The Crystal Structure of <i>Thermotoga maritima - Fig 1 </i> Class III Ribonucleotide Reductase Lacks a Radical Cysteine Pre-Positioned in the Active Site

<p>a) The radical generation and transfer pathways of all three classes of RNR are thought to converge on a completely conserved cysteine residue that transfers it to the substrate. The class I, II and III enzymes are coloured mauve, pink and gold respectively. The finger loops of all three classes and the C-terminal loop of the class III RNRs, as exemplified by the enzyme from bacteriophage T4, are shown in cartoon representation. The position of the glycyl radical in class III is marked by a sphere. The two hydrogen-bonded Tyr residues that end the proton-coupled electron transfer chain (PCET) in class I are shown in mauve, with the terminal oxygen atom shown as a sphere. The 5’-deoxyadenosine moiety generated by cleavage of the C-Co bond in AdoCbl by class II RNRs is shown with the 5’-C atom shown as a pink sphere. The GDP substrate bound to the class II enzyme is shown as sticks with the C3’ atom marked with a gray sphere. b) Overall structure of the tmNrdD dimer. The left-hand monomer is coloured grey while the right-hand monomer is coloured as a spectrum from deep blue at the N-terminus to deep red at the C-terminus. The allosteric effector dATP and the substrate CTP are shown in space-filling representation. c) Comparison of the structures of tmNrdD and the previously determined structure of NrdD from bacteriophage T4 [<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0128199#pone.0128199.ref009" target="_blank">9</a>]. The T4 structure is coloured dark blue and tmNrdD is coloured red. d) Depiction of the active site area where the tips of the finger loop (blue) and the C-terminal loop (orange) meet. The position of the glycyl radical is marked by an orange sphere and Ile359 at the tip of the finger loop by a blue sphere. The substrate CTP is shown in stick representation. The Zn-binding domain is shown in yellow.