Gluco-1 H-imidazole: A New Class of Azole-Type β-Glucosidase Inhibitor

Abstract

Gluco-azoles competitively inhibit glucosidases by transition-state mimicry and their ability to interact with catalytic acid residues in glucosidase active sites. We noted that no azole-type inhibitors described, to date, possess a protic nitrogen characteristic for 1 H-imidazoles. Here, we present gluco-1 H-imidazole, a gluco-azole bearing a 1 H-imidazole fused to a glucopyranose-configured cyclitol core, and three close analogues as new glucosidase inhibitors. All compounds inhibit human retaining β-glucosidase, GBA1, with the most potent ones inhibiting this enzyme (deficient in Gaucher disease) on a par with glucoimidazole. None inhibit glucosylceramide synthase, cytosolic β-glucosidase GBA2 or α-glucosidase GAA. Structural, physical and computational studies provide first insights into the binding mode of this conceptually new class of retaining β-glucosidase inhibitors.

Figure 1

(a) Known cyclitol-azole type glycosidase inhibitors and their inhibitory potencies against porcine NAG (1) and sweet almond β-glucosidase (2–5).⁻ (b) 1H-Imidazoles subject of this study.

Scheme 1. Synthesis of 1H-Imidazoles 6 and 7

Reagents and conditions: (a) RuCl₃·3H₂O (7 mol %), NaIO₄, EtOAc, MeCN, H₂O, 0 °C, 90 min, 11a 40%, 11b 32%; (b) MsCl, N-methyl-imidazole, Et₃N, CHCl₃, rt, 16 h, then NaN₃, DMF, 100 °C, 16 h, 12a 74%, 12b 58%; (c) PtO₂, H₂, THF, rt, 16 h, 13a 80%, 13b 96%; (d) trimethyl orthoformate, HFIP, rt, 16 h, 14a 76%, 14b 87%; or trimethyl orthovalerate, HFIP, rt, 16 h, 16a 74%, 16b 78%; (e) for 15: IBX, DMSO, 45 °C, 16 h, 75% from 14a, 71% from 14b; for 17: (COCl)₂, DMSO, DCM, −60 °C, 1 h, 76% from 16a; 70% from 16b; (f) Pd(OH)₂/C, H₂, HCl, MeOH, quant. 6; quant. 7.

Figure 2

(a) Gluco-1H-imidazole 6 in complex with TmGH1, with direct H-bonding interactions shown. (b) Overlay of 5 (pink) and 6 (cyan) within the TmGH1 active site (chain B from each structure). (c) Gluco-1H-imidazole 6 in complex with TxGH116. (d) Overlay of 5 (salmon) and 6 (blue) within the TxGH116 active site. Electron densities are REFMAC maximum-likelihood/σ_A weighted 2Fo–Fc contoured to 0.38 (TmGH1) or 0.48 (TxGH116) e^–/ų.

Figure 3

Interactions of gluco-1H-imidazole 6 and classical glucoimidazole 5 with the catalytic residues. (a) Prototropic tautomerism of 6. (b) Positive charge is delocalized onto the “apical” carbon in protonated 6. (c) In 5, positive charge is delocalized onto the anomeric equivalent carbon, ideally located for charge–charge interaction with the nucleophile residue. Mulliken charges are annotated in red.