Comparative molecular docking studies of lupeol and lupenone isolated from Pueraria lobata that inhibits BACE1: Probable remedies for Alzheimer's disease

Objective: To discover lead lupane triterpenoid's potential isolated from Pueraria lobata roots against beta-site amyloid precursor protein cleaving enzyme 1 (BACE1), which serve as a rate limiting step in amyloid beta (Ab) production altering the course of Alzheimer's disease. In addition, enzyme kinetics study and molecular docking were conducted to establish the inhibition type and structure activity relationship. Methods: A systematic study of 70% ethanolic P. lobata root extract was employed to identify its BACE1 inhibitory potential. Further, BACE1 inhibitory potential of two lupane terpenoids, yielded from ethanolic extract, was assessed. In order to determine their inhibition mode, Lineweaver-Burk plots and Michaelis-Menten model for BACE1 was performed. AutoDock 4.2 program in addition determined the molecular interaction of BACE1 with isolated terpenoids. Results: Considering the inhibitory potential of 70% ethanolic extract of P. lobata against BACE1 (IC50 = 80.35 mg/mL), lupeol and lupenone were subsequently isolated and exhibited notable or moderate BACE1 inhibitory activity with IC50 values of 5.12 and 62.98 mu mol/L, respectively, as compared to the positive control quercetin (IC50 = 21.28 mmol/L). The enzyme kinetics study enabled us to identify both compounds as competitive inhibitors, where lupeol displayed a very potent inhibition against BACE1 with low inhibition constant (Ki) value of 1.43 mu mol/L, signifying greater binding affinity. In order to understand the binding mechanism and structure-activity relationship of two triterpene-based BACE1 inhibitors, we employed computer aided docking studies which evidently revealed that hydroxyl group of lupeol formed two hydrogen bonds with the ASP32 (catalytic aspartic residue) and SER35 residues of BACE1 with the binding energy of (-8.2 kcal/mol), while the ketone group of lupenone did not form any hydrogen bonds with BACE1 giving evidence for less binding affinity. These results in turn have predicted the dependence of the inhibitory activity in the presence of hydroxyl group which has provided a new basis for BACE1 blockade. Conclusions: Our results have successfully explored the molecular mechanism of lupane triterpenoids via BACE1 inhibition, suggesting that lupeol in particular could be utilized as a useful therapeutic and preventive agent to mitigate Alzheimer's disease.