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Chemical Sciences Seminars

Structural Characterization of Ribonucleotide Reductase Intermediate X Using 35 GHz Pulsed-ENDOR Spectroscopy

by Dr. Muralidharan Shanmugam (Northwestern University, USA)

Wednesday, July 31, 2013 from to (Asia/Kolkata)
at Colaba Campus ( AG-66 )
TIFR, Colaba Mumbai 400005
Description RNR-Intermediate X:
We previously used continuous wave (CW) and pulsed electron nuclear double resonance (ENDOR) techniques to identify the protonated oxygenic ligand(s) and their disposition within the FeIII/FeIV spin coupled cluster of the intermediate X, a direct precursor of the essential diferric-tyrosine radical, which initiates the catalysis of conversion of ribonucleotides to their corresponding deoxy-ribonuleotides. We reported that the active-site of X contains FeIII-bound terminal OHx (x = 1 or 2) and a bridging oxo ligand; [(OHx)FeIII-O-FeIV]. This inorganic core of X was confirmed by the subsequent 17O-ENDOR studies of X prepared in 17OH2-water and 17O2-gas. This has been questioned by recent reports, but these results do not themselves agree on the active-site of X; (i) Extensive DFT calculations performed on the various model compounds, followed by their comparison with the available experimental data led Han, W.-G.; et al. to conclude that the core of X contains two bridging oxo and a terminal FeIII-bound water (ii) based on MCD experiments and DFT calculations, Mitic, N.; et al. concluded that the core of X contains an oxo bridge and postulated the possible presence of a bridging hyroxo plus a terminal FeIII-bound hydroxyl.
Our very recent 1,2H-ENDOR studies of X prepared in H2O/D2O buffer concluded that there is no bridging hydroxo in the active-site of X, either alone or in addition to the FeIII-bound terminal aqua ligand OHx (x = 1 or 2) and in agreement with our proposal, we made over a decade ago. In this presentation, we will address the issue of how many oxygenic species are present in the inorganic core of X and their disposition within the FeIII/FeIV spin coupled cluster of X. This will be followed up immediately with discussion of future research plans and the implications of the research work.