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The auxiliary [4Fe-4S] cluster of the Radical SAM heme synthase from Methanosarcina barkeri is involved in electron transfer.

ABSTRACT: The heme synthase AhbD catalyzes the oxidative decarboxylation of two propionate side chains of iron-coproporphyrin III to the corresponding vinyl groups of heme during the alternative heme biosynthesis pathway occurring in sulfate-reducing bacteria and archaea. AhbD belongs to the family of Radical SAM enzymes and contains two [4Fe-4S] clusters. Whereas one of these clusters is required for substrate radical formation, the role of the second iron-sulfur cluster is not known. In this study, the function of the auxiliary cluster during AhbD catalysis was investigated. Two single cluster variants of AhbD from M. barkeri carrying either one of the two clusters were created. Using these enzyme variants it was shown that the auxiliary cluster is not required for substrate binding and formation of the substrate radical. Instead, the auxiliary cluster is involved in a late step of AhbD catalysis most likely in electron transfer from the reaction intermediate to a final electron acceptor. Moreover, by using alternative substrates such as coproporphyrin III, Cu-coproporphyrin III and Zn-coproporphyrin III for the AhbD activity assay it was observed that the central iron ion of the porphyrin substrate also participates in the electron transfer from the reaction intermediate to the auxiliary [4Fe-4S] cluster. Altogether, new insights concerning the completely uncharacterized late steps of AhbD catalysis were obtained.

SUBMITTER: Kuhner M

PROVIDER: S-EPMC6013774 | biostudies-literature | 2016 Jul

REPOSITORIES: biostudies-literature

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Project description:Sulfatases catalyze the cleavage of a variety of cellular sulfate esters via a novel mechanism that requires the action of a protein-derived formylglycine cofactor. Formation of the cofactor is catalyzed by an accessory protein and involves the two-electron oxidation of a specific cysteinyl or seryl residue on the relevant sulfatase. Although some sulfatases undergo maturation via mechanisms in which oxygen serves as an electron acceptor, AtsB, the maturase from Klebsiella pneumoniae, catalyzes the oxidation of Ser72 on AtsA, its cognate sulfatase, via an oxygen-independent mechanism. Moreover, it does not make use of pyridine or flavin nucleotide cofactors as direct electron acceptors. In fact, AtsB has been shown to be a member of the radical S-adenosylmethionine superfamily of proteins, suggesting that it catalyzes this oxidation via an intermediate 5'-deoxyadenosyl 5'-radical that is generated by a reductive cleavage of S-adenosyl- l-methionine. In contrast to AtsA, very little in vitro characterization of AtsB has been conducted. Herein we show that coexpression of the K. pneumoniae atsB gene with a plasmid that encodes genes that are known to be involved in iron-sulfur cluster biosynthesis yields soluble protein that can be characterized in vitro. The as-isolated protein contained 8.7 +/- 0.4 irons and 12.2 +/- 2.6 sulfides per polypeptide, which existed almost entirely in the [4Fe-4S] (2+) configuration, as determined by Mossbauer spectroscopy, suggesting that it contained at least two of these clusters per polypeptide. Reconstitution of the as-isolated protein with additional iron and sulfide indicated the presence of 12.3 +/- 0.2 irons and 9.9 +/- 0.4 sulfides per polypeptide. Subsequent characterization of the reconstituted protein by Mossbauer spectroscopy indicated the presence of only [4Fe-4S] clusters, suggesting that reconstituted AtsB contains three per polypeptide. Consistent with this stoichiometry, an as-isolated AtsB triple variant containing Cys --> Ala substitutions at each of the cysteines in its CX 3CX 2C radical SAM motif contained 7.3 +/- 0.1 irons and 7.2 +/- 0.2 sulfides per polypeptide while the reconstituted triple variant contained 7.7 +/- 0.1 irons and 8.4 +/- 0.4 sulfides per polypeptide, indicating that it was unable to incorporate an additional cluster. UV-visible and Mossbauer spectra of both samples indicated the presence of only [4Fe-4S] clusters. AtsB was capable of catalyzing multiple turnovers and exhibited a V max/[E T] of approximately 0.36 min (-1) for an 18-amino acid peptide substrate using dithionite to supply the requisite electron and a value of approximately 0.039 min (-1) for the same substrate using the physiologically relevant flavodoxin reducing system. Simultaneous quantification of formylglycine and 5'-deoxyadenosine as a function of time indicates an approximate 1:1 stoichiometry. Use of a peptide substrate in which the target serine is changed to cysteine also gives rise to turnover, supporting approximately 4-fold the activity of that observed with the natural substrate.

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The auxiliary [4Fe-4S] cluster of the Radical SAM heme synthase from Methanosarcina barkeri is involved in electron transfer.

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