Figure 6.

Scheme of the proposed model for different transport modes mediated by GR-WT. (A) Structure section of BR-WT dark state (PDB:1M0L, 1.47 Å). Water molecules at the cytoplasmic side are shown according to Freier et al. (37). (B) Model of the predicted dark state structure of GR-WT based on the known Xanthorhodopsin structure (PDB:3DDL, 1.90 Å) for standard conditions (pH_o 7.5, 0 mV). The water cluster near D121, D253, and the protonated Schiff base is assumed to be similar to BR. Between E132 and S77, at least one water molecule is inserted that is also in close distance to the backbone of K257. (C) Structural section of BR-WT during the late photocycle N-intermediate (PDB:1P8U, 1.62 Å). The late N-intermediate is characterized by reprotonation of the RSB from the primary proton donor D96 via a water chain. Full assembly of the water chain between D96 and the Schiff base is developed during the M to N transition. Protein hydrogens are not shown. (D and E) Model of proton translocation in GR-WT show as dark-state model without (D) and with (E) applied negative electrochemical gradients. (Arrows) Assumed proton movements.