Variable composition of heme oxygenases with different regiospecificities in Pseudomonas species

Abstract

Heme oxygenases (HO) degrade heme yielding iron, carbon monoxide and one of four possible biliverdin (BV) isomers. Pseudomonas aeruginosa PAO1 is thus far the only organism to contain two HOs with different regiospecificities: BphO and PigA. While BphO cleaves heme to exclusively yield BV IXα, PigA produces the BV isomers IXβ and IXδ. We bioinformatically identified putative HOs in diverse Pseudomonas strains, tested their enzymatic functionality and determined their regiospecificity. Surprisingly, even high amino acid sequence identities to the P. aeruginosa HOs were not sufficient to correctly predict the HO regiospecificity in all cases. Based on our results, Pseudomonas strains differ in their HO composition containing either BphO or PigA or both HO types. Concomitantly with the existence of bphO is the occurrence of at least one gene encoding a bacterial phytochrome implying that only BV IXα is the sufficient phytochrome chromophore. In contrast, pigA genes are organized in gene clusters associated with iron utilization implying a role of PigA in iron acquisition. However, at least in strains containing no PigA this function maybe fulfilled by BphO. Only a combination of homology searches and analyses of genetic environments is appropriate for a reliable prediction of the regiospecificity of Pseudomonas HOs.

In vitro heme biotransformation by the HupZ enzyme from Group A streptococcus

“…Most of the characterized HOs in bacteria are associated with iron acquisition and are negatively regulated by iron (Nobles and Maresso 2011;Tong and Guo 2009). BphO, however, appears to be an exception as it is expressed independently of iron availability and serves only to produce the precursor for the phytochrome chromophore (Barker et al 2012;Gisk et al 2012;Wegele et al 2004).…”

“…Finally, many bacterial HOs cannot reduce and liberate the iron in vitro and thus catalyze the formation of ferricbiliverdin complex as the final product. These include HemO from N. meningitidis (Zhu et al 2000b), BphO from P. aeruginosa (Wang et al 2007), PigA from Pseudomonas species (Gisk et al 2012), and HutZ from V. cholerae (Uchida et al 2012). Considering the absence of sequence homology between HupZ and HO-1, it seems likely that the interactions between HupZ and the eukaryotic CPR are inefficient which may explain the sluggish iron release.…”

“…4). Difficulties in releasing iron from the linear tetrapyrrole in vitro were reported for several HOs (Gisk et al 2012;Reniere et al 2010;Wang et al 2007;Wilks and Schmitt 1998). The efficiency of this final step in heme catalysis often appears to depend on the nature of the redox partner.…”

“…Like the mammalian enzymes, these bacterial HOs have a single compact domain that consists mostly of a-helices, with the heme inserted in between two of the a-helices (Wilks and Heinzl 2014). These bacterial enzymes degrade heme to a-biliverdin, with the exception of PigA, which produces a mixture of band d-biliverdin isomers (Barker et al 2012;Gisk et al 2012;Ratliff et al 2001;Wegele et al 2004). Most of the characterized HOs in bacteria are associated with iron acquisition and are negatively regulated by iron (Nobles and Maresso 2011;Tong and Guo 2009).…”

Phytochrome evolution in 3D: deletion, duplication, and diversification

“…HO may thus have provided a means for living cells to suppress this toxic combination. HO generates BV, carbon monoxide, and free iron as reaction products, permitting iron scavenging (Ratliff et al , ; Fujii et al , ; Gisk et al , ; Duanmu et al , ). Coupling BV to a photoreceptor would provide an elegant means for bacteria to use a single pathway for both sensing and detoxifying the toxic combination of light and oxygen.…”