Could tyrosine and tryptophan serve multiple roles in biological redox processes?

Single-step electron tunnelling reactions can transport charges over distances of 15-20 Åin proteins. Longer-range transfer requires multi-step tunnelling processes along redox chains, often referred to as hopping. Long-range hopping via oxidized radicals of tryptophan and tyrosine, which has been identified in several natural enzymes, has been demonstrated in artificial constructs of the blue copper protein azurin. Tryptophan and tyrosine serve as hopping way stations in high-potential charge transport processes. It may be no coincidence that these two residues occur with greater-than-average frequency in O(2)- and H(2)O(2)-reactive enzymes. We suggest that appropriately placed tyrosine and/or tryptophan residues prevent damage from high-potential reactive intermediates by reduction followed by transfer of the oxidizing equivalent to less harmful sites or out of the protein altogether.