Coupling of retinal, protein, and water dynamics in squid rhodopsin

Abstract

The light-induced isomerization of the retinal from 11-cis to all-trans triggers changes in the conformation of visual rhodopsins that lead to the formation of the activated state, which is ready to interact with the G protein. To begin to understand how changes in the structure and dynamics of the retinal are transmitted to the protein, we performed molecular dynamics simulations of squid rhodopsin with 11-cis and all-trans retinal, and with two different force fields for describing the retinal molecule. The results indicate that structural rearrangements in the binding pocket, albeit small, propagate toward the cytoplasmic side of the protein, and affect the dynamics of internal water molecules. The sensitivity of the active-site interactions on the retinal force-field parameters highlights the coupling between the retinal molecule and its immediate protein environment.

Figure 1

Retinal binding pocket in MD simulations of squid rhodopsin in the dark state. (A) Overlap of the geometries at the end of Sim1 (cyan), Sim2 (purple), and Sim3 (yellow) on the starting crystal structure (orange) (15). For simplicity, only retinal and selected amino-acid side-chain atoms are depicted explicitly. (B) Protein amino acids within 6 Å of retinal heavy atoms. Retinal heavy atoms are shown as sticks and transparent surface representation. Bold amino-acid labels represent contacts not present in the crystal structure and found in Sim1 (cyan circle), Sim2 (purple circle), and/or Sim3 (yellow circle); labels in italics represent contacts present in the crystal structure and not found in Sim1 (cyan circle), Sim2 (purple circle), and/or Sim3 (yellow circle). Retinal-protein interactions were identified from average positions over the last 10 ns of simulation. Protein domains for helices TM1-7 and EL2 are indicated to identify amino-acid positions in the protein structure.

Figure 2

Binding pocket of squid rhodopsin in the all-trans retinal state. (A) Configurations at the end of Sim4 (cyan circle), Sim5 (purple circle), and/or Sim6 (yellow) compared with the starting QM/MM-optimized all-trans geometry. The largest deviation of the retinal conformation from the starting configuration was observed in Sim5, where the polyene chain and the β-ionone ring adjust their geometries after reorientation of the protonated Schiff base. (B) Snapshots at 0, 10, 20, and 40 ps of Sim5, showing the rotation of the N-H bond (depicted as blue-white sticks) from TM2 toward EL2. The rotation switches the H-bond acceptor from N87 to E180, which is the closest anionic residue to the protonated Schiff base. (Dashed lines) H-bond interactions. A rotation of the Schiff base N-H bond base has been detected in the metarhodopsin I state of bovine rhodopsin (54). (C) Protein amino acids within 6 Å of retinal heavy atoms in the all-trans models. Retinal is shown as sticks and transparent surface representations. Amino-acid labels in bold indicate contacts not present in the crystal structure and found in Sim4 (cyan circle), Sim5 (purple circle), and/or Sim6 (yellow circle); labels in italics represent contacts present in the squid rhodopsin crystal structure (15) and not found in Sim1 (cyan circle), Sim2 (purple circle), and/or Sim3 (yellow circle). Retinal-protein interactions were identified from averaged positions over the last 10 ns of simulation. Protein domains for helices TM1–7 and EL2 are shown to indicate amino-acid positions in the protein structure.

Figure 3

Isodensity surfaces (in red) of the IWM in the interhelical region. Density values of 0.1 molecules/ų were traced over grid points at 0.2 Å (see text). The calculations were performed using the last 15 ns of each trajectory. Retinal heavy atoms and amino acids D80, S84, S122, W274, and Y315 are depicted as licorice (with carbon atoms, cyan; with nitrogen, light blue; with oxygen, red; and with hydrogen atoms, light gray).