• cytochrome;
  • encounter complex;
  • NMR;
  • plastocyanin;
  • transient complex

Electron transfer proteins transport electrons safely between large redox enzymes. The complexes formed by these proteins are among the most transient. The biological function requires, on the one hand, sufficient specificity of the interaction to allow for rapid and selective electron transfer, and, on the other hand, a fast turnover of the complex. Recent progress in the characterization of the nature of these complexes has demonstrated that the encounter state plays an important role. This state of initial binding is dominated by electrostatic interactions, and consists of an ensemble of orientations. Paramagnetic relaxation enhancement NMR and chemical shift perturbation analysis provide ways for the experimental characterisation of the encounter state. Several studies that have used these techniques have shown that the surface area sample in the encounter state can be limited to the immediate environment of the final, specific complex. The encounter complex can represent a large fraction and, in some small complexes, no specific binding is detected at all. It can be concluded that, in electron transfer protein complexes, a fine balance is sought between the low-specificity encounter state and the high-specificity productive complex to meet the opposing requirements of rapid electron transfer and a high turnover rate.