EVIDENCE FOR A BAND-III ANALOG IN THE NEAR-INFRARED ABSORPTION-SPECTRA OF CYTOCHROME-C-OXIDASE

Ground state near-infrared absorption spectra of fully reduced unliganded and fully reduced CO (a2+ CuA+ a32+-CO CuB+) cytochrome c oxidase were investigated. Flash-photolysis time-resolved absorption difference spectra of the mixed-valence (a3+ CuA2+ a32+-CO CuB+) and the fully reduced CO complexes were also studied. A band near 785 nm (ε ∼50 M-1cm-1) was observed in the fully reduced unliganded enzyme and the CO photoproducts. The time-resolved 785 nm band disappeared on the same timescale (t 1 2 ∼7 ms) as CO recombined with cytochrome a32+. This band, which is attributed to the unliganded five coordinate ferrous cytochrome a32+, has some characteristics of band III in deoxy-hemoglobin and deoxy-myoglobin. A second band was observed at ∼710 nm (ε ∼80 M-1cm-1) in the fully reduced unliganded and the fully reduced CO complexes. This band, which we assign to the low spin ferrous cytochrome a, appears to be affected by the ligation state at the cytochrome a32+ site. © 1992.