TRYPTOPHAN RADICALS FORMED BY IRON OXYGEN REACTION WITH ESCHERICHIA-COLI RIBONUCLEOTIDE REDUCTASE PROTEIN R2 MUTANT Y122F/

The active state of the small subunit, protein R2, of ribonucleotide r eductase is formed by the reaction of apoprotein with Fe2+ and O-2, wh ereby the diferric site and a stable phenoxy free radical on a tyrosyl residue (Tyr(122)) is formed. The corresponding reaction was studied in the mutant Y122F R2. It leads to a normal iron site, but the reduct ion equivalent from Tyr(122) now has to be supplied from elsewhere. EP R spectroscopy shows formation of several paramagnetic species on diff erent time scales. Using apoprotein with deuterium-labeled tryptophan residues, at least two species could be assigned to tryptophan free ra dicals. This is the first EPR observation of relatively stable protein -linked tryptophan radicals at room temperature and at 77 K. These try ptophan radicals may be involved as redox intermediates in long range electron transfer within the protein structure.