Fig. 2: The structure of CriT.

a, Ribbon representation of a selected CriT from the crystal structure of Bcs3-CMP. Two different orientations show the overall architecture and the central location of the active site. Colored elements highlight HB-1 (orange), HB-2 (pink-orange), HB-3 (gold) and CriT-CAT (gray). Loops participating in the active site are colored as follows: loop 1 (tan, residues 678–680, connecting β20-α37), loop 2 (green, residues 705–707, connecting β21-α38), loop 3 (red, residues 818–823, connecting β22-α45), loop 4 (orange, residues 848–857, connecting β23-β24), loop 5 (pink, residues 863–869, connecting β24-α46) and loop 6 (light blue, residues 880–887, connecting α46-β25). Note the presence of the two phosphates found in the active site. b, Corresponding surface representation of CriT using the color code shown in a reveals the surface topology and active site. Note that HB-2 and HB-3 generate a donut shape surrounding the active site (dashed circle). c, Corresponding Coulombic charge surface representation reveals an intense positive character at the active site entrance. d–f, Detail of the active site in ribbon (d) and surface representation (e, f) showing the docked PRPP and Mg²⁺. Relevant active site residues participating in substrate binding are indicated. Specifically, the side chains of R708, R712 and T825, and main chains of residues 821–825 (part of loop 3, red, (d)) coordinate the PRPP 5-phosphate group, whereas the side chains of S678, S707, S884 and R679 form the pocket that accounts for the interaction with the β-phosphate of the pyrophosphate moiety. Note that the side chains of E818 and D680 accommodate the ribose moiety, while positively charged residues surrounding the binding site hold the phosphate moieties in place. g, A cross-section reveals the PRPP-Mg²⁺ binding site cavity, where R708 appears to be involved in positioning the ribose ring.