Could a diiron-containing four-helix-bundle protein have been a primitive oxygen reductase?

To fulfil the title hypothesis, such a protein would have to harbour a binu clear transition metal site that would allow the complete reduction of diox ygen to water, thus playing an important role in early oxygen defence mecha nisms in primordial anaerobes. The hypothesis here raised is based on data concerning the oxygen reductase activity of the four-helix diiron protein r ubrerythrin (see schematic picture), and on sequence- and structure-based p hylogenetic relations between this and other diiron proteins. Interestingly , an evolutionary relationship between such an early system and the alterna tive oxidases present in extant eukaryotes can be depicted.