Molecular Switch Controlling the Binding of Anionic Bile Acid Conjugates to Human Apical Sodium-Dependent Bile Acid Transporter

Abstract

The human apical sodium-dependent bile acid transporter (hASBT) may serve as a prodrug target for oral drug absorption. Synthetic, biological, NMR and computational approaches identified the structure-activity relationships of mono- and dianionic bile acid conjugates for hASBT binding. Experimental data combined with a conformationally-sampled pharmacophore/QSAR modeling approach (CSP-SAR) predicted that dianionic substituents with intramolecular hydrogen bonding between hydroxyls on the cholane skeleton and the acid group on the conjugate's aromatic ring increased conjugate hydrophobicity and improved binding affinity. Notably, the model predicted the presence of a conformational molecular switch, where shifting the carboxylate substituent on an aromatic ring by a single position controlled binding affinity. Model validation was performed by effectively shifting the spatial location of the carboxylate by inserting a methylene adjacent to the aromatic ring, resulting in the predicted alteration in binding affinity. This work illustrates conformation as a determinant of ligand binding affinity to a biological transporter.

Mechanistic interpretation of conventional Michaelis–Menten parameters in a transporter system

“…ASBT Ki vs Km values of 50 compounds from previous studies (9, 10, 11, 13). Each compound is an ASBT substrate and inhibitor.…”

“…To determine if the model-supported conclusion Km = Ki is also supported by observed data, parameters of 50 previously tested non-native bile acids were analyzed (9, 10, 11, 13). In addition to examining previous data, Matlab error simulations were performed to evaluate the extent that error explains observed differences between Km and Ki.…”

A substrate pharmacophore for the human sodium taurocholate co-transporting polypeptide

“…CDCA-L-Val-OH and CDCA-L-Glu-γ-Benzyl Ester were synthesized as previously described with minor modification (Rais et al, 2010). Briefly, CDCA (5 g, 12.7 mmol) was added to 15mL dimethylformamide(DMF) along with N,N,N’,N’-Tetramethyl-O-(1H-benzotriazol-1-yl)uronium hexafluorophosphate (HBTU, 5.07 g, 13.37 mmol), 1-Hydroxybenzotriazole hydrate (HOBt, 0.86 g, 6.37 mmol), and triethylamine (TEA, 1.86 mL, 13.37 mmol).…”

Structural requirements of bile acid transporters: C-3 and C-7 modifications of steroidal hydroxyl groups

“…However, a single negative charge in the C-24 region promotes translocation across the transporter (Balakrishnan et al, 2006a). ASBT also accommodates various drug-like single ring scaffolds with different substituents attached to the bile acid (Gonzalez et al, 2009; Rais et al, 2010a; Rais et al, 2010b; Zheng et al, 2010). Using C-24 bile acid linkage chemistry, bile acid-based prodrugs of gabapentin, ketoprofen, and niacin have been shown to be ASBT substrates (Rais et al, 2011; Zheng and Polli, 2010).…”