Binding of sulfonium‐ion analogues of di‐epi‐swainsonine and 8‐epi‐lentiginosine to Drosophila Golgi α‐mannosidase II: The role of water in inhibitor binding

Abstract

Retaining glycosidases operate by a two-step catalytic mechanism in which the transition states are characterized by buildup of a partial positive charge at the anomeric center. Sulfonium-ion analogues of the naturally occurring glycosidase inhibitors, swainsonine and 8-epi-lentiginosine, in which the bridgehead nitrogen atom is replaced by a sulfonium-ion, were synthesized in order to test the hypothesis that a sulfonium salt carrying a permanent positive charge would be an effective glycosidase inhibitor. Initial prediction based on computational docking indicated three plausible binding modes to Drosophila Golgi alpha-mannosidase II (dGMII), the most likely being close to that of swainsonine. Observation of the binding of di-epi-thioswainsonine and 8-epi-thiolentiginosine to dGMII from crystallographic data, however, revealed an orientation different from swainsonine in the active site. Screening these two compounds against dGMII shows that they are inhibitors with IC(50) values of 2.0 and 0.014 mM, respectively. This dramatic difference in affinity between the two compounds, which differ by only one hydroxyl group, is rationalized in terms of bound water molecules and the water molecule substructure in the active site, as identified by comparison of high resolution X-ray crystal structures of several dGMII-inhibitor complexes.

Structural Investigation of the Binding of 5‐Substituted Swainsonine Analogues to Golgi α‐Mannosidase II

“…In that case addition of a single hydroxyl group to 8-epi-thiolentiginosine changed the IC 50 from 14 mm to 2000 mm. [18] We had not previously observed this Arg228-mediated loss of water in other dGMII:inhibitor complexes, and this water is present in the complexes with 2-4. Interestingly, in the complexes of dGMII with the three least potent inhibitors (5-7) Arg228 is displaced to a significant extent and the critical water is missing in the electron density (results not shown).…”

“…We have previously shown [18] that removal of another conserved water molecule in the active site (indicated by an asterisk in Figure 4 C and D) caused by the displacement of Arg228 results in considerable loss of inhibitor potency (PDB ID: 2OW6). In that case addition of a single hydroxyl group to 8-epi-thiolentiginosine changed the IC 50 from 14 mm to 2000 mm.…”

“…[4,[13][14][15]18] Further, in a study of mannostatin analogues, interaction with the Arg876O appeared to be a key component. [19] In that study, removal of the thiomethyl group of mannostatin A (or a hydroxyl group in an analogue) to which Arg876O formed hydrogen bonds, resulted in a greater than 1000-fold reduction in inhibitor potency even though other interactions remained the same.…”

“…In the case of the b-substituted noncarbonyl-containing 7 the movement of Arg228 is even greater (by 0.5 ) than was observed in PDB: 2OW6. [18] The C(5)a substituents form new hydrophobic interactions with Arg876…”

α-d-Mannose derivatives as models designed for selective inhibition of Golgi α-mannosidase II

“…5.1.4.1. Benzyl 2,3,4,6-tetra-O-acetyl-a-D-mannopyranosyl sulfone (10). Yield 0.20 g; 96%; white solid; mp 169e172 C; lit [27] Tables 2 and 3.…”