Modeling Substrate Entry into the P‑Glycoprotein Efflux Pump at the Blood–Brain Barrier

We
report molecular dynamics simulations of rhodamine entry into
the central binding cavity of P-gp in the inward open conformation.
Rhodamine can enter the inner volume via passive transport across
the luminal membrane or lateral diffusion in the lipid bilayer. Entry
into the inner volume is determined by the aperture angle at the apex
of the protein, with a critical angle of 27° for rhodamine. The
central binding cavity has an aqueous phase with a few lipids, which
significantly reduces substrate diffusion. Within the central binding
cavity, we identified regions with relatively weak binding, suggesting
that the combination of reduced mobility and weak substrate binding
confines rhodamine to enable the completion of the efflux cycle. Tariquidar,
a P-gp inhibitor, aggregates at the lower arms of the P-gp, suggesting
that inhibition involves steric hindrance of entry into the inner
volume and/or steric hindrance of access of ATP to the nucleotide-binding
domains.