A novel conformer of oxidized Paracoccus pantotrophus cytochrome cd(1) observed by freeze-quench NIR-MCD spectroscopy

Paracoccus pantotrophus cytochrome cd(1) is a physiological nitrite reducta se and an in vitro hydroxylamine reductase. The oxidised "as isolated" form of the enzyme has bis-histidinyl coordinated c-heme and upon reduction its coordination changes to histidine/ methionine. Following treatment of redu ced enzyme with hydroxylamine, a novel, oxidised, conformer of the enzyme i s obtained. We have devised protocols for freeze-quench near-ir-MCD spectro scopy that have allowed us to establish unequivocally the c-heme coordinati on of this species as His/Met. Thus it is shown that the catalytically comp etent, hydroxylamine reoxidised, form of P. pantotrophus cytochrome cd(1) h as different axial ligands to the c-heme than "as isolated" enzyme. (C) 200 0 Academic Press.