Facilitated intramolecular electron transfer in the Escherichia coli bo-type ubiquinol oxidase requires chloride.

Previous flow-flash measurements using the bo-type ubiquinol oxidase of Escherichia coli have revealed that facilitated heme B-heme O intramolecular electron transfer initiated upon reaction of the fully-reduced enzyme with dioxygen proceeds with a rate constant higher than 5 x 10(4) s-1 at pH 7.4 and 20 degrees C. Depletion of chloride anions from the enzyme by HPLC performed in the present study considerably decreased the rate constant to approximately 700 s-1, but the reaction of either dioxygen or carbon monoxide at the binuclear center was not affected at all kinetically. These results strongly suggest that Cl- is essential in maintaining a subtle molecular structure around the heme B and heme O that enables facilitated intramolecular electron transfer. Furthermore, a series of absorption spectra of the enzyme collected on time scales from microseconds to milliseconds during its single turnover indicate that as heme-heme intramolecular electron transfer is retarded by depletion of Cl-, an alternative electron transfer pathway is invoked. We discuss a possible role of novel bound Cl- in electron transfer from bound quinol to the binuclear center to accomplish dioxygen reduction.