title: Mechanism of thiosulfate oxidation in the SoxA family of cysteine-ligated cytochromes.
abstract: 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.
authors: From the Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom and.
authors: Daniel B
authors: DB
authors: Grabarczyk
authors: the Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, United Kingdom.
authors: Paul E
authors: PE
authors: Chappell
authors: From the Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom and.
authors: Bianca
authors: B
authors: Eisel
authors: the Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, United Kingdom.
authors: Steven
authors: S
authors: Johnson
authors: the Sir William Dunn School of Pathology, University of Oxford, South Parks Road, Oxford OX1 3RE, United Kingdom susan.lea@path.ox.ac.uk.
authors: Susan M
authors: SM
authors: Lea
authors: From the Department of Biochemistry, University of Oxford, South Parks Road, Oxford OX1 3QU, United Kingdom and ben.berks@bioch.ox.ac.uk.
authors: Ben C
authors: BC
authors: Berks
mesh_terms: D000595:Amino Acid Sequence; D001419:Bacteria / Q000378:metabolism; D001426:Bacterial Proteins / Q000737:chemistry / Q000378:metabolism*; D001483:Base Sequence; D003545:Cysteine / Q000378:metabolism*; D003580:Cytochromes / Q000737:chemistry / Q000378:metabolism*; D017931:DNA Primers; D013058:Mass Spectrometry; D008969:Molecular Sequence Data; D010084:Oxidation-Reduction; D011487:Protein Conformation; D017386:Sequence Homology, Amino Acid; D013056:Spectrophotometry, Ultraviolet; D013885:Thiosulfates / Q000378:metabolism*
chemical_list: D001426:Bacterial Proteins; D003580:Cytochromes; D017931:DNA Primers; D013885:Thiosulfates; D003545:Cysteine
keywords: Bacterial Metabolism; Cytochrome; Electron Transport; Heme; Metalloenzyme; Sox System; Sulfur Oxidizing Bacteria; Thiosulfate Dehydrogenase