Spin-label studies of the conformational properties of heme proteins: A review

This review briefly summarizes the results of spin-label studies of conformational transitions in monomeric globins (myoglobin, leghemoglobin, and erythrocruorin) and in another heme-containing protein (cytochrome c) induced by ligands of the heme and by pH. In contrast to integral methods, the use of the spin-labelling technique for proteins of known spatial structure makes it possible to obtain information about the conformational behavior of specific parts of the protein structure and to draw conclusions about the nature of conformational changes in the protein during its functioning. The experimental results fit well with a model that represents a Mb-like structure as being composed of three independent rigid helical fragments: AE (ABCDE), F, and GH, whose mutual arrangement is controlled by N- and C-terminal salt bridges. Synchronous displacement of these fragments relative to each other due either to the ligand attachment and structural changes in the heme complex or to disturbances in ionic interactions at the N- and/or C-termini of the structure induced by pH or allosteric effecters might serve as a structural basis for homo- and heterotropic regulations in hemoglobin. Similarly, the spin label method allows for tracing and obtaining important information about local conformational events in cytochrome c which are induced by pH in the range of 5-13 and are accompanied by the substitution of the heme sixth ligand (Met-80) by Lys-79 (pK 9.3) and then by Tyr-67 (pK 11.1). The changes in the local conformation and dynamics of native cytochrome c create prerequisites for global changes in its structure upon alkaline denaturation. It was found that substituting the protein ligand Met-80 by the external ligand cyanide markedly alters the dynamic properties of the polypeptide chain segment 65-75 adjacent to Met-80.