FIGURE 3.

Transition between structures P_II^ADP1 and P_II^ADP3 and differential binding mode of ADP. A, superposition of the P_II^ADP1 (orange and yellow) and P_II^ADP3 (dark blue and magenta) structures shown as schematics with side views of the S1 effector binding sites. Secondary structure elements are marked with α1-α2 and β1-β4 (with * for the second molecule). The ADP cofactors are represented by stick structures and color-coded according to the protein backbone (orange and blue). B, close-up of the S1 site of P_II^ADP3 highlighting residues (labeled and represented by stick structures) involved in ADP binding. C, comparison of the structures of the S2 binding site of P_II^ADP1 and P_II^ADP3. The color codes are the same as for A, but additional subunits are light orange (P_II^ADP1) and light blue (P_II^ADP3). Significant structural changes in B-loop, T-loop, and C termini induced by ADP binding are enclosed in dotted rectangles. D, close-up of the S2 site of P_II^ADP3. ADP is bound only via main-chain interactions to residues of the B-loop and T-loop.