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Thiosulfate dehydrogenase (TsdA) catalyzes the oxidation of two thiosulfate molecules to form tetrathionate and is predicted to use an unusual cysteine-ligated heme as the catalytic cofactor. We have determined the structure of Allochromatium vinosum TsdA to a resolution of 1.3 Å. This structure confirms the active site heme ligation, identifies a thiosulfate binding site within the active site cavity, and reveals an electron transfer route from the catalytic heme, through a second heme group to the external electron acceptor. We provide multiple lines of evidence that the catalytic reaction proceeds through the intermediate formation of a S-thiosulfonate derivative of the heme cysteine ligand: the cysteine is reactive and is accessible to electrophilic attack; cysteine S-thiosulfonate is formed by the addition of thiosulfate or following the reverse reaction with tetrathionate; the S-thiosulfonate modification is removed through catalysis; and alkylating the cysteine blocks activity. Active site amino acid residues required for catalysis were identified by mutagenesis and are inferred to also play a role in stabilizing the S-thiosulfonate intermediate. The enzyme SoxAX, which catalyzes the first step in the bacterial Sox thiosulfate oxidation pathway, is homologous to TsdA and can be inferred to use a related catalytic mechanism.

Original publication




Journal article


J Biol Chem

Publication Date





9209 - 9221


Bacterial Metabolism, Cytochrome, Electron Transport, Heme, Metalloenzyme, Sox System, Sulfur Oxidizing Bacteria, Thiosulfate Dehydrogenase, Amino Acid Sequence, Bacteria, Bacterial Proteins, Base Sequence, Cysteine, Cytochromes, DNA Primers, Mass Spectrometry, Molecular Sequence Data, Oxidation-Reduction, Protein Conformation, Sequence Homology, Amino Acid, Spectrophotometry, Ultraviolet, Thiosulfates