PqqE is a radical S-adenosyl-l-methionine (SAM) enzyme that catalyzes the initial reaction of pyrroloquinoline quinone (PQQ) biosynthesis. PqqE belongs to the SPASM (subtilosin/PQQ/anaerobic sulfatase/mycofactocin maturating enzymes) subfamily of the radical SAM superfamily and contains multiple Fe–S clusters. To characterize the Fe–S clusters in PqqE from Methylobacterium extorquens AM1, Cys residues conserved in the N-terminal signature motif (CX3CX2C) and the C-terminal seven-cysteine motif (CX9–15GX4CXnCX2CX5CX3CXnC; n = an unspecified number) were individually or simultaneously mutated into Ser. Biochemical and Mössbauer spectral analyses of as-purified and reconstituted mutant enzymes confirmed the presence of three Fe–S clusters in PqqE: one [4Fe–4S]2+ cluster at the N-terminal region that is essential for the reductive homolytic cleavage of SAM into methionine and 5′-deoxyadenosyl radical, and one each [4Fe–4S]2+ and [2Fe–2S]2+ auxiliary clusters in the C-terminal SPASM domain, which are assumed to serve for electron transfer between the buried active site and the protein surface. The presence of [2Fe–2S]2+ cluster is a novel finding for radical SAM enzyme belonging to the SPASM subfamily. Moreover, we found uncommon ligation of the auxiliary [4Fe–4S]2+ cluster with sulfur atoms of three Cys residues and a carboxyl oxygen atom of a conserved Asp residue.

Saichana N., Tanizawa K., Ueno H., Pechousek J., Novak P., Frebortova J.: „Characterization of auxiliary iron-sulfur clusters in a radical S-adenosylmethionine enzyme PqqE from Methylobacterium extorquens AM1“, FEBS OPEN BIO, 7(12) (2017) 1864-1879. UPOL IF: 2,143. DOI: 10.1002/2211-5463.12314