Characterisation of an unusual cysteine pair in the Rieske carnitine monooxygenase CntA catalytic site

Abstract

Rieske monooxygenases undertake complex catalysis integral to marine, terrestrial and human gut‐ecosystems. Group‐I to ‐IV Rieske monooxygenases accept aromatic substrates and have well‐characterised catalytic mechanisms. Nascent to our understanding are Group‐V members catalysing the oxidation/breakdown of quaternary ammonium substrates. Phylogenetic analysis of Group V highlights a cysteine residue‐pair adjacent to the mononuclear Fe active site with no established role. Following our elucidation of the carnitine monooxygenase CntA structure, we probed the function of the cysteine pair Cys206/Cys209. Utilising biochemical and biophysical techniques, we found the cysteine residues do not play a structural role nor influence the electron transfer pathway, but rather are used in a nonstoichiometric role to ensure the catalytic iron centre remains in an Fe(II) state.