The mechanism of salivary amylase hydrolysis: role of residues at subsite S2'

Spectroscopic and in silico evaluation of hesperetin, aglycone flavanone, as a prospective regulatory ligand for human salivary α-amylase

The insight into the binding mechanism of hesperetin, an aglycone flavanone, with human salivary α-amylase (HSAA), simulated under physiological salivary condition, was explored using various spectroscopic approaches and in silico method. Hesperetin effectively quenched the intrinsic fluorescence of HSAA and the quenching was mixed quenching mechanism. The interaction perturbed the HSAA intrinsic fluorophore microenvironment and the enzyme global surface hydrophobicity. The negative values of ΔG for thermodynamic parameters and in silico study revealed the spontaneity of HSAA-hesperetin complex while the positive values of enthalpy change (ΔH) and entropy change (ΔS) showed noticeable involvement of hydrophobic bonding in the stabilization of the complex. Hesperetin was a mixed inhibitor for HSAA with a KI of 44.60 ± 1.63 μM and having apparent inhibition coefficient (α) of 0.26. Macromolecular crowding, given rise to microviscosity and anomalous diffusion, regulated the interaction. Sodium ion (Na+) created high ionic strength, also, modulated the interaction. The in silico study proposed the preferential binding of hesperetin at the active cleft domain of HSAA with the least energy of -8.0 kcal/mol. This work gives a novel insight on the potentials of hesperetin as a future prospective medicinal candidate in the management of postprandial hyperglycemic condition.Communicated by Ramaswamy H. Sarma.

A Structure—Activity Relationship Study of the Inhibition of α-Amylase by Benzoic Acid and Its Derivatives

Phenolic acids are widely found in fruits and vegetables. The inhibitory effect of phenolic acids on α-amylase, a key enzyme for starch digestion, has attracted the attention of researchers. To further investigate the effects of different substituents on the benzene ring of phenolic acid on the inhibition of α-amylase activity, in vitro experiments and molecular docking were used. The structure-activity relationships of 17 phenolic acids with benzoic acid as the parent nucleus were analyzed by determining their half inhibitory concentration (IC₅₀) toward α-amylase. The results showed that 2,3,4-trihydroxybenzoic acid had the strongest inhibitory effect on α-amylase with an IC₅₀ value of 17.30 ± 0.73 mM. According to the structure-activity analysis, the hydroxyl group at the 2-position on the benzene ring had a strong positive effect on the inhibitory activity of α-amylase, while methoxylation at the 2-position and hydroxylation at the 5-position had a negative effect. Molecular docking revealed that hydrogen bonding and hydrophobic interactions were involved in the inhibition, with hydrogen bonding being the primary force. These findings provide a more comprehensive understanding of phenolic acids as inhibitors of α-amylase and provide new ideas for the design of dietary formulations for diabetic patients.

Human taste detection of glucose oligomers with low degree of polymerization

Studies have reported that some animals, including humans, can taste mixtures of glucose oligomers (i.e., maltooligosaccharides, MOS) and that their detection is independent of the known T1R2/T1R3 sweet taste receptor. In an effort to understand potential mechanisms underlying the taste perception of glucose oligomers in humans, this study was designed to investigate: 1) the variability of taste sensitivity to MOS with low degree-of-polymerization (DP), and 2) the potential role of hT1R2/T1R3 in the MOS taste detection. To address these objectives, a series of food grade, narrow-DP-range MOS were first prepared (DP 3, 3–4, 5–6, and 6–7) by fractionating disperse saccharide mixtures. Subjects were then asked to discriminate these MOS stimuli as well as glucose (DP 1) and maltose (DP 2) from blanks after the stimuli were swabbed on the tongue. All stimuli were presented at 75 mM with and without a sweet taste inhibitor (lactisole). An α-glucosidase inhibitor (acarbose) was added to all test stimuli to prevent oral digestion of glucose oligomers. Results showed that all six stimuli were detected with similar discriminability in normal tasting conditions. When the sweet receptor was inhibited, DP 1, 2, and 3 were not discriminated from blanks. In contrast, three higher-DP paired MOS stimuli (DP 3–4, 5–6, and 6–7) were discriminated from blanks at a similar degree. Overall, these results support the presence of a sweet-independent taste perception mechanism that is stimulated by MOS greater than three units.

ZnO nanoparticles and their acarbose-capped nanohybrids as inhibitors for human salivary amylase

The authors report a controlled synthesis of biocompatible ZnO and acarbose-capped nanohybrids, and examined the inhibition activities of these nanosystems with human salivary α-amylase (HSA) activity. XRD measurements reveal ZnO present in wurtzite phase with hexagonal structure. The average size of ZnO particles for the two studied nanosystems was estimated to lie between 10 to 12 nm using Scherrer equation. These particles depict the onset of absorption at about 320 nm and the band-gap emission at about 370 nm, which are fairly blue shifted as compared with the bulk ZnO and have been understood due to the size quantisation effect. The inhibitory action of thioglycerol capped ZnO nanoparticles (SP1) and acarbose drug (used for diabetes type II) capped ZnO (SP2) for HSA was observed to 61 and72%, respectively. The inhibition activity of the SP1 alone was found to be very similar to that of acarbose and the coating of these particles with drug (SP2) demonstrated an enhancement in inhibition activity of the enzyme by about 30%. From the inhibition studies, it is confirmed that these nanosystems showed better inhibition activity at physiological temperature and pH. These nanosystems are projected to have potential applications in diabetes type II control.

Transcriptome analysis provides insights for understanding the adverse effects of endosulfan in Drosophila melanogaster

Indiscriminate use of agrochemicals worldwide, particularly, persistent organic pollutants (POPs), is of concern. Endosulfan, a POP, is used by various developing/developed nations and is known to adversely affect the development and the hormonal profiles of humans and animals. However, little is known about the molecular players/pathways underlying the adverse effects of endosulfan. We therefore analyzed the global gene expression changes and subsequent adverse effects of endosulfan using Drosophila. We used Drosophila melanogaster keeping in view of its well annotated genome and the wealth of genetic/molecular reagents available for this model organism. We exposed third instar larvae of D. melanogaster to endosulfan (2.0 μg mL(-1)) for 24 h and using microarray, we identified differential expression of 256 genes in exposed organisms compared to controls. These genes are associated with cellular processes such as development, stress and immune response and metabolism. Microarray results were validated through quantitative PCR and biochemical assay on a subset of genes/proteins. Taking cues from microarray data, we analyzed the effect of endosulfan on development, emergence and survival of the organism. In exposed organisms, we observed deformities in hind-legs, reminiscent of those observed in higher organisms exposed to endosulfan. In addition, we observed delayed and/or reduced emergence in exposed organisms when compared to their respective controls. Together, our studies not only highlight the adverse effects of endosulfan on the organism but also provide an insight into the possible genetic perturbations underlying these effects, which might have potential implications to higher organisms.

Probing the role of aromatic residues at the secondary saccharide-binding sites of human salivary alpha-amylase in substrate hydrolysis and bacterial binding

Human salivary alpha-amylase (HSAmy) has three distinct functions relevant to oral health: (1) hydrolysis of starch, (2) binding to hydroxyapatite (HA), and (3) binding to bacteria (e.g., viridans streptococci). Although the active site of HSAmy for starch hydrolysis is well-characterized, the regions responsible for bacterial binding are yet to be defined. Since HSAmy possesses several secondary saccharide-binding sites in which aromatic residues are prominently located, we hypothesized that one or more of the secondary saccharide-binding sites harboring the aromatic residues may play an important role in bacterial binding. To test this hypothesis, the aromatic residues at five secondary binding sites were mutated to alanine to generate six mutants representing either single (W203A, Y276A, and W284A), double (Y276A/W284A and W316A/W388A), or multiple [W134A/W203A/Y276A/W284A/W316A/W388A; human salivary alpha-amylase aromatic residue multiple mutant (HSAmy-ar)] mutations. The crystal structure of HSAmy-ar as an acarbose complex was determined at a resolution of 1.5 A and compared with the existing wild-type acarbose complex. The wild-type and the mutant enzymes were characterized for their abilities to exhibit enzyme activity, starch-binding activity, HA-binding activity, and bacterial binding activity. Our results clearly showed that (1) mutation of aromatic residues does not alter the overall conformation of the molecule; (2) single or double mutants showed either moderate or minimal changes in both starch-binding activity and bacterial binding activity, whereas HSAmy-ar showed significant reduction in these activities; (3) starch-hydrolytic activity was reduced by 10-fold in HSAmy-ar; (4) oligosaccharide-hydrolytic activity was reduced in all mutants, but the action pattern was similar to that of the wild-type enzyme; and (5) HA binding was unaffected in HSAmy-ar. These results clearly show that the aromatic residues at the secondary saccharide-binding sites in HSAmy play a critical role in bacterial binding and in starch-hydrolytic functions of HSAmy.

Inhibitory effects of tannin on human salivary α-amylase

Here, we first report on the effectiveness and specificity of tannin inhibition of 2-chloro-4-nitrophenyl-4-O-beta-d-galactopyranosylmaltoside hydrolysis that is catalyzed by human salivary alpha-amylase (HSA). Tannin was gallotannin in which quinic acid was esterified with 2-7 units of gallic acid. A number of studies establish that polyphenols-like tannins-may prevent oral diseases, e.g., dental caries. Kinetic analyses confirmed that the inhibition of hydrolysis is a mixed non-competitive type and only one molecule of tannin binds to the active site or the secondary site of the enzyme. Since Dixon plots were linear, product formation could be excluded from the enzyme-substrate-inhibitor complex (ESI). Kinetic constants calculated from secondary plots and non-linear regression are almost identical, thereby confirming the suggested model. Kinetic constants (K(EI) = 9.03 microgmL(-1), K(ESI) = 47.84 microgmL(-1)) show that tannin is as an effective inhibitor of HSA as acarbose and indicate a higher stability for the enzyme-inhibitor complex than ESI.

Human salivary alpha-amylase Trp58 situated at subsite -2 is critical for enzyme activity

The nonreducing end of the substrate-binding site of human salivary alpha-amylase contains two residues Trp58 and Trp59, which belong to beta2-alpha2 loop of the catalytic (beta/alpha)(8) barrel. While Trp59 stacks onto the substrate, the exact role of Trp58 is unknown. To investigate its role in enzyme activity the residue Trp58 was mutated to Ala, Leu or Tyr. Kinetic analysis of the wild-type and mutant enzymes was carried out with starch and oligosaccharides as substrates. All three mutants exhibited a reduction in specific activity (150-180-fold lower than the wild type) with starch as substrate. With oligosaccharides as substrates, a reduction in k(cat), an increase in K(m) and distinct differences in the cleavage pattern were observed for the mutants W58A and W58L compared with the wild type. Glucose was the smallest product generated by these two mutants in the hydrolysis oligosaccharides; in contrast, wild-type enzyme generated maltose as the smallest product. The production of glucose by W58L was confirmed from both reducing and nonreducing ends of CNP-labeled oligosaccharide substrates. The mutant W58L exhibited lower binding affinity at subsites -2, -3 and +2 and showed an increase in transglycosylation activity compared with the wild type. The lowered affinity at subsites -2 and -3 due to the mutation was also inferred from the electron density at these subsites in the structure of W58A in complex with acarbose-derived pseudooligosaccharide. Collectively, these results suggest that the residue Trp58 plays a critical role in substrate binding and hydrolytic activity of human salivary alpha-amylase.

Inhibition of human salivary α-amylase by glucopyranosylidene-spiro-thiohydantoin

This study is the first report on the effectiveness and specificity of glucopyranosylidene-spiro-thiohydantoin (G-TH) inhibitor on the 2-chloro-4-nitrophenyl-4-O-beta-D-galactopyranosyl-maltoside (GalG(2)CNP) hydrolysis catalysed by human salivary alpha-amylase (HSA). The inhibition of hydrolysis is a mixed-noncompetitive type. In any case, only one molecule of inhibitor binds to HSA. Since our substrate and inhibitor are small molecules the long enough active site facilitates accommodating both of them simultaneously. However, the product formation can be excluded from enzyme-substrate-inhibitor complex (ESI) since Dixon plots are linear. Kinetic constants calculated from secondary plots and nonlinear regression are almost entirely equal, confirming the fidelity of the suggested model. Kinetic constants (K(1i)=7.3mM, L(1i)=2.84 mM) show that G-TH is not such a potent inhibitor of HSA as acarbose and indicate higher stability for ESI than for enzyme-inhibitor complex.

Introducing Transglycosylation Activity into Human Salivary α-Amylase (HSA)

Synthesis of 4-nitrophenyl 1-thio-beta-D-maltoside, maltotrioside, and maltotetraoside in yields up to 60% has been achieved by a Tyr151Met (Y151M) mutant of human salivary alpha-amylase. Y151M is capable of transferring maltose and maltotriose residues from a maltotetraose donor onto different p-nitrophenyl glycosides. (1)H and (13)C NMR studies revealed that the mutated enzyme preserved the stereo- and regioselectivity. The glycosylation took place at position 4 of the glycosyl acceptor, forming the alpha(1-4)glycosidic bond, exclusively. [reaction: see text]

Subsite mapping of human salivary alpha-amylase and the mutant Y151M

This study characterizes the substrate-binding sites of human salivary alpha-amylase (HSA) and its Y151M mutant. It describes the first subsite maps, namely, the number of subsites, the position of cleavage sites and apparent subsite energies. The product pattern and cleavage frequencies were determined by high-performance liquid chromatography, utilizing a homologous series of chromophore-substituted maltooligosaccharides of degree of polymerization 3-10 as model substrates. The binding region of HSA is composed of four glycone and three aglycone-binding sites, while that of Tyr151Met is composed of four glycone and two aglycone-binding sites. The subsite maps show that Y151M has strikingly decreased binding energy at subsite (+2), where the mutation has occurred (-2.6 kJ/mol), compared to the binding energy at subsite (+2) of HSA (-12.0 kJ/mol).

Probing the role of a mobile loop in substrate binding and enzyme activity of human salivary amylase

Mammalian amylases harbor a flexible, glycine-rich loop 304GHGAGGA(310), which becomes ordered upon oligosaccharide binding and moves in toward the substrate. In order to probe the role of this loop in catalysis, a deletion mutant lacking residues 306-310 (Delta306) was generated. Kinetic studies showed that Delta306 exhibited: (1) a reduction (>200-fold) in the specific activity using starch as a substrate; (2) a reduction in k(cat) for maltopentaose and maltoheptaose as substrates; and (3) a twofold increase in K(m) (maltopentaose as substrate) compared to the wild-type (rHSAmy). More cleavage sites were observed for the mutant than for rHSAmy, suggesting that the mutant exhibits additional productive binding modes. Further insight into its role is obtained from the crystal structures of the two enzymes soaked with acarbose, a transition-state analog. Both enzymes modify acarbose upon binding through hydrolysis, condensation or transglycosylation reactions. Electron density corresponding to six and seven fully occupied subsites in the active site of rHSAmy and Delta306, respectively, were observed. Comparison of the crystal structures showed that: (1) the hydrophobic cover provided by the mobile loop for the subsites at the reducing end of the rHSAmy complex is notably absent in the mutant; (2) minimal changes in the protein-ligand interactions around subsites S1 and S1', where the cleavage would occur; (3) a well-positioned water molecule in the mutant provides a hydrogen bond interaction similar to that provided by the His305 in rHSAmy complex; (4) the active site-bound oligosaccharides exhibit minimal conformational differences between the two enzymes. Collectively, while the kinetic data suggest that the mobile loop may be involved in assisting the catalysis during the transition state, crystallographic data suggest that the loop may play a role in the release of the product(s) from the active site.