Non-equivalence and inverse allosteric response of the alpha and beta chains in haemoglobins. An electron spin resonance study of NO-ligated Hb Kansas

Abstract

The electron spin resonance (ESR) spectra of 15NO- and 14NO-ligated Hb Kansas have been measured at 77 K in the range of pH 5 TO 10. At low pH the ESR spectrum is the composite of a type I and a type II spectrum which changes to another composite of type I and type II spectrum at high pH. For the definition of type I and type II spectra and the correlation of these types with two tertiary conformation states see Overkamp et al., Z. Naturforsch. 31 c, 524 [1976]. Both, the type I and the type II spectra observed at low and high pH respectively are different with regard to g-tensors and hyperfine-splitting constants. Therefore at intermediate pH values the ESR spectra of NO-Hb Kansas are the composites of four spectral components. The assignments of the four spectral components to the alpha and the beta chains are arrived at from the comparison of the ESR spectra of the alpha2Mmet beta2NO and of the alpha2MNObeta2NO species of Hb M Iwate. alpha and beta chains are both characterized by a pH-dependent spectral transition from a type I to a type II spectrum. The chains are non-equivalent with regard to both the type I and the type II spectra. The type I spectra assigned to the alpha and the beta chains are characterized by G*ZZ=2.0095 with a hyperfine splitting of alpha*ZZ(15NO)=2.36 mT and gZZ-2.0085 with a hyperfine splitting of aZZ(15NO)=2.41 mT respectively. The type II spectra assigned to the alpha and the beta chains are characterized by g*'ZZ-2.005 and a hyperfine splitting of alpha*'ZZ(15NO)-3.07 mT and g'ZZ-2.005 and a hyperfine splitting of alpha'ZZ(15NO)=3.31 mT. The change of the hyperfine splitting at gZZ during the transition from type I to type II corresponds to an increase of the spin density at the NO by about 25% in both types of chains. Comparison of type I spectra of the NO-ligated alpha and beta chains respectively demonstrates that the spin density at the NO is larger in the beta chains that in the alpha chains. The spectral types are correlated with functional states defined by the kinetics of N0-binding. Binding of inosital hexaphosphate has no influence on the ESR spectra in the whole range of pH as it is expected if NO-Hb Kandas is in the quaternary T structure.