Enzyme hydrolysis, glycosides

It was of interest to determine whether glycosides of 6-deoxy-D-xylo-hex-5-enopyranose were susceptible to enzyme hydrolysis by / -glucosi-dase. Since aromatic glucosides are hydrolyzed by this enzyme at a much faster rate than aliphatic glycosides, phenyl 6-deoxy-/ -D-rt/Zo-hex-5-enopyranoside (18) was prepared (20). Phenyl / -D-glucopyranoside was converted to the 6-tosylate by selective esterification and then, by conventional procedures, transformed to phenyl 2,3,4-tri-0-acetyl-6-deoxy-... 

The implications of the above observations may be important, especially if similar trends are observed in pyranose anomers. For example, with respect to the mechanism of acid-catalyzed hydrolysis of pyranosides, endocyclic C-0 bond cleavage (preceeded by 05 protonation) may be assisted in P-anomers in which the Cl-01 bond is equatorial, since the 04-Cl bond may already be extended in these anomers. By a similar argument, exocyclic C-0 scission (preceeded by 01 protonation) may be assisted in the hydrolysis of a-pyranosides in which the Cl-01 is axial and extended, thus resembling the transition state. Post and Karplus have recently suggested that enzyme-catalyzed glycoside hydrolysis of P-pyranosides may indeed take place by ring oxygen protonation, followed by endocyclic C-0 bond scission. 

The main cyanogenic glycoside in laurel is prunasin, the P-o-glucoside of benzaldehyde cyanohydrin. The enzymic hydrolysis of prunasin may be visualized as an acid-catalysed process, first of all hydrolysing the acetal linkage to produce glucose and the cyanohydrin. Further hydrolysis results in reversal of cyanohydrin formation, giving HCN and benzaldehyde. 

Enzymic hydrolysis (25-40°C) at the heterosidic bond of the chromogenic substrates was followed either continuously (via formation of 2 -chloro,4 -nitrophenol) at pH 5.5 (O.D. 405 nm) or discontinuously (4-methylumbel-liferone fluorescence at pH 10, emission at A > 435, excitation at A366 nm). Reaction rates were calculated from the linear increase of O.D. (em = 9000 M-1cm-1) or fluorescence (standardization with 4-methylumbelliferone) versus time. Alternatively, an HPLC method was used to follow the formation of chromophoric reaction products, phenols and glycosides (1). Concentrations were calculated from peak heights after appropriate standardization. 

It has been pointed out in Chapter 2 (p. 39) (see also 1-4) that there is good evidence which indicates that in the hydrolysis of B-glycosides by lysozyme, the substrate must take a boat conformation in order to produce the half-chair oxocarbonium ion 0+2- -3i)- Lysozyme is therefore a good example which provides evidence that stereoelectronic effects play a key role in enzymic hydrolysis. 

Wintergreen Gaultheria procumbens (Ericacae) or Betula lenta (Betulaceae) leaves bark 0.7-1.5 0.2-0.6 methyl salicylate (98%) flavour, antiseptic, antirheumatic prior to distillation, plant material is macerated with water to allow enzymic hydrolysis of glycosides methyl salicylate is now produced synthetically... 

Cyanogenic glycosides, which are widely distributed in higher plants, are a bound form of toxic hydrogen cyanide, which is released from the glucoside following enzyme hydrolysis.214... 

Giinata, Y.Z., Bitteur, S., Brillouet, J.-M., Bayonove, C. L. Cordonnier, R. E. (1988). Sequential enzymic hydrolysis of potentially aromatic glycosides from grapes. Carbohydr. Research, 184, 139-149. 

For enzyme hydrolysis a glycosidic extract prepared from 1500 mL juice or skin extract was hydrolyzed in pH 5 buffer (162 mL) at 37°C for 16 h with Rohapect C (12 mg, Rohm, Darmstadt, Germany). 

Figure 4. Top trace Gas chromatogram of aglycons generated by enzyme hydrolysis of C retained monoterpene glycosides. Lower trace The same glycosidic material treated with denatured enzyme. For peak assignments refer to Figure 1. B benzyl alcohol P=2-phenylethanol and i.s. internal standard (octan-l-ol).

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