Characterization of the paramagnetic iron‐containing redox centres of Thiosphaera pantotropha periplasmic nitrate reductase

Electron paramagnetic resonance spectroscopy signals attributable to low‐spin haem c in the oxidised protein and [4Fe–4S] 1+ in the dithionite‐reduced protein were identified, at low temperature, in Thiosphaera pantotropha periplasmic nitrate reductase. Spin integration of these signals as well as elemental analysis suggest a stoichiometry of 1.3–1.6 c ‐haem and 1 [4Fe–4S] cluster per enzyme molecule. The E m (at pH 7.4) of the [4F–4S] 2+,1+ couple, −160 mV, means that it is unlikely to be physiologically reducible. Peptide sequences from the 90 kDa subunit indicate that the enzyme is a member of the family of molybdopterin guanine dinucleotide‐binding polypeptides, the majority of which possess a putative [4Fe–4S] cluster binding sequence and thus may also bind a (low potential) iron—sulphur cluster.