Retaining Glycosidase Families

2 GH 2. The tetrameric lacZ (3-galactosidase of Escherichia coli has been intensively investigated for over 40 years, initially because of its relationship to Monod s classic work on enzyme induction. A very large set of data accumulated by Wallenfels and Weil unfortunately had to be discarded because it was obtained in inhibitory Tris buffers.The first glycosyl-glycosidase intermediate to become kinetically accessible was with this en-zyme the 2-fluoro-2-deoxy-a-D-galactopyranosyl enzyme, the parent enzyme and ES complexes have since been crystallised and its structure determined. 

The enzyme is Mg dependent, activation of the hydrolysis of O-glycosides in both loose and tight complexes being seen. The Mg (presumably 

Mg decreased trifluoroethyl galactoside by 10 -fold but for ethyl gal-actoside by 10 -fold. If the same substituted ethanols used as leaving groups were also used to intercept the galactosyl-enzyme, a value of jSjmc of —0.19 0.10 was obtained.Combination of this value with the jSig value gave a jSeq value of —0.56 0.05 for the equilibrium of eqn. (5.50)  

That the thermodynamic, in addition to the kinetic, stability of glycosides should be so dependent on leaving group ability was surprising. 

Although the E461G mutant undergoes the normal sort of acid-base mutant catalytic rescue with nucleophiles, giving p-galactosyl azide, acetate and butyrate in the presence of azide, acetate and butyrate anions, the smaller formate ion, when bound in the hole left by truncation of residue 461 from CH2-CH2-COO- to H, acts as a general base, rather than a nucleophile and accelerates production of (3-galactopyranose.  

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Although one can write a canonical mechanism for retaining O-glycosidases, there are departures from this mechanism in acid-catalytic machinery and the nature of the nucleophile (for some families, not the carboxylate shown). In Table 5.4 are set out the nature of the catalytic groups, the protonation trajectory and the conformations of substrate and glycosyl enzyme, where at least two of these are known. 

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