¹⁸O and secondary²H kinetic isotope effects confirm the existence of two pathways for acid-catalysed hydrolyses of α-arabinofuranosides

Abstract

The l80 kinetic isotope effect on the HCI0,-catalysed hydrolysis of 4-nitrophenyl [l -180]a-arabinofuranoside (kle/k,& is 1.023 f 0.003 at 80.0 "C; that for isopropyl [l -180] -a-arabinofuranoside is 0.988 at 30.2"C and the secondary deuterium effect on the hydrolysis of [2-*H]propan-2-yl a-arabinofuranoside (k,/k,) is 0.979. The nitrophenyl glycoside reacts with exocyclic C-0 cleavage and the propan-2-yl glycoside by endocyclic C-0 cleavage.

Purification and mechanistic properties of an extracellular α-l-arabinofuranosidase from Monilinia fructigena

“…Determination of 180 kinetic isotope effect. The 180 kinetic isotope effect for hydrolysis of p-nitrophenyl a-L-arabinofuranoside was measured at a substrate concentration of 8 mm (1OKm), with the labelled glycoside described by Bennet et al (1985), and the procedure used by Hosie & Sinnott (1985) for the equivalent measurement on a-glucosidase, except that the reaction was monitored at a wavelength of 345 nm, the isosbestic point of p-nitrophenol and its anion. Successive comparisons of 13-17 pairs of rates of hydrolysis of labelled and unlabelled substrates, with recrystallization of the substrates between each set of measurements, gave isotope effects of 1.0535+0.034, 1.0312 +0.014, 1.0395 +0.014 and 1.0299 +0.007.…”

“…We therefore now present data on the catalytic mechanism of an ac-L-arabinofuranosidase of Monilinia fructigena. Whereas ring-opening during non-enzymic hydrolyses of O-glycopyranosides lacks chemical precedent, ring-opening during acid catalysis of the hydrolysis of O-arabinofuranosides of electron-donating aglycones has been firmly established (Lonnberg & Kulonpiiii, 1977;Bennet et al, 1985). If ring-opening can be dismissed in a system chemically biassed towards it, then it can 'probably be dismissed in general as a mechanistic possibility for glycosidases.…”

Hydrolytic decomposition of glycosides in aqueous acids

“…An inverse effect is characteristic of an endocyclic cleavage, whereas a normal effect refers to the exocyclic cleavage. 14 The values obtained for isopropyl and 4-nitrophenyl glycosides of α-arabinofuranose (1a) are 0.987 and 1.023, respectively, 15 and hence fully consistent with endocyclic and exocyclic cleavage, respectively. β-deuterium effect of 0.979 is also consistent with the endocyclic cleavage of the isopropyl acetal of α-arabinofuranose.…”

Glycoside hydrolase stabilization of transition state charge: new directions for inhibitor design

“…This result clearly differentiates this class of inhibitor from pyranosides where positive charge development occurs mainly on the two adjacent positions to the anomeric carbon atom within the six-membered ring: the ring oxygen in the pyranosylium ion (C + -O 4 C]O + ) and carbon-2 (via hyperconjugation). 56,57 In addition, we note that the computed FES is relatively at in the vicinity of the quadratic region of the TS (Fig. 3A), a conclusion that suggests that the enzyme is able to stabilize positive charge development on C5 via a plethora of different TS structures.…”