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different from those of the native protein.The reaction of a5RuH202+ with sperm whale myoglobin was allowed to proceed for 24 h under anaerobic conditions at pH 7, and the Ru-labeled protein was then oxidized and purified by standard procedures, to give Ru3Mb. * Absences of the imidazole C-2 proton resonances of His-12, His-81, and His-113 in the high-field N M R spectrum of Ru3Mb pinpoint those residues as the sites of attachment of the a5Ru3+ groups (Figure l).**J3The three attached a5Ru3+ groups do not appear strongly to perturb the M b conformation, as judged by comparative measurements of electronic and vibrational spectra. The ability of the heme in Ru3Mb to bind anions, however, is enhanced greatly over that of the native protein. Cyanide, for example, binds strongly both to the Fe"' and Fe" forms of Ru3Mb.14 The electrostatic influence of the three aSRu3+ groups is likely responsible for this impressive change in anion affinity, as it is for the very high PI value (-9.2) for Ru3Mb.The reduction of O2 by a variety of organic substrates is catalyzed quite effectively by Ru3Mb. Good substrates for this L( synthetic oxidation-reduction enzyme" include ascorbate and durohydroquinone. Comparisons of reactivity parameters (K,,,,

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