Research Update:

E. coli ribonucleotide reductase (RNR) catalyzes the conversion of nucleoside diphosphates (NDPs) to dNDPs and is composed of two homodimeric subunits: R1 and R2. Jie Ge et al carried out pre-steady-state experiments with wild type R1 or C754S/C759S-R1 and R2 to determine which step(s) are rate-limiting and whether both active sites of R1 can catalyze nucleotide reduction. The results reveal that the rate-determining step is a physical step prior to rapid nucleotide reduction and rapid tyrosine reoxidation to a tyrosyl radical. In addition to the slow conformational change(s) prior to chemistry, re-reduction of the active site disulfide to dithiol or a conformational change accompanying this process can also be rate-limiting. This work was published in Biochemistry, 42 (34), 10071-10083. [PDF]

Escherichia coli class I ribonucleotide reductase consists of two subunits: R1 and R2. R1 possesses the active site, while R2 harbors the essential diferric-tyrosyl radical cofactor.The tyrosyl radical on R2 is proposed to generate a transient thiyl radical on R1, 35 Å distant, through amino acid radical intermediates (See the figure above). Cyril Yee, Michelle Chang et al studied the putative long-range proton-coupled electron transfer (PCET) using intein technology. Y356, a putative intermediate in the pathway, was replaced with 2,3-difluorotyrosine (F2Y, pKa = 7.8). pH rate profiles (pH 6.5-9.0) of wild-type and F2Y-R2 were very similar. Thus, a proton can be lost from the putative PCET pathway without affecting nucleotide reduction. The current model involving H transfer is thus unlikely. This work was published in J. Am. Chem Soc.125(35), 10506-10507, 2003. [PDF].

To learn more about ribonucleotide reductase (RNR), go to the RNR webpage