Energetics of Substrate Binding and Conformational Change of the Bile Acid Transporter ASBTNm

Abstract

The apical sodium-dependant bile acid transporter (ASBT) utilises the sodium gradient to drive the re-absorption of bile acids from the intestine, operating through an alternating access mechanism. It is of interest as a potential target for the treatment of hypercholesterolaemia as well as for drug delivery. Several structures of bacterial homologues are available, including that from Neisseria meningitidis (ASBTNM) in an inward-facing conformation with sodium and the bile acid taurocholate bound. However, less is known about the dynamic details of the ASBT transport cycle, including the nature of the conformational transition and the interplay of the substrates. We studied the behaviour of ASBTNM on an atomistic scale with molecular dynamics (MD) simulations. Multi-microsecond simulations were used to investigate the stable inward- and outward-facing states and substrate binding sites. The energetics of the conformational transition and of sodium and taurocholate binding to ASBTNM, and how this is altered in different substrate-bound configurations, were calculated using a two-stage enhanced sampling approach, in which semi-quantitative energetic landscapes were first obtained from short metadynamics simulations and then used to guide parameter selection for more accurate Bias Exchange Umbrella Sampling (BEUS) simulations. Together, these results represent an important step towards understanding the complete transport cycle of ASBT.