Electron transfer reactivity of the Arabidopsis thaliana sulfhydryl oxidase AtErv1.

The redox reactivity of the three disulfide bridges and the flavin present in each protomer of the wild-type Arabidopsis thaliana mitochondrial sulfhydryl oxidase (AtErv1) homodimer has been investigated. Pulse radiolytically produced CO2- radical ions were found to reduce the disulfide bridges to yield disulfide radicals, RSS*R-. Rates and absorption changes due to formation or decay of RSS*R- and the flavin quinone, semiquinone, and hydroquinone were measured and analyzed. During the first 100 micros following the pulse, the flavin was reduced to the semiquinone by intramolecular electron transfer from the active site disulfide radical. The semiquinone and the remaining disulfide radicals then reacted by much slower, 40 ms to 40 s, inter-homodimer electron transfer reactions, culminating in reduced flavin and dithiols. The dithiols were then subject to oxidation by enzyme molecules via their intrinsic enzymatic activity, at a rate comparable to the slower intermolecular processes in the 10-s time regime. Mutants of AtErv1 lacking each of the three individual cysteine pairs were studied to determine the involvement of the respective disulfide groups in these reactions. Elimination of the active site disulfide bridge increased the stability of the flavin semiquinone making it a long-lived product. Relevance of these observations to the design and function of the sulfhydryl oxidases is discussed.