Synergistic activation of beta-galactosidase by Na and Cs

Activation and thermal stabilization of a recombinant γ-glutamyltranspeptidase from Bacillus licheniformis ATCC 27811 by monovalent cations

The regulation of enzyme activity through complexation with certain metal ions plays an important role in many biological processes. In addition to divalent metals, monovalent cations (MVCs) frequently function as promoters for efficient biocatalysis. Here, we examined the effect of MVCs on the enzymatic catalysis of a recombinant γ-glutamyltranspeptidase (BlrGGT) from Bacillus licheniformis ATCC 27,811 and the application of a metal-activated enzyme to L-theanine synthesis. The transpeptidase activity of BlrGGT was enhanced by Cs⁺ and Na⁺ over a broad range of concentrations with a maximum of 200 mM. The activation was essentially independent of the ionic radius, but K⁺ contributed the least to enhancing the catalytic efficiency. The secondary structure of BlrGGT remained mostly unchanged in the presence of different concentrations of MVCs, but there was a significant change in its tertiary structure under the same conditions. Compared with the control, the half-life (t_1/2) of the Cs⁺-enriched enzyme at 60 and 65 °C was shown to increase from 16.3 and 4.0 min to 74.5 and 14.3 min, respectively. The simultaneous addition of Cs⁺ and Mg²⁺ ions exerted a synergistic effect on the activation of BlrGGT. This was adequately reflected by an improvement in the conversion of substrates to L-theanine by 3.3-15.1% upon the addition of 200 mM MgCl₂ into a reaction mixture comprising the freshly desalted enzyme (25 μg/mL), 250 mM L-glutamine, 600 mM ethylamine, 200 mM each of the MVCs, and 50 mM borate buffer (pH 10.5). Taken together, our results provide interesting insights into the complexation of MVCs with BlrGGT and can therefore be potentially useful to the biocatalytic production of naturally occurring γ-glutamyl compounds. KEY POINTS: • The transpeptidase activity of B. licheniformis γ-glutamyltranspeptidase can be activated by monovalent cations. • The thermal stability of the enzyme was profoundly increased in the presence of 200 mM Cs⁺. • The simultaneous addition of Cs⁺and Mg²⁺ions to the reaction mixture improves L-theanine production.

β-Galactosidase from Kluyveromyces lactis: Characterization, production, immobilization and applications - A review

A review on the enzyme β-galactosidase from Kluyveromyces lactis is presented, from the perspective of its structure and mechanisms of action, the main catalyzed reactions, the key factors influencing its activity, and selectivity, as well as the main techniques used for improving the biocatalyst functionality. Particular attention was given to the discussion of hydrolysis, transglycosylation, and galactosylation reactions, which are commonly mediated by this enzyme. In addition, the products generated from these processes were highlighted. Finally, biocatalyst improvement techniques are also discussed, such as enzyme immobilization and protein engineering. On these topics, the most recent immobilization strategies are presented, emphasizing processes that not only allow the recovery of the biocatalyst but also deliver enzymes that show better resistance to high temperatures, chemicals, and inhibitors. In addition, genetic engineering techniques to improve the catalytic properties of the β-galactosidases were reported. This review gathers information to allow the development of biocatalysts based on the β-galactosidase enzyme from K. lactis, aiming to improve existing bioprocesses or develop new ones.

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.

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.

Mechanism of Na(+) binding to thrombin resolved by ultra-rapid kinetics

The interaction of Na(+) and K(+) with proteins is at the basis of numerous processes of biological importance. However, measurement of the kinetic components of the interaction has eluded experimentalists for decades because the rate constants are too fast to resolve with conventional stopped-flow methods. Using a continuous-flow apparatus with a dead time of 50 micro s we have been able to resolve the kinetic rate constants and entire mechanism of Na(+) binding to thrombin, an interaction that is at the basis of the procoagulant and prothrombotic roles of the enzyme in the blood.

Rapid kinetics of Na+ binding to thrombin

The kinetic mechanism of Na(+) binding to thrombin was resolved by stopped-flow measurements of intrinsic fluorescence. Na(+) binds to thrombin in a two-step mechanism with a rapid phase occurring within the dead time of the spectrometer (<0.5 ms) followed by a single-exponential slow phase whose k(obs) decreases hyperbolically with increasing [Na(+)]. The rapid phase is due to Na(+) binding to the enzyme E to generate the E:Na(+) form. The slow phase is due to the interconversion between E(*) and E, where E(*) is a form that cannot bind Na(+). Temperature studies in the range from 5 to 35 degrees C show significant enthalpy, entropy, and heat capacity changes associated with both Na(+) binding and the E to E(*) transition. As a result, under conditions of physiologic temperature and salt concentrations, the E(*) form is negligibly populated (<1%) and thrombin is almost equally partitioned between the E (40%) and E:Na(+) (60%) forms. Single-site Phe mutations of all nine Trp residues of thrombin enabled assignment of the fluorescence changes induced by Na(+) binding mainly to Trp-141 and Trp-215, and to a lesser extent to Trp-148, Trp-207, and Trp-237. However, the fast phase of fluorescence increase is influenced to different extents by all Trp residues. The distribution of these residues over the entire thrombin surface demonstrates that Na(+) binding induces long-range effects on the structure of the enzyme as a whole, contrary to the conclusions drawn from recent structural studies. These findings elucidate the mechanism of Na(+) binding to thrombin and are relevant to other clotting factors and enzymes allosterically activated by monovalent cations.

A study of the relationships of interactions between Asp-201, Na+or K+, and galactosyl C6 hydroxyl and their effects on binding and reactivity of β-galactosidase

The interactions between Na+(and K+) and Asp-201 of β-galactosidase were studied. Analysis of the changes in Kmand Vmaxshowed that the Kdfor Na+of wild type β-galactosidase (0.36 ± 0.09 mM) was about 10× lower than for K+(3.9 ± 0.6 mM). The difference is probably because of the size and other physical properties of the ions and the binding pocket. Decreases of Kmas functions of Na+and K+for oNPG and pNPG and decreases of the Kiof both shallow and deep mode inhibitors were similar, whereas the Kmand Kiof substrates and inhibitors without C6 hydroxyls remained constant. Thus, Na+and K+are important for binding galactosyl moieties via the C6 hydroxyl throughout catalysis. Na+and K+had lesser effects on the Vmax. The Vmaxof pNPF and pNPA (substrates that lack a C6 hydroxyl) did not change upon addition of Na+or K+, showing that the catalytic effects are also mediated via the C6 hydroxyl. Arrhenius plots indicated that Na+, but not K+, caused k3(degalactosylation) to increase. Na+also caused the k2(galactosylation) with oNPG, but not with pNPG, to increase. In contrast, K+caused the k2values with both oNPG and pNPG to increase. Na+and K+mainly altered the entropies of activation of k2and k3with only small effects on the enthalpies of activation. This strongly suggests that only the positioning of the substrate, transition states, and covalent intermediate are altered by Na+and K+. Further evidence that positioning is important was that substitution of Asp-201 with a Glu caused the Kmand Kivalues to increase significantly. In addition, the Kdvalues for Na+or K+were 5 to 8 fold higher. The negative charge of Asp-201 was shown to be vital for Na+and K+binding. Large amounts of Na+or K+had no effect on the very large Kmand Kivalues of D201N-β-galactosidase and the Vmaxvalues changed minimally and in a linear rather than hyperbolic way. D201F-β-galactosidase, with a very bulky hydrophobic side chain in place of Asp, essentially obliterated all binding and catalysis.Key words: β-galactosidase, sodium, potassium, binding, aspartic acid.

Stabilities of uncomplemented and complemented M15 beta-galactosidase (Escherichia coli) and the relationship to alpha-complementation

M15 beta-galactosidase (Escherichia coli) is a mutant form of beta-galactosidase having residues 11-41 deleted. It is an inactive dimer but can be complemented to the active tetrameric form by the addition of a peptide containing the deleted residues. The activities of uncomplemented and complemented M15 beta-galactosidases decreased starting at 42 degrees C--uncomplemented over a narrow temperature range, complemented over a broad range. This is because uncomplemented protein is a simple dimer while complemented is a mix of interacting oligomers at high temperatures. The effects of added components on stability and alpha-complementation are best explained by binding effects on equilibria between native forms and forms susceptible to inactivation. Mg2+ stabilized complemented protein but destabilized uncomplemented protein (10x less Mg2+ was needed for complemented protein). Alpha-complementation increased somewhat at low Mg2+ but decreased at high Mg2+. These effects can be explained by differential Mg2+ binding to the native and susceptible forms. The enhancement of both stability and alpha-complementation by Na+ can be explained by preferential binding of Na+ to the native forms of both the uncomplemented and complemented proteins. Low 2-mercaptoethanol concentrations stabilized uncomplemented M15 beta-galactosidase, but high concentrations destabilized it. All concentrations destabilized complemented M15 beta-galactosidase. Alpha-complementation was enhanced by 2-mercaptoethanol. Thus, there is a correlation between stability of the uncomplemented protein and alpha-complementation at low 2-mercaptoethanol owing to interactions with native forms. The lack of correlation at higher 2-mercaptoethanol probably results from precipitation by 2-mercaptoethanol. In contrast to irreversible thermal inactivation, differences in reversible stability in urea were small. This suggests that quaternary structure and Mg2+ and Na+ sites are lost at low urea concentrations and are unimportant at the urea concentrations that result in reversible denaturation.

Caesium accumulation by microorganisms: uptake mechanisms, cation competition, compartmentalization and toxicity

The continued release of caesium radioisotopes into the environment has led to a resurgence of interest in microbe-Cs interactions. Caesium exists almost exclusively as the monovalent cation Cs+ in the natural environment. Although Cs+ is a weak Lewis acid that exhibits a low tendency to form complexes with ligands, its chemical similarity to the biologically essential alkali cation K+ facilitates high levels of metabolism-dependent intracellular accumulation. Microbial Cs+ (K+) uptake is generally mediated by monovalent cation transport systems located on the plasma membrane. These differ widely in specificity for alkali cations and consequently microorganisms display large differences in their ability to accumulate Cs+; Cs+ appears to have an equal or greater affinity than K+ for transport in certain microorganisms. Microbial Cs+ accumulation is markedly influenced by the presence of external cations, e.g. K+, Na+, NH4+ and H+, and is generally accompanied by an approximate stoichiometric exchange for intracellular K+. However, stimulation of growth of K(+)-starved microbial cultures by Cs+ is limited and it has been proposed that it is not the presence of Cs+ in cells that is growth inhibitory but rather the resulting loss of K+. Increased microbial tolerance to Cs+ may result from sequestration of Cs+ in vacuoles or changes in the activity and/or specificity of transport systems mediating Cs+ uptake. The precise intracellular target(s) for Cs(+)-induced toxicity has yet to be clearly defined, although certain internal structures, e.g. ribosomes, become unstable in the presence of Cs+ and Cs+ is known to substitute poorly for K+ in the activation of many K(+)-requiring enzymes.

Protein hydration changes during catalysis: a new mechanism of enzymic rate-enhancement and ion activation/inhibition of catalysis

There exists a linear correlation between the effect of a salt on the rate of an enzymic reaction and its effect on the activation volume (delta V++) of the reaction. Salts that increase delta V++ invariably decrease the rate of the reaction, and vice versa. The salt effects on reaction rate are, however, much larger than would be predicted solely on the basis of pressure-volume work changes deriving from the observed alterations in delta V++. Different inorganic salts affect reaction rates and activation volumes in a manner that reflects the salts' positions in the Hofmeister series. These observations, taken in conjunction with data on the effects of salts on protein functional group (aminoacid side-chains and peptide linkages) hydration, lead us to propose the following hypothesis to account for salt activation and inhibition of catalysis. Aminoacid side-chains and peptide linkages located on or near the protein surface change their exposure to water during conformational events in catalysis. These protein group transfers are accompanied by large volume and energy changes that are due largely to changes in the organization of water around these groups. When these transfer processes occur during the rate-limiting step in catalysis, these energy and volume changes can contribute to the free energy of activation (delta G++) and the activation volume of the reaction. By influencing the degree to which water can organize around transferred protein groups, salts can modify both the delta G++ (rate) and the delta V++ of a reaction.