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Biophysics of structure and mechanism

, Volume 7, Issue 3, pp 171–186 | Cite as

pH Dependence of oxy and deoxy cobalt-substituted leghemoglobin from soybean

An electron spin resonance study
  • M. Christahl
  • A. Raap
  • K. Gersonde
Article

Abstract

In leghemoglobin a, which is the major hemoglobin component in soybean root nodules, the haem iron has been replaced by cobalt. The electron spin resonance (ESR) of frozen solutions of the cobalt-substituted leghemoglobin has been studied at 77 K in the deoxy and oxy forms respectively. Both ligation states exhibit rhombic g tensors. The hyperfine constants of 59Co, 14N-imidazole (residue of the proximal histidine) and 14N-pyrroles are determined for the three principal directions of the g tensor. Both, the oxy and the deoxy state exhibit pH-dependent changes of the hyperfine structures. For oxy cobalt leghemoglobin a quantitative analysis of the pH titration and of the ESR parameters of the low and high-pH forms respectively are performed. The interconversion of the low and the high-pH forms is controlled by a proton-dissociating group with pK=6.4 which is most probably the distal histidine. g tensors and hyperfine constants are compared with those described for oxy cobalt myoglobin crystal spectra [34] allowing assignments of the low and high-pH species of leghemoglobin to stereoelectronic structures with non-equivalent and equivalent dioxygen atoms respectively. Hydrogen-bonding of the distal histidine with dioxygen favours the structure with equivalent oxygen atoms. The pH dependence of the deoxy form is interpreted as interaction of the proximal imidazole with the central cobalt atom.

Key words

Cobalt-substituted leghemoglobin Electron spin resonance Hyperfine constants pH dependence 

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Copyright information

© Springer-Verlag 1981

Authors and Affiliations

  • M. Christahl
    • 1
  • A. Raap
    • 1
  • K. Gersonde
    • 1
  1. 1.Abteilung Physiologische ChemieRheinisch-WestfÄlische Technische Hochschule AachenAachenFederal Republic of Germany

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