Model of E. coli ribonucleotide reductase based on the X-ray structures of its α₂ and β₂ subunits. The model is viewed with its twofold axis vertical and is drawn in ribbon form with its α subunits pink and red, and its β subunits light and dark blue. The C-terminal segment of the β subunit that was bound to the α subunit in its X-ray structure is cyan. A portion of the activity site is orange. GDP (green) bound in the catalytic site, TTP (yellow-green) bound in the specificity site, and the Fe₂O core of the diferric cluster (Fe brown and O red) are drawn in space-filling form. The sidechains implicated in forming the radical-transfer pathway are shown in stick form with C green, N blue, O red and S yellow. Click on the "Show" button to more clearly see a view of the diferric cluster and radical-transfer pathway to the the thiyl radical. [Courtesy of JoAnne Stubbe, MIT. PDBids 1RIB, 3R1R, and 4R1R.]