It was revealed that the solvent-accessible surface was displaced outward from the molecular surface by a distance equal to the probe radius [8].
Each histone has a N-terminal tail that projects outward, making it accessible to modifying enzymes that adjust the structure of the chromatin, depending upon the enzymatic process and the amino acid at which it occurs.
Furthermore, among the three mutated residues shown in Figures 6, I25 (green) is the only one facing outward from the channel axis and accessible from the periphery of the protein.
Furthermore, among the three mutated residues shown in Figure 6, I23 (green) is the only one facing outward from the channel axis and accessible from the periphery of the protein (see Author response image 4B and D).
In the apo or proton-bound state, the ion-binding site near the center of TMH 6 would be accessible from the extracellular side, making this the outward-open state.
I23 is the only one among then that is facing outward from the channel axis and is accessible from the periphery of the protein.
Starting from the outward open conformation the binding site is accessible to the extracellular side of the membrane, and the intracellular gate is closed and stabilised by a possible salt bridge between Lys126 (TM4) and Glu400 (TM10).
In the outward facing conformation, the central binding site should be accessible to the extracellular side of the membrane to receive peptides, in a conformation potentially resembling that of the fucose permease FucP (Dang et al, 2010).
This row of images demonstrates that the drug binding surfaces of Pgp are exposed to the cytoplasmic face of the membrane and cytoplasm in the fully opened inward conformation and that the drug binding surfaces become progressively less accessible to the cytoplasm as the transporter moves toward the fully opened outward conformation.
As a result of the helix bundle rotation, the binding site with the bound citrate moves by 16 Å from the external membrane surface in the outward-facing state to a position where it is accessible from the cytoplasmic membrane surface in the inward-facing state.
