Bifunctional diheme cytochrome c thiosulfate dehydrogenases/tetrathionate reductases (TsdA) exhibit different catalytic properties depending on the source organism. In the human food-borne intestinal pathogen Campylobacter jejuni , TsdA functions as a tetrathionate reductase enabling respiration with tetrathionate as an alternative electron acceptor. Here, evidence is provided that Cys138 and Met255 serve as the sixth ligands of Heme 1 and Heme 2, respectively, in the oxidized Cj TsdA wt protein. Replacement of Cys138 resulted in a virtually inactive enzyme, confirming Heme 1 as the active site heme. Significantly, TsdA variants carrying amino acid exchanges in the vicinity of the electron-transferring Heme 2 (Met255, Asn254 and Lys252) exhibited markedly altered catalytic properties of the enzyme, showing these residues play a key role in the physiological function of TsdA. The growth phenotypes and tetrathionate reductase activities of a series of Δ tsdA/*tsdA complementation strains constructed in the original host C. jejuni 81116, showed that in vivo , the TsdA variants exhibited the same catalytic properties as the pure, recombinantly produced enzymes. However, variants that catalyzed tetrathionate reduction more effectively than the wild-type enzyme did not allow better growth.
- axial heme ligation
- reaction directionality
- tetrathionate reductase
- ©2016 The Author(s)
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