Unexpected weak magnetic exchange coupling between haem and non-haem iron in the catalytic site of nitric oxide reductase (NorBC) from Paracoccus denitrificans

Bacterial NOR (nitric oxide reductase) is a major source of the powerful greenhouse gas N2O. NorBC from Paracoccus denitrificans is a heterodimeric multi-haem transmembrane complex. The active site, in NorB, comprises high-spin haem b3 in close proximity with non-haem iron, FeB. In oxidized NorBC, the active site is EPR-silent owing to exchange coupling between FeIII haem b3 and FeBIII (both S=5/2). On the basis of resonance Raman studies [Moënne-Loccoz, Richter, Huang, Wasser, Ghiladi, Karlin and de Vries (2000) J. Am. Chem. Soc. 122, 9344-9345], it has been assumed that the coupling is mediated by an oxo-bridge and subsequent studies have been interpreted on the basis of this model. In the present study we report a VFVT (variable-field variable-temperature) MCD (magnetic circular dichroism) study that determines an isotropic value of J=-1.7 cm-1 for the coupling. This is two orders of magnitude smaller than that encountered for oxo-bridged diferric systems, thus ruling out this configuration. Instead, it is proposed that weak coupling is mediated by a conserved glutamate residue.