KINETIC EVIDENCE THAT A RADICAL TRANSFER PATHWAY IN PROTEIN R2 OF MOUSE RIBONUCLEOTIDE REDUCTASE IS INVOLVED IN GENERATION OF THE TYROSYL FREE-RADICAL

Class I ribonucleotide reductases consist of two subunits, R1 and R2, The active site is located in B1; active R2 contains a diferric center and a tyrosyl free radical (Tyr(.)), both essential for enzymatic act ivity. The proposed mechanism for the enzymatic reaction includes the transport of a reducing equivalent, i.e. electron or hydrogen radical, across a 35-Angstrom distance between Tyr(.) in R2 and the active sit e in R1, which are connected by a hydrogen-bonded chain of conserved, catalytically essential amino acid residues. Asp(266) and Trp(103) in mouse R2 are part of this radical transfer pathway. The differic/Tyr(. ) site in R2 is reconstituted spontaneously by mixing iron-free apoR2 with Fe(II) and O-2. The reconstitution reaction requires the delivery of an external reducing equivalent to form the diferric/Tyr(.) site. Reconstitution kinetics were investigated in mouse ape-wild type R2 an d the three mutants D266A, W103Y, and W103F by rapid freeze-quench ele ctron paramagnetic resonance with greater than or equal to 4 Fe(II)/R2 at various reaction temperatures. The kinetics of Tyr(.) formation in D266A and W103Y is on average 20 times slower than in wild type R2, M ore strikingly, Tyr(.) formation is completely suppressed in W103F, No change in the reconstitution kinetics was found starting from Fe(II)- preloaded proteins, which shows that the mutations do not affect the r ate of iron binding. Our results are consistent with a reaction mechan ism using Asp266 and Trp(103) for delivery of the external reducing eq uivalent. Further, the results with W103F suggest that an intact hydro gen-bonded chain is crucial for the reaction, indicating that the exte rnal reducing equivalent is a H-.. Finally, the formation of Tyr(.) is not the slowest step of the reaction as it is in Escherichia coli R2, consistent with a stronger interaction between Tyr(.) and the iron ce nter in mouse R2, A new electron paramagnetic resonance visible interm ediate named mouse X, strikingly similar to species X found in E. coli R2, was detected only in small amounts under certain conditions. We p ropose that it may be an intermediate in a side reaction leading to a diferric center without forming the neighboring Tyr ..