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| - Indirect Determination of the Rate of Iron Uptake into the Apoprotein of theRibonucleotide Reductase of E. coli
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| - The second-order rate constant kapo for uptake of FeII by the apoprotein of RibonucleotideReductase R2 has been measured by letting that reaction compete with the uptake of FeII by ferrozine(rate constant kFz). The rate of the FeII/ferrozine reaction was studied at high ferrozine concentrations,and an effective first-order rate constant kFz for the disappearance of FeII determined in the presence ofbovine serum albumin as a viscogen. Solutions of apoprotein and ferrozine in various ratios were mixedwith FeII solutions in a stopped-flow apparatus, and the growth of the 562 nm FeII(ferrozine)3 absorbancemonitored. Attempts to fit the data to a variety of kinetic schemes imply that uptake of the second FeIIby apo is slower than uptake of the first, suggesting that the rate-determining step in the activation of R2is a conformational change after the uptake of the first iron. The resulting value of kapo is 1.8(1) × 106M-1 s-1.
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