A comparison is made between the voltammetric behavior of three electron-transfer proteins and that of their derivatives obtained by attachment of a single Ru(NH3)53+/2+ group to specific histidine (imidazole) residues. The native proteins‒plastocyanin (PCu) from the green alga Scenedesmus obliquus (舠blue舡 Cu2+/1+ center, Mr ∼ 10.5 × 103), cytochrome c55l from Pseudomonas stutzeri (porphyrin Fe3+/2+ center, Mr ∼ 9.25 × 103), and (to a lesser extent) high-potential iron–sulfur protein (HiPIP) from Chromatium vinosum ([4Fe–4S]3+/2+ center, Mr ∼ 9.5 × 103)‒require the presence of a cationic reagent (neomycin is used here) to promote their interaction and electron exchange with the pyrolytic graphite-“edge” electrode. By contrast, each of the derivatives PCu-(His59)Ru(NH3)5, HiPIP(His42)Ru(NH3)5, and c555(His47)Ru(NH3)5 displays well-defined peak-type cyclic voltammograms without inclusion of such reagents in the electrolyte. The results indicate the importance of localized (as opposed to overall) protein surface charge as a determining factor underlying protein–electrode interactions that lead to reversible electron exchange. It is shown that reduction potentials of the intrinsic and Ru centers in such derivatives may be significantly different from the respective values for native proteins and the complex [Ru(NH3)5(imid)]3+/2+.