Perturbations at the high spin heme b center in the membrane-bound nitric oxide reductase

The effects of lowering pH from 7 to 5 on the absorption, circular dichrois m (MCD) and EPR spectra were studied for Paracoccus halodenitrificans nitri c oxide reductase (NOR). Intensities of the characteristic bands for the hi gh spin heme b, that at 592 nm in the absorption spectrum and those at 591 (+) and 606 (-) in the MCD spectrum decreased considerably. Concomitant cry ogenic EPR spectrum indicated a drastic increase in the signal intensity du e to the high spin heme b at g similar to 6, of which less than 5% had been EPR detectable at pH 7. Cyanide (x40) bound to the high spin heme b center in the reduced NOR irrespective of pH, while a much larger amount of azide (x1000) was necessary to bind to the reduced NOR at an acidic pH, ca. 5. B ased on these results the structure and function of the high spin heme b ce nter as the active site of NOR was discussed. (C) 2001 Elsevier Science B.V . All rights reserved.