Mechanism of dithionite reduction of the R2 protein of E. coli and mouse ribonucleotide reductase: evidence for reduction of FeIII
2 prior to the tyrosyl radical |
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| Authors: | A Mark Dobbing Joo-Yeon Han A G Sykes |
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| Institution: | (1) Department of Chemistry, The University of Newcastle, Newcastle upon Tyne, NE1 7RU, UK, GB |
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| Abstract: | Dithionite has been found to reduce directly (without mediators) the Escherichia coli R2 subunit of ribonucleotide reductase. With dithionite (∼10 mM) in large excess, the reaction at 25 °C is complete in ∼10 h.
Preparations of E. coli R2 have an FeIII
2 (met-R2) component in this work at ∼40% levels, alongside the fully active enzyme FeIII
2 . . . Tyr*, which has a tyrosyl radical at Tyr-122. In the pH range studied (7–8) the kinetics are biphasic. Rate laws for
both phases give S2O4
2–] and not S2O4
2–]1/2 dependencies, and saturation kinetics are observed for the first time in R2 studies. No dependence on pH was detected. The
kinetics (25 °C) of the first phase are reproduced in separate experiments using only met-R2, with association of S2O4
2– to met-R2, K=330 M–1, occurring prior to electron transfer, k
et=4.8×10–4 s–1, I=0.100 M (NaCl). The second phase assigned to the reaction of FeIII
2 . . . Tyr* with S2O4
2– gives K=800 M–1 and k
et=5.6×10–5 s–1. Bearing in mind the substantially smaller reduction potential for FeIII
2 compared to Tyr*, this is a quite remarkable finding, with implications similar to those already reported for the reaction
of R2 with hydrazine, but with additional information provided by the saturation kinetics. The similarity in rates for the
two phases (∼fourfold difference) suggests that reduction of FeIII
2 is occurring in both cases, and since S2O4
2– is involved a two-equivalent change is proposed with the formation of FeII
2 . . . Tyr* in the case of active R2. As a sequel to the second phase, intramolecular reduction of the strongly oxidising
Tyr* by the FeII
2 is rapid, and further decay of FeIIFeIII is also fast. There is no stable mouse met-R2 form, and the single-phase reaction with dithionite gives saturation kinetics
with K=208 M–1 and k
et=1.7±10–3 s–1. Mechanistic implications, including the applicability of a pathway for electron transfer via FeA, are considered.
Received: 25 February 1998 / Received: 20 August 1998 |
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