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

Identification of acacia gum fermenting bacteria from pooled human feces using anaerobic enrichment culture

Commercial acacia gum (AG) used in this study is a premium-grade free-flowing powder. It is a gummy exudate composed of arabinogalactan branched polysaccharide, a biopolymer of arabinose and galactose. Also known as food additive, acacia gum (E414), which is presently marketed as a functional dietary fiber to improve overall human gut health. The health effects may be related to the luminal pH regulation from the short-chain fatty acids (SCFA) production. Studies suggested that amylolytic and butyrogenic pathways are the major factors determining the SCFA outcome of AG in the lower gut. However, the primary bacteria involved in the fermentation have not been studied. This study aimed to investigate the putative primary degraders of acacia gum in the gut ecosystem. Isolation and identification of gum-fermenting bacteria were performed through enrichment culture fermentation. The experiment was conducted in an anaerobic chamber for 144 h in three stages. The study was conducted in triplicate using an anaerobic chamber system. This culture system allows specific responses to support only bacteria that are responsible for gum fermentation among the gut microbiota. Five bacterial strains were isolated and found to be gum-fermenting bacteria. Based on the 16s RNA sequence, the isolates matched to butyrate-producing Escherichia fergusonii, ATCC 35469.

Crystal structure of 4-nitro-phenyl 6-O-ethyl-β-d-galacto-pyran-oside monohydrate

The synthesis and crystal structure of the title compound, C14H19NO8·H2O, prepared in three steps from 6-O-ethyl-1,2;3,4-di-O-iso-propyl-idene-α-d-galacto-pyran-ose using protecting-group strategies employed in carbohydrate chemistry, is reported. The asymmetric unit consists of a single galactoside mol-ecule, in which the pyran-oid ring has a 4C1 conformation and the 4-nitro-phenyl moiety is essentially planar. In the crystal, each carbohydrate is surrounded by other d-galactose residues and water mol-ecules, linked by O-H⋯O hydrogen bonds involving all hy-droxy groups, giving a two-dimensional substructure lying parallel to (100) and extended into three dimensions by C-H⋯O inter-actions.

LacZ β-galactosidase: structure and function of an enzyme of historical and molecular biological importance

This review provides an overview of the structure, function, and catalytic mechanism of lacZ β-galactosidase. The protein played a central role in Jacob and Monod's development of the operon model for the regulation of gene expression. Determination of the crystal structure made it possible to understand why deletion of certain residues toward the amino-terminus not only caused the full enzyme tetramer to dissociate into dimers but also abolished activity. It was also possible to rationalize α-complementation, in which addition to the inactive dimers of peptides containing the "missing" N-terminal residues restored catalytic activity. The enzyme is well known to signal its presence by hydrolyzing X-gal to produce a blue product. That this reaction takes place in crystals of the protein confirms that the X-ray structure represents an active conformation. Individual tetramers of β-galactosidase have been measured to catalyze 38,500 ± 900 reactions per minute. Extensive kinetic, biochemical, mutagenic, and crystallographic analyses have made it possible to develop a presumed mechanism of action. Substrate initially binds near the top of the active site but then moves deeper for reaction. The first catalytic step (called galactosylation) is a nucleophilic displacement by Glu537 to form a covalent bond with galactose. This is initiated by proton donation by Glu461. The second displacement (degalactosylation) by water or an acceptor is initiated by proton abstraction by Glu461. Both of these displacements occur via planar oxocarbenium ion-like transition states. The acceptor reaction with glucose is important for the formation of allolactose, the natural inducer of the lac operon.

Ser-796 of β-galactosidase (Escherichia coli) plays a key role in maintaining a balance between the opened and closed conformations of the catalytically important active site loop

A loop (residues 794-803) at the active site of β-galactosidase (Escherichia coli) opens and closes during catalysis. The α and β carbons of Ser-796 form a hydrophobic connection to Phe-601 when the loop is closed while a connection via two H-bonds with the Ser hydroxyl occurs with the loop open. β-Galactosidases with substitutions for Ser-796 were investigated. Replacement by Ala strongly stabilizes the closed conformation because of greater hydrophobicity and loss of H-bonding ability while replacement with Thr stabilizes the open form through hydrophobic interactions with its methyl group. Upon substitution with Asp much of the defined loop structure is lost. The different open-closed equilibria cause differences in the stabilities of the enzyme·substrate and enzyme·transition state complexes and of the covalent intermediate that affect the activation thermodynamics. With Ala, large changes of both the galactosylation (k(2)) and degalactosylation (k(3)) rates occur. With Thr and Asp, the k(2) and k(3) were not changed as much but large ΔH(‡) and TΔS(‡) changes showed that the substitutions caused mechanistic changes. Overall, the hydrophobic and H-bonding properties of Ser-796 result in interactions strong enough to stabilize the open or closed conformations of the loop but weak enough to allow loop movement during the reaction.

Importance of Arg-599 of β-galactosidase (Escherichia coli) as an anchor for the open conformations of Phe-601 and the active-site loop

Structural and kinetic data show that Arg-599 of β-galactosidase plays an important role in anchoring the "open" conformations of both Phe-601 and an active-site loop (residues 794-803). When alanine was substituted for Arg-599, the conformations of Phe-601 and the loop shifted towards the "closed" positions because interactions with the guanidinium side chain were lost. Also, Phe-601, the loop, and Na+, which is ligated by the backbone carbonyl of Phe-601, lost structural order, as indicated by large B-factors. IPTG, a substrate analog, restored the conformations of Phe-601 and the loop of R599A-β-galactosidase to the open state found with IPTG-complexed native enzyme and partially reinstated order. ᴅ-Galactonolactone, a transition state analog, restored the closed conformations of R599A-β-galactosidase to those found with ᴅ-galactonolactone-complexed native enzyme and completely re-established the order. Substrates and substrate analogs bound R599A-β-galactosidase with less affinity because the closed conformation does not allow substrate binding and extra energy is required for Phe-601 and the loop to open. In contrast, transition state analog binding, which occurs best when the loop is closed, was several-fold better. The higher energy level of the enzyme•substrate complex and the lower energy level of the first transition state means that less activation energy is needed to form the first transition state and thus the rate of the first catalytic step (k2) increased substantially. The rate of the second catalytic step (k3) decreased, likely because the covalent form is more stabilized than the second transition state when Phe-601 and the loop are closed. The importance of the guanidinium group of Arg-599 was confirmed by restoration of conformation, order, and activity by guanidinium ions.

Role of Met-542 as a guide for the conformational changes of Phe-601 that occur during the reaction of β-galactosidase (Escherichia coli)

The Met-542 residue of β-galactosidase is important for the enzyme's activity because it acts as a guide for the movement of the benzyl side chain of Phe-601 between two stable positions. This movement occurs in concert with an important conformational change (open vs. closed) of an active site loop (residues 794-803). Phe-601 and Arg-599, which interact with each other via the π electrons of Phe-601 and the guanidium cation of Arg-599, move out of their normal positions and become disordered when Met-542 is replaced by an Ala residue because of the loss of the guide. Since the backbone carbonyl of Phe-601 is a ligand for Na(+), the Na(+) also moves out of its normal position and becomes disordered; the Na(+) binds about 120 times more poorly. In turn, two other Na(+) ligands, Asn-604 and Asp-201, become disordered. A substrate analog (IPTG) restored Arg-599, Phe-601, and Na(+) to their normal open-loop positions, whereas a transition state analog d-galactonolactone) restored them to their normal closed-loop positions. These compounds also restored order to Phe-601, Asn-604, Asp-201, and Na(+). Binding energy was, however, necessary to restore structure and order. The K(s) values of oNPG and pNPG and the competitive K(i) values of substrate analogs were 90-250 times higher than with native enzyme, whereas the competitive K(i) values of transition state analogs were ~3.5-10 times higher. Because of this, the E•S energy level is raised more than the E•transition state energy level and less activation energy is needed for galactosylation. The galactosylation rates (k₂) of M542A-β-galactosidase therefore increase. However, the rate of degalactosylation (k₃) decreased because the E•transition state complex is less stable.

Direct and indirect roles of His-418 in metal binding and in the activity of beta-galactosidase (E. coli)

The active site of ss-galactosidase (E. coli) contains a Mg(2+) ion ligated by Glu-416, His-418 and Glu-461 plus three water molecules. A Na(+) ion binds nearby. To better understand the role of the active site Mg(2+) and its ligands, His-418 was substituted with Asn, Glu and Phe. The Asn-418 and Glu-418 variants could be crystallized and the structures were shown to be very similar to native enzyme. The Glu-418 variant showed increased mobility of some residues in the active site, which explains why the substitutions at the Mg(2+) site also reduce Na(+) binding affinity. The Phe variant had reduced stability, bound Mg(2+) weakly and could not be crystallized. All three variants have low catalytic activity due to large decreases in the degalactosylation rate. Large decreases in substrate binding affinity were also observed but transition state analogs bound as well or better than to native. The results indicate that His-418, together with the Mg(2+), modulate the central role of Glu-461 in binding and as a general acid/base catalyst in the overall catalytic mechanism. Glucose binding as an acceptor was also dramatically decreased, indicating that His-418 is very important for the formation of allolactose (the natural inducer of the lac operon).

Synthesis and biological evaluation of (11)C-labeled beta-galactosyl triazoles as potential PET tracers for in vivo LacZ reporter gene imaging

In our aim to develop LacZ reporter probes with a good retention in LacZ expressing cells, we report the synthesis and preliminary evaluation of two carbon-11 labeled beta-galactosyl triazoles 1-(beta-d-galactopyranosyl)-4-(p-[(11)C]methoxyphenyl)-1,2,3-triazole ([(11)C]-6) and 1-(beta-d-galactopyranosyl)-4-(6-[(11)C]methoxynaphthyl)-1,2,3-triazole ([(11)C]-13). The precursors for the radiolabeling and the non-radioactive analogues (6 and 13) were synthesized using straightforward 'click' chemistry. In vitro incubation experiments of 6 with beta-galactosidase in the presence of o-nitrophenyl beta-d-galactopyranoside (ONPG) showed that the triazolic compound was an inhibitor of beta-galactosidase activity. Radiolabeling of both precursors was performed using [(11)C]methyl iodide as alkylating agent at 70 degrees C in DMF in the presence of a small amount of base. The logP values were -0.1 and 1.4, respectively, for [(11)C]-6 and [(11)C]-13, the latter therefore being a good candidate for increased cellular uptake via passive diffusion. Biodistribution studies in normal mice showed a good clearance from blood for both tracers. [(11)C]-6 was mainly cleared via the renal pathway, while the more lipophilic [(11)C]-13 was excreted almost exclusively via the hepatobiliary system. Despite the lipophilicity of [(11)C]-13, no brain uptake was observed. Reversed phase HPLC analysis of murine plasma and urine revealed high in vivo stability for both tracers. In vitro evaluation in HEK-293T cells showed an increased cell uptake for the more lipophilic [(11)C]-13, however, there was no statistically higher uptake in LacZ expressing cells compared to control cells.

β-Galactosidase (Escherichia coli) has a second catalytically important Mg2+ site

It is shown here that Escherichia coli beta-galactosidase has a second Mg2+ binding site that is important for activity. Binding of Mg2+ to the second site caused the k(cat) (with oNPG as the substrate) to increase about 100 s(-1); the Km was not affected. The Kd for binding the second Mg2+ is about 10(-4)M. Since the concentration of free Mg2+ in E. coli is about 1-2 mM, the second site is physiologically significant. Non-polar substitutions (Ala or Leu) for Glu-797, a residue in an active site loop, eliminated the k(cat) increase. This indicates that the second Mg2+ site is near to Glu-797. The Ki values of transition state analogs were decreased by small but statistically significant amounts when the second Mg2+ site was occupied and Arrhenius plots showed that less entropic activation energy is required when the second site is occupied. These inhibitor and temperature results suggest that binding of the second Mg2+ helps to order the active site for stabilization of the transition state.

A structural view of the action of Escherichia coli (lacZ) beta-galactosidase

The structures of a series of complexes designed to mimic intermediates along the reaction coordinate for beta-galactosidase are presented. These complexes clarify and enhance previous proposals regarding the catalytic mechanism. The nucleophile, Glu537, is seen to covalently bind to the galactosyl moiety. Of the two potential acids, Mg(2+) and Glu461, the latter is in better position to directly assist in leaving group departure, suggesting that the metal ion acts in a secondary role. A sodium ion plays a part in substrate binding by directly ligating the galactosyl 6-hydroxyl. The proposed reaction coordinate involves the movement of the galactosyl moiety deep into the active site pocket. For those ligands that do bind deeply there is an associated conformational change in which residues within loop 794-804 move up to 10 A closer to the site of binding. In some cases this can be inhibited by the binding of additional ligands. The resulting restricted access to the intermediate helps to explain why allolactose, the natural inducer for the lac operon, is the preferred product of transglycosylation.

Structural analyses of new tri- and tetrasaccharides produced from disaccharides by transglycosylation of purified Trichoderma viride beta-glucosidase

A new beta-glucosidase was partially purified from Trichoderma viride cellulase. This beta-glucosidase catalyzed a transglycosylation reaction of cellobiose to give beta-D-Glc-(1-->6)-beta-D-Glc-(1-->4)-D-Glc (1, yield: 18.8%) and beta-D-Glc-(1-->6)-beta-D-Glc-(1-->6)-beta-D-Glc-(1-->4)-D-Glc (2, 3.7%), regioselectively. Furthermore, the enzyme regioselectively converted laminaribiose and gentiobiose into beta-D-Glc-(1-->6)-beta-D-Glc-(1-->3)-D-Glc (3, 15.3%) and beta-D-Glc-(1-->6)-beta-D-Glc-(1-->6)-D-Glc (4, 20.2%), respectively. The structures (1-4) of the products were determined by 1H and 13C NMR spectroscopies. This high regio- and stereoselectively of the beta-glucosidase could be applied for oligosaccharide synthesis.

Characterization of a strictly specific acid beta-galactosidase from Achatina achatina

An acid beta-galactosidase was isolated from the digestive juice of Achatina achatina and purified to homogeneity by anion exchange, gel-filtration and hydroxyapatite chromatographies. This enzyme is soluble, as are the cytosolic beta-galactosidases, functions at acid pH like the lysosomal enzymes but differs from the other soluble animal beta-galactosidases in that it is highly specific for the beta-D-galactosyl residue. In addition, it cleaves the beta1-4 linkage much faster than the beta1-3 and beta1-6 linkages. The enzyme is a monomeric glycoprotein with a molecular mass of 120-125 kDa and the carbohydrate moiety makes up approximately 6% (w/w) of the protein. The amino acid composition displays an important amount of acidic/amide and hydroxy amino acid residues and a low content of basic residues. The enzyme activity is markedly affected by the ionic strength of the medium and the rate-pH curve was shifted towards higher pH values in the presence of added salt. Acid beta-galactosidase is capable of catalysing transgalactosylation reactions. The yields of galactosylation of hydroxy amino acid derivatives, catalysed by the enzyme in the presence of lactose as the glycosyl donor, were higher than those reported previously with conventional sources of beta-galactosidases. In addition, the pH optimum is different for the hydrolysis (pH 3.2) and transgalactosylation (pH 5.0) reactions. On the basis of this work, the enzyme could be used as a tool in the structural analysis of D-galactose-containing oligosaccharide chains, as well as for the synthesis of glycoconjugates.

Directed evolution of a fucosidase from a galactosidase by DNA shuffling and screening

An efficient beta-fucosidase was evolved by DNA shuffling from the Escherichia coli lacZ beta-galactosidase. Seven rounds of DNA shuffling and colony screening on chromogenic fucose substrates were performed, using 10,000 colonies per round. Compared with native beta-galactosidase, the evolved enzyme purified from cells from the final round showed a 1,000-fold increased substrate specificity for o-nitrophenyl fucopyranoside versus o-nitrophenyl galactopyranoside and a 300-fold increased substrate specificity for p-nitrophenyl fucopyranoside versus p-nitrophenyl galactopyranoside. The evolved cell line showed a 66-fold increase in p-nitrophenyl fucosidase specific activity. The evolved fucosidase has a 10- to 20-fold increased kcat/Km for the fucose substrates compared with the native enzyme. The DNA sequence of the evolved fucosidase gene showed 13 base changes, resulting in six amino acid changes from the native enzyme. This effort shows that the library size that is required to obtain significant enhancements in specificity and activity by reiterative DNA shuffling and screening, even for an enzyme of 109 kDa, is within range of existing high-throughput technology. Reiterative generation of libraries and stepwise accumulation of improvements based on addition of beneficial mutations appears to be a promising alternative to rational design.

Binding energy and catalysis. Fluorinated and deoxygenated glycosides as mechanistic probes of Escherichia coli (lacZ) beta-galactosidase

Kinetic parameters for the hydrolysis of a series of deoxy and deoxyfluoro analogues of 2',4'-dinitrophenyl beta-D-galactopyranoside by Escherichia coli (lacZ) beta-galactosidase have been determined and rates found to be two to nine orders of magnitude lower than that for the parent compound. These large rate reductions result primarily from the loss of transition-state binding interactions due to the replacement of sugar hydroxy groups, and such interactions are estimated to contribute at least 16.7 kJ (4 kcal).mol-1 to binding at the 3, 4 and 6 positions and more than 33.5 kJ (8 kcal).mol-1 at the 2 position. The existence of a linear free-energy relationship between log(kcat./Km) for these compounds and the logarithm of the first-order rate constant for their spontaneous hydrolysis demonstrates that electronic effects are also important and provides direct evidence for oxocarbonium ion character in the enzymic transition state. A covalent intermediate which turns over only extremely slowly (t1/2 = 45 h) accumulates during hydrolysis of the 2-deoxyfluorogalactoside, and kinetic parameters for its formation have been determined. This intermediate is nonetheless catalytically competent, since it re-activates much more rapidly in the presence of the transglycosylation acceptors methanol or glucose, thereby providing support for the notion of a covalent intermediate during hydrolysis of the parent substrates.

Histidines, histamines and imidazoles as glycosidase inhibitors

This present study reports the ability of a range of derivatives of L-histidine, histamine and imidazole to act as inhibitors of sweet-almond beta-glucosidase, yeast alpha-glucosidase and Escherichia coli beta-galactosidase. The addition of a hydrophobic group to the basic imidazole nucleus greatly enhances binding to both the alpha- and beta-glucosidases. L-Histidine (beta-naphthylamide (Ki 17 microM) is a potent competitive inhibitor of sweet-almond beta-glucosidase as is omega-N-acetylhistamine (K1 35 microM), which inhibits the sweet-almond beta-glucosidase at least 700 times more strongly than either yeast alpha-glucosidase or Escherichia coli beta-galactosidase, and suggests potential for the development of selective reversible beta-glucosidase inhibitors. A range of hydrophobic omega-N-acylhistamines were synthesized and shown to be among the most potent inhibitors of sweet-almond beta-glucosidase reported to date.

Thermal denaturation of beta-galactosidase and of two site-specific mutants

The thermal denaturation of wild-type beta-galactosidase and two beta-galactosidases with substitutions at the active site was studied by kinetics, differential scanning calorimetry, electrophoresis, molecular exclusion chromatography, and circular dichroism. From the results, a model is developed for thermal denaturation of beta-galactosidase which includes the reversible dissociation of ligands, reversible formation of an inactive tetramer, irreversible dissociation of the inactive tetramer to inactive monomers, and subsequent aggregation of inactive monomers to dimers and larger aggregates. Under some conditions, partial reversibility of the activity loss could be demonstrated, and several intermediates in the thermal denaturation process were trapped by quenching and observed by electrophoresis and molecular exclusion chromatography. The ligands Mg2+ and phenylethyl thio-beta-D-galactoside increase the stability of beta-galactosidase to heat denaturation by shifting the ligand binding equilibrium according to Le Chatelier's principle, thus decreasing the concentration of the ligand-free tetramer which can proceed to subsequent steps. Circular dichroism results indicated that beta-galactosidase is dominated by beta-sheet with lower amounts of alpha-helix. Large changes in secondary structure begin to occur only after activity has been lost. Single amino acid changes at the active site can have significant effects on thermal stability of beta-galactosidases. Some of the effects result from increased thermal stability of the ligand-free enzyme itself. Other effects result from changes in ligand binding, but the magnitude of the resulting changes in stability is not related to the strength of ligand binding in a simple fashion.

Derivatives of methyl beta-lactoside as substrates for and inhibitors of beta-D-galactosidase from E. coli

The 2'-,4'-, and 6'-deoxy derivatives of methyl beta-lactoside have been synthesised by deoxygenation at positions 2', 4', and 6', and the 3'-deoxy derivative was obtained by a glycosylation reaction. The 2'-O-methyl, 2'-O-benzyl, 2'-amino-2'-deoxy, and 1'-deuterio derivatives have been synthesized also. Only the 6'-deoxy and 1'-deuterio derivatives were substrates for the beta-D-galactosidase from E. coli, and the 2'-deoxy- and 2'-amino-2'-deoxy derivatives were potent inhibitors for the hydrolysis of methyl beta-lactoside by the enzyme.

A simple strategy for changing the regioselectivity of glycosidase-catalysed formation of disaccharides

The regioselectivity of glycosidase-catalysed formation of disaccharides can be changed by using alpha- or beta-glycosyl acceptors with various aglycons. The preponderant formation of other than (1----6) linkages can be effected with glycosidases which normally give (1----6) linkages. Thus, an alpha-D-galactosidase can be induced to catalyse the formation mainly of alpha-(1----2)-, alpha-(1----3), or alpha-(1----6)-linked digalactosides. Both the structure of the aglycon and the configuration of the glycosidic linkage can have a pronounced influence on the regioselectivity of disaccharide formation. Enzymic syntheses, in yields of 20-30%, are described of alpha-D-Galp-(1----3)-alpha-D-Galp-OMe, beta-D-Galp-(1----3)-beta-D-Galp-OMe, beta-D-Galp-(1----6)-alpha-D-Galp-OMe, alpha-D-Manp-(1----2)-alpha-D-Manp-OMe, alpha-D-Manp-(1----6)-alpha-D-Manp-OMe, alpha-D-Galp-(1----2)-alpha-D-Galp-OPhNO2-o, alpha-D-Galp-(1----3)-alpha-D-Galp-OPhNO2-p, alpha-D-Manp-(1----2)-alpha-D-Manp-OPhNO2-p, and alpha-D-Manp-(1----2)-alpha-D-Manp-(1----2)-alpha-D-Manp+ ++-OMe. Soluble and immobilised enzymes have been used.

Strong inhibitory effect of furanoses and sugar lactones on beta-galactosidase Escherichia coli

Various sugars and their lactones were tested for their inhibition of beta-galactosidase (Escherichia coli). L-Ribose, which in the furanose form has a hydroxyl configuration similar to that of D-galactose at positions equivalent to the 3- and 4-positions of D-galactose, was a very strong inhibitor, and D-lyxose, which in the furanose form also resembles D-galactose, was a much better inhibitor than expected. Structural comparisons prelude the pyranose forms of these sugars from being significant contributors to the inhibition, and inhibition at different temperatures (at which there are different furanose concentrations) strongly supported the conclusion that the furanose form is inhibitory. Studies with sugar derivatives that can only be in the furanose form also supported the conclusion. This is the first report of the inhibitory effect of furanose on beta-galactosidase. Lactones were also inhibitory. Every lactone tested was much more inhibitory than was its parent sugar. D-Galactonolactone was especially good. Experiments indicated that it was D-galactono-1,5-lactone rather than D-galactono-1,4-lactone which was inhibitory. Inhibition of beta-galactosidases from mammalian sources by lactones has been reported previously, but this is the first report of the effect of beta-galactosidase from E. coli. Since furanoses in the envelope form are analogous (in some ways) to half-chair or sofa conformations and since lactones with six-membered rings probably have half-chair or sofa conformations, the results indicate that beta-galactosidase probably destabilizes its substrate into a planar conformation of some type and that the galactose in the transition state may, therefore, also be quite planar.(ABSTRACT TRUNCATED AT 250 WORDS)

Reversion reactions of beta-galactosidase (Escherichia coli)

The reversion reactions of beta-galactosidase (Escherichia coli) produced beta-galactosyl-galactoses and beta-galactosyl-glucoses. About 10 beta-galactosyl-galactose and 10 beta-galactosyl-glucose gas-liquid chromatographic peaks were detected and it is thus very likely that every possible isomer of beta-galactosyl-galactose and beta-galactosyl-glucose was formed by the reversion reactions (taking into account both anomers for each isomer). The presence of lactose and allolactose among the beta-galactosyl-glucoses was confirmed with standards. An important finding relating to the role of allolactose as an inducer of the lac operon was that allolactose (beta-D-galactosyl-(1----6)-D-glucose) was the only disaccharide formed initially, and at equilibrium it was present in the largest amount (50%). Obviously the enzyme is specific in its ability to form allolactose, and allolactose is the most stable beta-galactosyl-glucose, both important inducer properties. The equilibrium constant (concentration of disaccharides divided by the concentration of reactants at equilibrium) of the reaction was about 9.5 mM-1. This is the first report of an equilibrium constant for the beta-galactosidase reaction. Of mechanistic significance is the fact that only three compounds were able to replace D-galactose as a reversion reactant. Two of these (L-arabinose and D-fucose) had alterations at carbon 6. The 6 position, therefore, is not essential for reactivity. The third compound was D-galactal. Any other sugars tested (even with very minor changes relative to D-galactose) did not react. Of special consequence is the 2 position. The results strongly suggest that there has to be either an equatorial hydroxyl at the 2 position of a sugar or a special reactivity (as with D-galactal) in order for the enzyme to catalyze the beta-galactosidase reaction.

m-Fluorotyrosine substitution in beta-galactosidase; evidence for the existence of a catalytically active tyrosine

The pH profiles of beta-galactosidase, having tyr replaced by m-fluorotyrosine, were compared to those of normal enzyme. The inflection point on the alkaline side was lowered about 1.5 pH units in the fluoro-enzyme, corresponding to the difference in the phenolic pKa values of m-fluorotyrosine and tyr. When glycosidic bond breakage was rate-limiting, the Vm at pH 7.0 was higher for the fluoro-enzyme. When hydrolysis was rate-limiting or when acceptors which made transgalactosylis rate-limiting were used, the Vm was lower for the fluoro-enzyme. This shows that a tyr in beta-galactosidase is a general-acid catalyst in the glycosidic bond breaking reaction and a tyr (probably the same one) is a general-base catalyst in the hydrolytic reaction.

Binding and reactivity at the "glucose" site of galactosyl-beta-galactosidase (Escherichia coli)

A large number of sugars and alcohols were tested to see how well they bound and how readily they reacted at the "glucose" site of the galactosyl form of beta-galactosidase. Two classes of compounds were found to bind well to the galactosyl form of the enzyme. One class contained sugars and alcohols similar in structure to D-glucose in its pyranose ring form, and the other class was composed of relatively hydrophobic sugars and alcohols. On the other hand, several factors seemed to control k4. Large k4 values were found for straight-chain alcohols as compared to the values for the corresponding ring sugars. Also, if the acceptors had hydroxyl groups at the end of the molecule, the reactivity (k4) was greater than if hydroxyl groups were only in the middle of the molecule. In addition, if there was a hydroxyl at an asymmetric carbon next to a terminal hydroxymethyl group, it was necessary that it be in the same orientation as the D configuration of glucose; otherwise, the k4 was low. Overall, the results showed that it is the binding effect, more than the reactivity, which is responsible for the specificity at the "glucose" site. More specifically, these studies showed that the reason glucose is such an ideal molecule for transgalactosylation is that it leaves the galactosyl form of the enzyme very slowly, that is, k-a is relatively small. Thus, glucose remains attached to the galactosyl form of beta-galactosidase for a sufficient time to allow transgalactosylation to occur, while other acceptors, despite being as reactive (or more reactive) in terms of their k4 values, dissociate from the "glucose" site of the galactosyl form of the enzyme very readily and thus are poor acceptors.