target for pathogens such as Mycobacterium tuberculosis.

Unusual Peroxide-Dependent, Heme-Transforming Reaction Catalyzed by HemQ

“…This idea is consistent with the previously documented interplay of HemY, HemH, and HemQ, where the kinetics of HemH/Y are altered by the presence of HemQ. 24 It could also explain how a catalase positive organism like S. aureus , which avidly detoxifies H 2 O 2 , might be able to manage and use H 2 O 2 as an essential enzyme substrate.…”

“…Other features typically assigned to vibrations involving the porphyrin pyrrole rings also track the extent of oxidative decarboxylation, including: the increase in ν 7 (pyrrole deformation mode), ν 8 (Fe−N pyrrole stretch), and ν 11 (pyrrole asymmetric folding mode) intensities and the loss in intensity of ν 15 and ν 5 (pyrrole symmetric fold). 20-24 These observations are consistent with changes occurring upon oxidative decarboxylation of propionate groups to yield vinyl substituents.…”

“…Consistent with that conclusion, a full eq of the product heme b was not observed with a stoichiometric addition of H 2 O 2 ( Figure 3 ), suggesting concurrent conversion of coproheme III, H 2 O 2 -mediated degradation of heme b , and possible catalytic disproportionation of H 2 O 2 . 9,24 Heme b degradation was driven to completion with the addition of ~100 eq of H 2 O 2 ( Figure S4 ).…”

Prokaryotic Heme Biosynthesis: Multiple Pathways to a Common Essential Product

“…It is now known that some archaea and sulfate-reducing bacteria synthesize siroheme and then convert it to protoheme (11,42), Gram-positive hemesynthesizing bacteria go through a set of enzymes that utilize a coproporphyrin intermediate (35,(43)(44)(45), and Gram-negative bacteria have a set of enzymes that go through a protoporphyrin intermediate (35). These various pathways to heme in prokaryotes have acquired a variety of common names, such as primitive (46), alternate (11), transitional (43), and classic.…”

The HemQ coprohaem decarboxylase generates reactive oxygen species: implications for the evolution of classical haem biosynthesis

“…Interestingly, the enzyme HemY from Bacillus subtilis has been shown to oxidise both coproporphyrinogen III and protoporphyrinogen IX [12,13]. Further studies reported that a haem-binding protein HemQ that is required for the terminal steps of haem synthesis in species of Actinobacteria and Firmicutes had peroxidase and catalase activity and could stimulate the oxidation of protoporphyrin IX by HemY [14]. S. aureus hemQ mutants were then shown to accumulate coproporphyrin III, and the HemQ enzyme was reported to lyse haem in the presence of peroxide/chlorite [15].…”