Abstract
Halophilic euryarchaea lack many of the genes necessary for the protoporphyrin-dependent heme biosynthesis pathway previously identified in animals and plants. Bioinformatic analysis suggested the presence of two heme biosynthetic processes, an Fe-coproporphyrinogen III (coproheme) decarboxylase (ChdC) pathway and an alternative heme biosynthesis (Ahb) pathway, in Haloferax volcanii. PitA is specific to the halophilic archaea and has a unique molecular structure in which the ChdC domain is joined to the antibiotics biosynthesis monooxygenase (ABM)-like domain by a histidine-rich linker sequence. The pitA gene deletion variant of H. volcanii showed a phenotype with a significant reduction of aerobic growth. Addition of a protoheme complemented the phenotype, supporting the assumption that PitA participates in the aerobic heme biosynthesis. Deletion of the ahbD gene caused a significant reduction of only anaerobic growth by denitrification or dimethylsulfoxide (DMSO) respiration, and the growth was also complemented by addition of a protoheme. The experimental results suggest that the two heme biosynthesis pathways are utilized selectively under aerobic and anaerobic conditions in H. volcanii. The molecular structure and physiological function of PitA are also discussed on the basis of the limited proteolysis and sequence analysis.
Highlights
Heme plays an important biochemical role as a coenzyme of the proteins catalyzing various redox reactions [1]
The phenotypic data on the pitA deletion variant presented in this paper is consistent with the suggestion of Dailey and Gerdes [8] that PitA might participate in the final step of the ChdC pathway in H. volcanii
Bioinformatic analysis of H. volcanii genome together with the phenotypic characterization of the ahbD deletion variant suggests the presence of a functioning anaerobic heme biosynthetic process, by the alternative heme biosynthesis (Ahb) pathway
Summary
Heme plays an important biochemical role as a coenzyme of the proteins catalyzing various redox reactions [1]. Two biosynthetic pathways, of which the biochemical processes differ from that of the protoporphyrin-dependent pathway, have been newly found in some bacteria and archaea (S1 Fig). An alternative heme biosynthesis (Ahb) pathway has been identified in anaerobic microbes including anaerobic sulfate- or nitrate-respiring bacteria. In the Ahb pathway, siroheme, the intermediate compound generated from uroporphyrinogen III, is converted to protoheme through oxidative decarboxylating reactions [4, 6]. The protoporphyrin-dependent pathway proceeds aerobically, and oxygen molecules (O2) are used for oxidative decarboxylation of the propionyl bases of the intermediate compounds [2]. All the reaction steps of the Ahb pathway proceed under anaerobic conditions, and the radical SAM enzymes catalyze the oxidative decarboxylation of the intermediates to yield protoheme [4,5,6]
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