SPIN-LABEL STUDIES OF THE CONFORMATIONAL PROPERTIES OF HEME-PROTEINS - A REVIEW

This review briefly summarizes the results of spin-label studies of co nformational transitions in monomeric globins (myoglobin, leghemoglobi n, and erythrocruorin) and in another heme-containing protein (cytochr ome c) induced by ligands of the heme and by pH. In contrast to integr al methods, the use of the spin-labelling technique for proteins of kn own spatial structure makes it possible to obtain information about th e conformational behavior of specific parts of the protein structure a nd to draw conclusions about the nature of conformational changes in t he protein during its functioning. The experimental results fit well w ith a model that represents a Mb-like structure as being composed of t hree independent rigid helical fragments: AE (ABCDE), F, and GH, whose mutual arrangement is controlled by N- and C-terminal salt bridges. S ynchronous displacement of these fragments relative to each other due either to the ligand attachment and structural changes in the heme com plex or to disturbances in ionic interactions at the N- and/or C-termi ni of the structure induced by pH or allosteric effecters might serve as a structural basis for homo- and heterotropic regulations in hemogl obin. Similarly, the spin label method allows for tracing and obtainin g important information about local conformational events in cytochrom e c which are induced by pH in the range of 5-13 and are accompanied b y 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 ch anges in its structure upon alkaline denaturation. It was found that s ubstituting the protein ligand Met-80 by the external ligand cyanide m arkedly alters the dynamic properties of the polypeptide chain segment 65-75 adjacent to Met-80.