et D1731 E302 E302 FIGURE 5 Hydrogen-bonding network among K-167, D-173.E-302 and bridging water molecules in DI K167 K167 conformational packing in the dynamic SPD-DI complex, so shown in Fig. 6, the interactions between ring A of SPD and we consider this packing effect in the dynamic SPD-D2 com- the surrounding residues of DI are mainly aromatic stacking plex to be a structural determinant in the antagonism of SpD with the side chains of w-3.28, W-7.40, and w-7.43, plus with the D2 receptor, which will be described in detail later. direct hydrogen bonding between the hydroxyl group on ring A of SPD with NEl of W-3.28(Fig. 6. C and E). In the SPD- The agonistic and antagonistic conformations of SPD D2 complex, ring A of SPD has much stronger electrostatic There is only a small difference between the initial confor- and hydrogen-bonding interactions with the protonated mations of SPD in the complexes with DI and D2. By mutual H-6.55. The role of H-6.55 in stabilizing the bent SPD con- conformational adjustment during the first -5 ns of dynam- formation in the D2 binding cavity becomes more important because the N-6.55 at the same position in the DI receptor is becomes quite different from that in the Di binding cavity not involved in any interactions with SPD (Fig. 6, C and D). The first adjustment was to the dihedral angle between rings A and D. In the SPD-D2 complex, ring A angle of 98), whereas the relative position of these two rings of Spo of the agonistic and antagonistic conformations of SPD becomes almost perpendicular to ring D(dihedral Energies PD-DI complex. The second adjustment is reflected in the conformations of SPD in the different receptors, Dl andrae SPD still ains planarity(dihedral angle of 150%)in the We next investigated the energetics of the different bioact different interactions between SPD with DI and D2. As Quantum mechanical methods were employed to calculate 39 A185e2,、539 V29 F538 332 H655 69 e 67426513 F65 43 W74 42 a 44 W740 惑 337s36648 651 E F 1 EL2 D FIGURE 6 Binding modes of SPD in the DI(A, C and E) and D2(B, D, and F)receptors (A and B)The models of DI-SPD and D2-SPD. The active sites were displayed as an electrostatic surface. (C and D) The important residues in the active sites of two complexes and SPD were rendered in stick representation (E and F) Schematic depiction of the main interactions between SPD with DI and D2. Biophysical Journal 93(5)1431-1441conformational packing in the dynamic SPD-D1 complex, so we consider this packing effect in the dynamic SPD-D2 com￾plex to be a structural determinant in the antagonism of SPD with the D2 receptor, which will be described in detail later. The agonistic and antagonistic conformations of SPD There is only a small difference between the initial confor￾mations of SPD in the complexes with D1 and D2. By mutual conformational adjustment during the first ;5 ns of dynam￾ics, the conformation of SPD in the D2 binding cavity becomes quite different from that in the D1 binding cavity (Fig. 6, C and D). The first adjustment was to the dihedral angle between rings A and D. In the SPD-D2 complex, ring A of SPD becomes almost perpendicular to ring D (dihedral angle of 98), whereas the relative position of these two rings in SPD still maintains planarity (dihedral angle of 150) in the SPD-D1 complex. The second adjustment is reflected in the different interactions between SPD with D1 and D2. As shown in Fig. 6, the interactions between ring A of SPD and the surrounding residues of D1 are mainly aromatic stacking with the side chains of W-3.28, W-7.40, and W-7.43, plus direct hydrogen bonding between the hydroxyl group on ring A of SPD with Nj1 of W-3.28 (Fig. 6, C and E). In the SPD￾D2 complex, ring A of SPD has much stronger electrostatic and hydrogen-bonding interactions with the protonated H-6.55. The role of H-6.55 in stabilizing the bent SPD con￾formation in the D2 binding cavity becomes more important because the N-6.55 at the same position in the D1 receptor is not involved in any interactions with SPD. Energies of the agonistic and antagonistic conformations of SPD We next investigated the energetics of the different bioactive conformations of SPD in the different receptors, D1 and D2. Quantum mechanical methods were employed to calculate FIGURE 5 Hydrogen-bonding network among K-167, D-173, E-302, and bridging water molecules in D1. FIGURE 6 Binding modes of SPD in the D1 (A, C, and E) and D2 (B, D, and F) receptors. (A and B) The models of D1-SPD and D2-SPD. The active sites were displayed as an electrostatic surface. (C and D) The important residues in the active sites of two complexes and SPD were rendered in stick representation. (E and F) Schematic depiction of the main interactions between SPD with D1 and D2. 1436 Fu et al. Biophysical Journal 93(5) 1431–1441