Magnetic Interactions Sense Changes in Distance between Heme <i>b</i>_L and the Iron−Sulfur Cluster in Cytochrome <i>bc</i>₁

During the operation of cytochrome <i>bc</i>₁, a key enzyme of biological energy conversion, the iron−sulfur head domain of one of the subunits of the catalytic core undergoes a large-scale movement from the catalytic quinone oxidation Q_o site to cytochrome <i>c</i>₁. This changes a distance between the two iron−two sulfur (FeS) cluster and other cofactors of the redox chains. Although the role and the mechanism of this movement have been intensely studied, they both remain poorly understood, partly because the movement itself is not easily traceable experimentally. Here, we take advantage of magnetic interactions between the reduced FeS cluster and oxidized heme <i>b</i>_L to use dipolar enhancement of phase relaxation of the FeS cluster as a spectroscopic parameter which with a unique clarity and specificity senses changes in the distance between those two cofactors. The dipolar relaxation curves measured by EPR at Q-band in a glass state of frozen solution (i.e., under the conditions trapping a dynamic distribution of FeS positions that existed in a liquid phase) of isolated cytochrome <i>bc</i>₁ were compared with the curves calculated for the FeS cluster occupying distinct positions in various crystals of cytochrome <i>bc</i>₁. This comparison revealed the existence of a broad distribution of the FeS positions in noninhibited cytochrome <i>bc</i>₁ and demonstrated that the average equilibrium position is modifiable by inhibitors or mutations. To explain the results, we assume that changes in the equilibrium distribution of the FeS positions are the result of modifications of the orienting potential gradient in which the diffusion of the FeS head domain takes place. The measured changes in the phase relaxation enhancement provide the first direct experimental description of changes in the strength of dipolar coupling between the FeS cluster and heme <i>b</i><sub>L