Dependence upon pH of steady-state parameters for the beta-galactosidase-catalysed hydrolyses of beta-D-galactopyranosyl derivatives of different chemical types

New class of quaternary ammonium salts, derivatives of methyl d-glucopyranosides

Reactions of two aromatic and two aliphatic amines with methyl 6-O-p-toluenesulfonyl-alpha-D-glucopyranoside or methyl 6-O-p-toluenesulfonyl-beta-D-glucopyranoside were performed on a micro-scale. The synthesis and preparative isolation methods have been developed for quaternary N-(methyl 2,3,4-tri-O-acetyl-6-deoxy-alpha- and -beta-D-glucopyranoside-6-yl)ammonium salts derived from three amines: trimethylamine, 2-methylpyridine, and pyridine. The reaction products were examined with 1H, 13C NMR spectroscopy. N-(Methyl 2,3,4-tri-O-acetyl-6-deoxy-beta-d-glucopyranoside-6-yl)trimethylammonium tosylate was additionally analyzed with X-ray crystallography.

Molecular and crystal structures of N-(beta-D-galactopyranosyl)pyridinium bromide and its per-O-acetylated derivative

1H NMR spectroscopy and X-ray diffraction data are described for N-(2,3,4,6-tetra-O-acetyl-beta-D-galactopyranosyl)pyridinium bromide and N-(beta-D-galactopyranosyl)pyridinium bromide. X-ray crystallography revealed that the O-acetylated salt crystallizes with two molecules of water and one molecule of ethanol.

The catalytic consequences of experimental evolution. Transition-state structure during catalysis by the evolved beta-galactosidases of Escherichia coli (ebg enzymes) changed by a single mutational event

1. The first chemical step in the hydrolysis of galactosylpyridinium ions by the evolvant ebg enzyme is less sensitive to leaving-group acidity than in the case of the wild-type ebg enzyme, implying less glycone-aglycone-bond fission at the transition state. 2. The first chemical step in the hydrolysis of aryl galactosides by ebg enzyme is probably less sensitive to leaving-group acidity than in the case of ebg enzyme, possibly as a consequence of resulting in more effective proton donation to the leaving aglycone. 3. alpha-Deuterium kinetic isotope effects of 1.1(0) and beta-deuterium kinetic isotope effects of 1.0(0) were measured for the hydrolysis of galactosyl-enzyme intermediates derived from ebg and ebg enzymes: these effects are not compatible with reaction of the sugar ring through a 4C1-like conformation, or with an ionic glycosyl-enzyme intermediate. 4. The variation with pH of steady-state kinetic parameters for hydrolysis of p-nitrophenyl galactoside by ebg and ebg enzymes and of 3-methylphenyl beta-galactoside, 3,4-dinitrophenyl beta-galactoside and beta-galactosyl-3-bromopyridinium ion by ebg enzyme was measured. The steep, non-classical, fall in activity against p-nitrophenyl galactoside at low pH observed with ebg and ebg enzymes is not observed with ebg enzymes.

Identification of an essential carboxylate group at the active site of lacZ beta-galactosidase from Escherichia coli

[3H] Conduritol C cis-epoxide (1,2-anhydro-epi-inositol, I) was synthesized as an active-site-directed inhibitor for lacZ beta-galactosidase from Escherichia coli. A considerable kinetic isotope effect was noted in the reduction by [3H]NaBH4 of the p-benzoquinone-derived precursor for I. Complete loss of beta-galactosidase activity occurred on incorporation of 4 mol I/mol beta-galactosidase tetramer. The inhibitor was very labile in the denatured enzyme at pH greater than 8, implying the formation of an ester bond between I and a carboxylate at the active site. The radioactive material released from the labeled enzyme was identified as allo-inositol. The stereochemistry of the expoxide reaction (trans-diaxial ring opening) is thus the same as for beta-glucosidases with the corresponding epoxides. The binding site for I was identified as Glu-461 by the isolation and partial sequence analysis of a radioactive octapeptide from the cyanogen bromide and pepsin fragments of the labeled enzyme. A failure to determine the N-terminal amino acid of the labeled peptide is ascribed to the great reactivity of the esterified gamma-carboxyl group of its N-terminal Glu-461 which causes rapid cyclisation of this residue to pyroglutamate, even under weakly basic conditions. The participation of the carboxylate of Glu-461 in catalysis is discussed.

Importance of hydroxyls at positions 3, 4, and 6 for binding to the "galactose" site of beta-galactosidase (Escherichia coli)

Values of kinetic inhibition constants representing dissociation constants of inhibitors interacting with the free form of beta-galactosidase ('galactose" site) were determined for a large number of monosaccharides and alcohols. The studies showed that when hydroxyl groups were present at positions 3, 4, and 6 and were in the same orientation as in galactose, binding to the 'galactose" site was good. Alterations in the position of or a lack of a hydroxyl at any of these three positions decreased binding dramatically. The overall binding specificity thus depends to a large extent on these three positions. Positions 3 and 4 were critical. A misorientation at either position eliminated most of the binding. Position 6 was a little more independent. The absence of a hydroxyl at that position did not totally eliminate the binding. The position 2 hydroxyl did not seem to be important for binding as its absence or orientation had only a small effect on the binding capacity. Studies regarding position 5 were inconclusive but since a minimum of four hydroxyl groups seemed important for binding and since positions 3, 4, and 6 are important the 5 position may also be of significance. The work also showed that p-nitrophenyl-beta-D-glucopyranoside, p-nitrophenyl-beta-L-arabinofuranoside, p-nitrophenyl-beta-D-xylopyranoside, cellobiose, and gentiobiose were not substrates in spite of the fact that the nitrophenyl groups made some of these bind quite well. Thus, not only are the presence and configuration of the hydroxyls at positions 3 and 4 important for binding, they are also important in catalysis. p-Nitrophenyl-beta-D-fucopyranoside was a substrate indicating that the 6 position hydroxyl is not as important for production binding as are the hydroxyls at positions 3 and 4.

The use of inhibitors in the study of glycosidases

The use of non-covalent as well as covalent inhibitors can be a useful tool to approach the mechanism of activity of glycosidases. An efficient method to determine the essential amino-acid groups directly or indirectly involved in the catalytic process is the use of active site directed irreversible inhibitors. Epoxide derivatives from conduritol B and conduritol C are the most important inhibitors in this group. The use of active site reversible inhibitors: cationic and basic glycosyl derivatives, glycals, glyconolactones, thioglycosides, is effective to study the different charges at the active site or the transition state during catalysis and also to detect conformational adaptability of an enzyme. Furthermore, inhibitors can be valuable tools to investigate various aspects of the physiological role of glycosidases.

Inactivation of beta-Galactosidase by iodination of tyrosine-253

Beta-Galactosidase is rapidly inactivated by iodination catalyzed by lactoperoxidase but is not inactivated in the presence of the substrate analogue, isopropyl beta-D-thiogalactoside (IPTG). Enzyme activity is lost upon the incorporation of 1 mol of iodine per mol of monomer, without dissociation of the tetrameric structure. Tryptic digests of beta-galactosidase iodinated with 125I in the presence and absence of IPTG were separated by high-performance liquid chromatography and were compared. One fraction was found to be more highly labeled in the digest from the inactivated protein. After isolation of the peptide, amino acid analysis indicated it to be Asp-Tyr-Leu-Arg, residues 252-255. Thus, Tyr-253 is the most reactive tyrosine in beta-galactosidase. This suggests that the conformation of this region of the protein may be altered by binding of IPTG to make Tyr-253 less accessible to iodination. Alternatively, Tyr-253 could be an active-site residue.