PH-induced cleavage of the proximal histidine to iron bond in the nitric oxide derivative of ferrous monomeric hemosproteins and of the ‘chelated’ protoheme model compound

Abstract

The EPR properties of the nitric oxide deriveative of several monomeric hemoproteins (such as sperm whale Mb, horse Mb, Coryphaena hippurus Mb, Dermochelys coriacea Mb and Chironomus thummi thammi erythrocruorin) and of the “chelated‘ protoheme model compound have been investigated in the pH range from 7.0 to 3.9 At neutral pH, the EPR spectra of all the NO-Fe complexes investigated have a rhombic shape with a different degree of resolution of the superhyperline structure in the g z region of the spectrum. By lowering pH, EPR spectra undergo a reversible change, characterized by the appearance of a three-line pattern in the high magnetic field region, which is essentially complete at pH 3.9 This transition, which has p K, values of 4.7 and 5.0 for sperm whale Mb and for the ‘chelatedld’ protoheme model compound respectively, has been interpreted as indicative of the protonation of the N ϵ of the proximal histidine and of the cleavage of the FE-N ϵ bond