Conformation-controlled trans-effect of the proximal histidine in haemoglobins. An electron spin resonance study of monomeric nitrosyl-57Fe-haemoglobins.

A monomeric allosteric haemoglobin from Chironomus thummi thummi was reconstituted with 57Fe-haem. This reconstituted haemoglobin was found to be identical to the non-reconstituted material with regard to the O2-binding properties and the visible spectra. The 270 MHz proton magnetic resonance of the bis (cyano)-57Fe-haemin shows that the reconstituted haem is identical with the non-reconstituted haem. Furthermore it has been proved by proton magnetic resonance that in Chironomus haemoglobins the prosthetic group is proto-haem IX. The ESR spectrum of the native nitrosyl haemoglobin demonstrates rhombic symmetry of the haem iron (gxx=2.086, gyy=1.981, gzz=2.005) and hyperfine structures at gyy (aNepsilon=1.35 mT) and at gzz (a15NO=3.05 MT, a14NO=2.19 mT, aNepsilon=0.715 mT, a57Fe=0.38 mT). The spectrum is independent of pH and can be classified as a type II spectrum following the classification of ref. 2. NO-binding obviously stabilizes the tertiary structure of this haemoglobin in a "tense" conformation with a relatively strong sigma bond of the 5th ligand (Nepsilon of imidazole) and a relatively weak sigma bond of the 6th ligand (NO). Reaction of this haemoglobin with anionic, cationic and non-ionic detergents, respectively, leads to a transformation of the NO-ligated form into a "relaxed" conformation with a stretched or broken sigma bond of the 5th ligand (Nepsilon of imidazole) and a strong sigma bond of the 6th ligand (NO). The ESR spectrum of this modified NO-haemoglobin shows again a rhombic symmetry of the haem iron (gxx=2.10, gyy=2.06, gzz=2.010), but dramatically changes in the g tensors (low field shift), hyperfine structures and hyperfine splitting constants (a15NO=2.32 MT, a14NO=1.66 mT, a57Fe=0.48 mT). The hyperfine splitting is isotropic. Transition from the "tense" conformation to the "relaxed" conformation corresponds with an increase of the spin density at the iron atom by 26% and a decrease of the spin density at the NO ligand by 25%. The spin density at the Nepsilon of imidazole strongly decreases in the "relaxed" conformation, so that a hyperfine splitting of this ligand is not any more resolved. These results demonstrate the trans-effect of the proximal imidazole which in haemoglobins controls of the binding properties of the external ligand in trans-position.