Detection, formation, and relevance of hemichromes and hemochromes.

Publisher Summary Native hemoglobins contain one histidine coordinated to the iron on the proximal side of the heme and another histidine in the ligand pocket on the distal side of the heme. Oxygen located in this pocket in oxyhemoglobin can be replaced by other neutral ligands, such as carbon monoxide, nitric oxide, and alkylisocyanide in the reduced Fe(II) hemoglobins. The iron in deoxyhemoglobin is in a high-spin state, whereas all of the Fe(II) complexes are in a low-spin state. Oxidized Fe(III) hemoglobin ligands bound in this pocket include water, hydroxide, fluoride, azide, thiocyanate, imidazole, and cyanide. The spin state for these complexes depends on the strength of the axial ligand and can be high spin (example, fluoride), low spin (example, cyanide), or a mixture (example, hydroxide). Low-spin hemoglobin complexes also exist when the exogenous ligand is replaced by an endogenous amino acid side chain. These low-spin complexes define the Fe(II) hemochromes and Fe(III) hemichromes. For many other heme proteins, low-spin complexes involving axial coordination of two amino acid side chains are functional. This chapter discusses the methods for determining the presence of the low-spin complexes. The characterization of different types of hemichromes, primarily on the basis of electron paramagnetic resonance (EPR) is reviewed. The chapter discusses the different ways of forming hemi(hemo)chromes. The processes clearly dissociated with denaturation to those clearly indicative of native substates and the significance of hemi(hemo)chrome formation are also discussed. The information regarding hemoglobin that can be obtained from studies of hemi(hemo)chrome formation and the possible functional role of bis-histidine complex formation have also been described in the chapter.