Iron‐Nitrosyl Bond Configuration in Nitrosyl‐Hemoproteins: A Comparative EPR Study of Hemoglobin A and Hemoglobin Kansas

Hemoglobin A and hemoglobin Kansas can exist in alternative forms in which the affinity of heme iron for NO is mediated either by pH or the presence of allosteric modifiers. In this study, we show that the superhyperfine (SHF) pattern observed in the EPR which arises from the heme iron‐NO complex can be correlated with a change in the affinity of the ligated tetrameric protein for NO. When a combination three‐line and nine‐line SHF pattern is observed, the nitrosyl hemoprotein is in a low affinity form while a nine‐line pattern alone is an expression of high affinity. A theory is presented relating SHF patterns to a molecular orbital picture of the nitrosyl heme complex. It is suggested that the change of SHF pattern correlated with an alteration of the affinity state of ligated hemoglobin is related to a structural change of the proximal imidazole ligand to the heme.