The bacterium Deinococcus radiodurans survives γ-radiation doses about 20 times higher than those that kill Escherichia coli, and its resistance is attributed to low-molecular-weight (LMW) Mn2+Mn^{2+}–metabolite complexes that catalytically remove reactive oxygen species (ROS) and thereby protect proteins, including the DNA-repair machinery, from oxidative inactivation. Advanced paramagnetic resonance measurements on intact cells (Sharma et al., PNAS 2013) showed that intracellular Mn2+Mn^{2+} in D. radiodurans exists predominantly as LMW complexes coordinated by orthophosphate and roughly one nitrogenous ligand per Mn2+Mn^{2+} ion, but the measurements could not identify the ligands. The open core of this problem is threefold: uniquely assign the chemical identities of the in vivo nitrogenous and phosphate ligands; determine the structures and coordination geometries of the functional antioxidant complexes in living cells; and establish their in vivo ROS-scavenging mechanism (rates, stoichiometries, catalytic versus stoichiometric mode).

Background

The bacterium Deinococcus radiodurans survives acute γ-radiation doses (≈12,000 Gy; the gray, Gy, is the SI unit of absorbed radiation dose) roughly 20 times larger than the lethal dose for Escherichia coli. Ionizing radiation kills cells largely indirectly: energy deposited in water generates reactive oxygen species (ROS), including the superoxide radical O2∙−O_2^{\bullet-}, which oxidatively damage proteins. D. radiodurans can reassemble its extensively fragmented genome only because its proteins — among them the DNA-repair enzymes themselves — escape oxidative inactivation. The leading explanation is the manganese-antioxidant (proteome-protection) hypothesis: D. radiodurans accumulates millimolar intracellular Mn2+Mn^{2+}, which is innocuous where redox-active metals such as Fe2+Fe^{2+} promote ROS via Fenton chemistry (iron-catalyzed reactions that generate the highly reactive hydroxyl radical), and this Mn2+Mn^{2+} exists predominantly as low-molecular-weight (LMW) complexes with cellular metabolites — orthophosphate (Pi), peptides, nucleosides, and amino acids — that remove ROS and shield the proteome.

These in vivo complexes were probed by Sharma et al. (PNAS 2013) with advanced paramagnetic resonance techniques, which detect the paramagnetic Mn2+Mn^{2+} ion (S=5/2S=5/2) and its coordination sphere in intact frozen cells: electron-spin-echo EPR (echo-detected absorption spectra that separate high-symmetry, rapidly tumbling LMW complexes from low-symmetry enzyme-bound Mn2+Mn^{2+}), ENDOR (electron nuclear double resonance, measuring hyperfine couplings to nuclei such as 31P^{31}P and 1H^{1}H around the metal), and ESEEM (electron spin echo envelope modulation, which analogously reports on nearby 14N^{14}N nuclei). The study found that in D. radiodurans essentially all intracellular Mn2+Mn^{2+} is in LMW complexes coordinated by orthophosphate (with possible polyphosphate and nucleotide contributions) and by roughly one nitrogenous ligand per Mn2+Mn^{2+} ion, with a negligible EPR contribution from Mn-dependent superoxide dismutase (SodA, the enzyme that normally dismutates O2∙−O_2^{\bullet-}); speciation was unchanged by a 10 kGy dose. In E. coli, by contrast, most Mn2+Mn^{2+} occupies low-symmetry enzyme sites such as SodA, and irradiation creates new nitrogenous fragments that bind Mn2+Mn^{2+}. Follow-up work by the same collaboration showed that the fraction of Mn2+Mn^{2+} held in high-symmetry metabolite complexes (denoted H-Mn, as opposed to enzyme-bound, low-symmetry L-Mn), gauged by EPR of live cells, is the strongest known predictor of radiation resistance across the three domains of life, and that small-molecule Mn antioxidants can complement and even supplant MnSOD in vivo.

What remains missing is molecular specificity. The resonance measurements identify coordination-atom types (O, N, P) and hyperfine constants, not which metabolites occupy the ligand sites. In vitro, many candidates reproduce the function: manganous phosphate alone catalyzes superoxide disproportionation at physiological concentrations with well-characterized rates (Barnese et al. 2012); peptides bind Mn2+Mn^{2+} and enhance radioprotection; and the designed synthetic decapeptide DP1 (DEHGTAVMLK) with orthophosphate forms a ternary Mn2+Mn^{2+} complex of exceptional antioxidant potency — the basis of the Mn–decapeptide–phosphate (MDP) formulation now used to produce irradiated whole-cell vaccines. Yet none of these model systems has been shown to be the actual in vivo species. Moreover, high-field EPR quantification by a second laboratory (Bruch, Tabares, Un and colleagues) reported that in stationary-phase D. radiodurans the LMW Mn2+Mn^{2+} pool falls below 16 μM while protein-bound Mn (MnSOD plus an unidentified Mn protein, together ≈570 μM) dominates — with no loss of radioresistance — so even the growth-phase dependence and the relative sizes of the LMW and proteinaceous Mn pools remain contested between laboratories. Because Mn2+Mn^{2+} is substitutionally labile and rapidly exchanges ligands, procedures that disrupt cells can themselves scramble the speciation, which is why the source paper restricted its conclusions to intact cells.

Problem Statement

Sharma et al. (PNAS 2013) explicitly flagged as future work the molecular characterization of the in vivo Mn-antioxidant complexes of D. radiodurans that their spectroscopy could describe only at the coordination-atom level. The still-open core of that question, for D. radiodurans (with E. coli as the contrast organism), is threefold:

  1. Ligand identity. Uniquely assign the chemical identities of the nitrogenous ligands occupying the 14N^{14}N coordination sites of the intracellular LMW Mn2+Mn^{2+} complexes in living D. radiodurans cells — which specific peptides, nucleosides, nucleobases, free amino acids, or other metabolites they are — and decompose the composite 31P^{31}P spectral contribution among orthophosphate, polyphosphate, and nucleotide-bound phosphate.

  2. Structure and coordination geometry. Determine the precise molecular structures and coordination geometries of the functional antioxidant Mn2+Mn^{2+} complexes in vivo, including the number and identity of the coordinated donor atoms per complex, and how those structural details enable the ROS-scavenging behavior.

  3. Mechanism. Establish how these complexes scavenge reactive oxygen species in vivo: which ROS are scavenged, at what rates and stoichiometries, and whether the mode is catalytic (as manganous phosphate behaves in vitro) or stoichiometric.

The complexes sought are those present in intact, living cells: Mn2+Mn^{2+} exchanges ligands rapidly, so cell-disrupting procedures can themselves scramble the in vivo speciation (the source's own caveat). Synthetic Mn-peptide/phosphate complexes for radioprotection and vaccine formulations are outside the scope of this task.

References

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