ANION-LINKED AND PH-LINKED EFFECTS ON THE HEME-IRON GEOMETRY IN FERROUS NITROSYLATED MONOMERIC MYOGLOBINS

The cooperative effect of anions and proton concentration on the EPR s pectroscopic properties of the ferrous nitrosylated derivative of mono meric Mb from loggerhead sea turtle (Caretta caretta), sperm whale (Ph yseter catodon), and horse (Caballus caballus) has been investigated b etween pH 4.5 and 9.0, at 100 K. In the absence of anions, an EPR spec trum characteristic of the hexa-coordinated species of ferrous nitrosy lated Mb with an axial geometry is observed, which is unaffected by pH . On the other hand, a transition toward a species characterized by an EPR spectrum corresponding to a hexa-coordinated rhombic geometry tak es place in the presence of phosphate, acetate, citrate, sulfate, and chloride. Only the hexa-coordinated form characterized by the rhombic EPR spectrum appears then to undergo a pH-dependent transition toward the penta-coordinated species. Present results show clear-cut evidence for the spectroscopic coupling of proton and anion binding sites with the Mb reactive center, indicating that an allosteric mechanism might modulate the proximal HisF8-heme-NO geometry in monomeric hemoprotein s.