Heteroleptic Silver(I) and Gold(I) N-Heterocyclic Carbene Complexes: Structural Characterization, Computational Analysis, Tyrosinase Inhibitory, and Biological Effects.

Derivatization of (NHC)M-Cl (M = Ag, Au) with selected sulfur donors from the family of dialkyldithiophosphates and bis(2-mercapto-1-methylimidazolyl)borate ligands gave a series of heteroleptic mononuclear complexes. In single-crystal X-ray diffraction analysis, Ag(I) complexes adopted a trigonal planar geometry, while Au(I) complexes are near-linear. TD-DFT and hole-electron analyses of the selected complexes gave insight into the electronic features of the metal complexes. In vitro cellular tests were conducted on the human cancerous breast cell line MCF-7 using 2 and 8. The antibacterial activities of complexes 1, 2, 3, 7, 8, and IPr-Ag-Cl were also screened against Gram-positive (Staphylococcus aureus PTCC 1112) and Gram-negative (Escherichia coli PTCC 1330) bacteria. Antityrosinase and hemolytic effects of the selected compounds were also determined.

Correction to "Homobimetallic Au(I)-Au(I) and Heterotrimetallic Au(I)-Fe(II)-Au(I) Complexes with Dialkyldithiophosphates and Phosphine Ligands: Structural Characterization, DFT Analysis, and Tyrosinase Inhibitory and Biological Effects".

[This corrects the article DOI: 10.1021/acsomega.3c00645.].

Homobimetallic Au(I)-Au(I) and Heterotrimetallic Au(I)-Fe(II)-Au(I) Complexes with Dialkyldithiophosphates and Phosphine Ligands: Structural Characterization, DFT Analysis, and Tyrosinase Inhibitory and Biological Effects.

The role of bridging and terminal ligand electronic and steric properties on the structure and antiproliferative activity of two-coordinated gold(I) complexes was investigated on seven novel binuclear and trinuclear gold(I) complexes synthesized by the reaction of either Au2(dppm)Cl2, Au2(dppe)Cl2, or Au2(dppf)Cl2 with potassium diisopropyldithiophosphate, K[(S-OiPr)2], potassium dicyclohexyldithiophosphate, K[(S-OCy)2], or sodium bis(methimazolyl)borate, Na(S-Mt)2, which afforded air-stable gold(I) complexes. In 1-7, the gold(I) centers adopt a two-coordinated linear geometry and are structurally similar. However, their structural features and antiproliferative properties highly depend upon subtle ligand substituent changes. All complexes were validated by 1H, 13C{1H}, 31P NMR, and IR spectroscopy. The solid-state structures of 1, 2, 3, 6, and 7 were confirmed using single-crystal X-ray diffraction. A density functional theory geometry optimization calculation was used to extract further structural and electronic information. To investigate the possible cytotoxicities of 2, 3, and 7, in vitro cellular tests were carried out on the human cancerous breast cell line MCF-7. 2 and 7 show promising cytotoxicity.

In silico and in vitro studies of novel derivatives of tyrosol and raspberry ketone as the mushroom tyrosinase inhibitors.

Tyrosinase is the key enzyme for melanin biosynthesis. Overproduction and deposition of this pigment cause different problems in various industries including agriculture and food. Finding safe tyrosinase inhibitors thus attracts great research interest. The goal of this study is evaluation of inhibitory potencies of some novel synthetic derivatives of tyrosol and raspberry ketone on diphenolase activity of mushroom tyrosinase. The ligands inhibited enzyme activity and compound 4-(2-(4-(hydroxymethyl)-2-methyl-1,3-dioxolan-2-yl)ethyl)phenol (1d) exhibited the most inhibitory potency (77% inhibition, IC50 = 0.32 µmol L-1) via the mixed inhibition mode. This compound was also safe according to the results of in vitro analyses. The enzyme-ligands interactions were theoretically and experimentally investigated using molecular docking and fluorescence quenching approaches, respectively. Modes of quenching and related parameters were also determined and molecular docking data showed that the ligands bind to important sites of the enzyme. These compounds, especially 1d, can be suggested as efficient candidates for further investigations.

Pomegranate seed polyphenol-based nanosheets as an efficient inhibitor of amyloid fibril assembly and cytotoxicity of HEWL.

Poor water solubility and low bioavailability are considered as two main factors restricting therapeutic applications of natural polyphenols in relation to various disorders including amyloid-related diseases. Among various strategies developed to overcome these limitations, nanonization has attracted considerable attention. Herein, we compared the potency of bulk and nano forms of the polyphenolic fraction of pomegranate seed (PFPS) for modulating Hen Egg White Lysozyme (HEWL) amyloid fibril formation. Prepared PFPS nanosheets using direct oxidative pyrolysis were characterized by employing a range of spectroscopic and microscopic techniques. We found that the nano form can inhibit the assembly process and disintegrate preformed fibrils of HEWL much more effective than the bulk form of PFPS. Moreover, MTT-based cell viability and hemolysis assays showed the capacity of both bulk and nano forms of PFPS in attenuating HEWL amyloid fibril-induced toxicity, where the nano form was more effective. On the basis of thioflavin T results, a delay in the initiation of amyloid fibril assembly of HEWL appears to be the mechanism of action of PFPS nanosheets. We suggest that the improved efficiency of PFPS nanosheets in modulating the HEWL fibrillation process may be attributed to their increased surface area in accord with the surface-assistance model. Our results may present polyphenol-based nanosheets as a powerful approach for drug design against amyloid-related diseases.

Evaluation of inhibitory effects of some novel phenolic derivatives on the mushroom tyrosinase activity: Insights from spectroscopic analyses, molecular docking and in vitro assays.

Tyrosinase plays determinant role in enzymatic browning of vegetables and fresh-cut fruits. Development of new tyrosinase inhibitors is of great concern in food and agriculture. To discover new inhibitors, novel phenolic derivatives were synthesized and their inhibitory effects were investigated on activity of mushroom tyrosinase. All compounds showed potent inhibitory activities in their low concentrations and compound 4-(4-hydroxyphenyl)butan-2-one (1b) was found to be the most potent inhibitor (73.75% inhibition, IC50 value 5.6 µmol L-1). This ligand inhibited enzyme activity in a mixed pattern and kinetic parameters were also determined. In vitro assays revealed that this compound has not cytotoxicity/hemolytic effects and can be considered as safe for further investigations. Analysis of fluorescence spectra showed that all ligands quenched enzyme intrinsic fluorescence. The quenching mode and important binding parameters were also calculated. Enzyme-ligands interactions were also theoretically analyzed by molecular docking and results showed that the ligands interact with structurally/functionally critical residues.

Immunogenicity evaluation of the HIV-1 Tat containing polyepitope DNA vaccine adjuvanted with CpG-ODNs in mice.

OBJECTIVES: HIV-1 is still considered a serious threat to human health, and accessibility of a suitable and efficient vaccine is urgently needed to address the disease burden. DNA vaccines employ the cells of the vaccinated hosts for in situ production of the vaccines. This strategy is an alternative and effective approach for traditional vaccination against high-risk pathogens, e.g., HIV-1. On the other hand, polyepitope vaccines, containing several immunogenic and conserved epitopes from virus vital regulatory and structural proteins, could more efficiently induce cellular and humoral immune responses against different clades of the virus. MATERIALS AND METHODS: Herein, we comparatively investigated the immunogenic potency of the HIV-1 polytope DNA vaccine containing CpG oligodeoxynucleotides (CpG-ODNs) in BALB/c mice. To this end, after verifying the expression of the recombinant sequence in the eukaryotic HEK 293 cell line, it was amplified and extracted in the prokaryotic host cells (E. coli DH5α)) and then formulated and administered intramuscularly (IM) to the experimental mice (on days 0, 14, and 28) with and without CpG-ODNs adjuvant. RESULTS: Taken together, the results demonstrated that CpG-ODNs adjuvanted DNA vaccine could significantly elicit cellular and humoral immune responses in the immunized animals in comparison with the control ones (P<0.05). CONCLUSION: Regarding the obtained results and also considering the advantages of polytopic and DNA vaccines, this approach might be considered a new regimen in HIV-1/AIDS vaccination.

Physicochemical studies on the structural stability of the HIV-1 vaccine candidate recombinant Tat protein.

HIV-1 transactivator of transcription protein is one of the most promising AIDS vaccine candidates and plays central roles in the virus life cycle and pathogenesis. Understanding structural properties of vaccine candidate antigens leads to rational design of vaccines which improves their presentation to immune system and facilitates their manufacturing and storage. This study aims to investigate structural properties and stability of one variant of HIV-1 Tat recombinant protein using different spectroscopic, electrophoretic, and microscopic methods. Therefore, after the gene transformation, protein expression was optimized in E. coli cells and the C-terminal His6-tagged protein was purified using Ni-NTA resin. The structural stability of the pure protein was then investigated under different conditions including pH, Zn2+ ions, thermal and chemical stress. Acidic and alkaline pHs affects spectroscopic properties of the vaccine in different ways. The structure unfolding experiment shows relatively poor stability of the zinc-free protein sample compared to the ion-containing one. According to the quenching experiment and also thermal stability study results, the protein has attained more structural compactness in the presence of Zn2+. Secondary structure of the protein is mainly disordered and didn't significantly affect under various conditions. Finally, different degrees of oligomerization and aggregation were found under physiological conditions.

Variation in Terpene Profiles of Thymus vulgaris in Water Deficit Stress Response.

Thyme (Thymus spp.) volatiles predominantly consisting of monoterpenes and sesquiterpenes, serve as antimicrobial, antiseptic and antioxidant in phytomedicine. They also play a key role in plants as secondary metabolites via their potential role against herbivores, attracting pollinators and abiotic stress tolerance. Plant volatiles are affected by different environmental factors including drought. Here, the effect of prolonged water deficit stress on volatile composition was studied on the sensitive and tolerant thyme plant cultivars (T. vulgaris Var. Wagner and T. vulgaris Var. Varico3, respectively). Volatile sampling along with morpho-physiological parameters such as soil moisture, water potential, shoot dry weight, photosynthetic rate and water content measurements were performed on one-month-old plants subsequent to water withholding at 4-day intervals until the plants wilted. The tolerant and sensitive plants had clearly different responses at physiological and volatile levels. The most stress-induced changes on the plants' physiological traits occurred in the photosynthetic rates, where the tolerant plants maintained their photosynthesis similar to the control ones until the 8th day of the drought stress period. While the analysis of the volatile compounds (VOCs) of the sensitive thyme plants displayed the same pattern for almost all of them, in the tolerant plants, the comparison of the pattern of changes in the tolerant plants revealed that the changes could be classified into three separate groups. Our experimental and theoretical studies totally revealed that the most determinant compounds involved in drought stress adaptation included α-phellandrene, O-cymene, γ-terpinene and ß-caryophyelene. Overall, it can be concluded that in the sensitive plants trade-off between growth and defense, the tolerant ones simultaneously activate their stress response mechanism and continue their growth.

Boosting Tat DNA vaccine with Tat protein stimulates strong cellular and humoral immune responses in mice.

The aim of the present study was to evaluate the efficacy of a novel DNA priming-protein boosting regimen in simultaneous enhancing humoral and cellular immunogenicity of the HIV-1-Tat-based candidate vaccines in mice. The experimental BALB/c mice were successfully immunized with the HIV-1-Tat DNA vaccine and boosted with the corresponding protein vaccine over a two-week interval and the elicitation of cellular and humoral immune responses were simultaneously assessed. The results showed that the prime-boost immunization has significantly given rise to lymphocyte proliferation and CTL responses, as well as the levels of both IgG and IgG antibodies compared to the other candidate vaccines. The results of the Th polarization also revealed that the Th1: Th2 indexes in the mice vaccinated with the HIV-1 Tat protein, Tat DNA, and the prime-boost vaccines were 1.03, 1.19, and 1.25, respectively. The results suggest that co-administration of the HIV-1-Tat DNA with the corresponding protein may serve as a potential formulation for enhancing of Tat vaccineinduced immunity and has measurable effects on shaping vaccines' induced Th polarization.

pH-responsive hybrid magnetic polyelectrolyte complex based on alginate/BSA as efficient nanocarrier for curcumin encapsulation and delivery.

Curcumin-loaded hybrid magnetic nanoparticles (HMNPs) were formulated using a simple and green method. At first, surface of Fe3O4 nanoparticles was modified using alginate (ALG), and magnetic ALG/Fe3O4 nanoparticles with a size of 33 nm and a zeta potential of -25 mv were prepared. Then, a complex including bovine serum albumin (BSA) and poly((3-acrylamidopropyl)trimethylammonium chloride) (p(APTMACl)) was prepared and optimized. The behavior of the BSA/p(APTMACl) complex was evaluated as a function of pH and protein/polymer ratio by UV-Vis spectroscopy, zeta potential, and DLS analysis. Finally, the hybrid complex of BSA/p(APTMACl)@ALG/Fe3O4 were prepared using mixing of the BSA/p(APTMACl) and ALG/Fe3O4 systems with opposite charges via electrostatic interactions. The prepared BSA/p(APTMACl) @ALG/Fe3O4 complex was named hybrid magnetic nanoparticles (HMNPs) and characterized by UV-Vis spectroscopy, SEM, FT-IR, TGA, XRD, VSM, and AFM. Three samples including pure Fe3O4, ALG/Fe3O4, and the final HMNPs exhibited no cytotoxic effect on MCF-7 breast cancer cell using the MTT assay. MTT assay to test the antitumor activity of bare and encapsulated-curcumin against MCF-7 cells showed BSA/p(APTMACl)@ALG/Fe3O4 nanoparticles are able to stabilize the curcumin anti-cancer drug and also enhancing the antitumor activity of the encapsulated curcumin. After loading of curcumin with a high efficiency of 95%. The accumulative release profiles of curcumin were investigated at 37 °C in phosphate buffered saline (PBS, pH 3 and 7.4). The release of curcumin-loaded HMNPs followed the Korsmeyer-Peppas equation and anomalous (non-fickian) diffusion mechanism at pHs 3 and 7.4.

Evaluation of thiazolidinone derivatives as a new class of mushroom tyrosinase inhibitors.

Tyrosinase (EC 1.14.18.1) is a key copper-containing metalloenzyme widely distributed in nature and plays determinant role in melanin biosynthesis. The enzyme manifests two unusual catalytic properties including oxidase and monooxygenase activities. Its inhibitors may be applied to efficiently treat of hyperpigmentation and widely used in pharmaceutical and cosmetic products, as well as food supplements and insecticides. The present study aims to evaluate the inhibitory effects of some novel azo-hydrazone tautomeric dyes (4a-e) including bioactive thiazolidinone moiety on the activity of the mushroom tyrosinase. When L-3,4-dihydroxyphenylalanine (L-Dopa) was used as the substrate for the enzyme, the compounds 4d, 4a, and 4e showed strong inhibitory effects against the activity of the enzyme (61%, 56%, and 49% inhibition in the presence of 60µM of each compound, respectively). The IC50 values of the synthetized compounds were measured and their inhibition properties were also visualized by zymography. According to tyrosinase inhibitory activity, the compounds 4a, 4c, 4d and 4e exhibited strong inhibitory activities with IC50 values of 45.83, 140.25, 37.59, and 42.31µM, respectively, compared to the positive control kojic acid (29.44µM). Kinetic study of 4d compound (as the most potent inhibitor) revealed that the compound acts as a reversible competitive inhibitor of the enzyme with the Ki value of 31.0µM. We also simulated the molecular docking with the compound 4d and the results confirmed that the compound strongly interacts with the mushroom tyrosinase residues. All results totally suggest that thiazolidine derivatives, especially 4d, 4a, and 4e, can be considered as safe and efficient tyrosinase inhibitors. They also have the potential to be used in the correspond fields.

Formulation of chitosan with the polyepitope HIV-1 protein candidate vaccine efficiently boosts cellular immune responses in mice.

Human immunodeficiency virus-1 (HIV-1) continues to be a major global public health issue and priority. Despite the variety of antiretroviral therapies, it seems that an effective vaccine against HIV-1 is still very necessary. An ideal HIV-1 vaccine should be able to elicit both humoral and cellular immunities. In this respect, polyepitope vaccines, incorporated from several conserved regions of HIV-1 proteins, have received much attention recently. Herein, the immunogenicity of the HIV-1 polyepitope protein-based candidate vaccines was evaluated in BALB/c mice. Following the plasmid (pET23a-HIV-1-tat/pol/gag/env) preparation and transformation, the recombinant protein expression was optimized in Escherichia coli BL21 (DE3) host cells. After the HIV-1-top4 protein purification, chitosan and alum adjuvants were added to the vaccines formulations to reinforce the immunogenicity of the candidate vaccines. Mice were subcutaneously immunized three times at 2-week intervals with the candidate vaccines and the elicitation of both humoral and cellular immune responses were investigated. Taken together, the results showed that chitosan adjuvanted candidate vaccine conferred a stronger immunogenicity and elicited higher cellular responses than other candidate vaccines (P < 0.05). Thereby, it seems that co-utilizing of potent adjuvants with the HIV-1 polyepitope protein vaccines can help to open new avenues for strategies for HIV/AIDS vaccine design.

Lipidomics Unravels the Role of Leaf Lipids in Thyme Plant Response to Drought Stress.

Thymus is one of the best known genera within the Labiatae (Lamiaceae) family, with more than 200 species and many medicinal and culinary uses. The effects of prolonged drought on lipid profile were investigated in tolerant and sensitive thyme plants (Thymus serpyllum L. and Thymus vulgaris L., respectively). Non-targeted non-polar metabolite profiling was carried out using Fourier transform ion cyclotron resonance (FT-ICR) mass spectrometry with one-month-old plants exposed to drought stress, and their morpho-physiological parameters were also evaluated. Tolerant and sensitive plants exhibited clearly different responses at a physiological level. In addition, different trends for a number of non-polar metabolites were observed when comparing stressed and control samples, for both sensitive and tolerant plants. Sensitive plants showed the highest decrease (55%) in main lipid components such as galactolipids and phospholipids. In tolerant plants, the level of lipids involved in signaling increased, while intensities of those induced by stress (e.g., oxylipins) dramatically decreased (50-60%), in particular with respect to metabolites with m/z values of 519.3331, 521.3488, and 581.3709. Partial least square discriminant analysis separated all the samples into four groups: tolerant watered, tolerant stressed, sensitive watered and sensitive stressed. The combination of lipid profiling and physiological parameters represented a promising tool for investigating the mechanisms of plant response to drought stress at non-polar metabolome level.

The effects of CpG-ODNs and Chitosan adjuvants on the elicitation of immune responses induced by the HIV-1-Tat-based candidate vaccines in mice.

HIV1-Tat-based vaccines could elicit broad, durable and neutralizing immune responses and are considered as potential AIDS vaccines. The present study aims to formulate CpG-ODNs adjuvant and Chitosan with Tat protein to enhance the immunogenicity of HIV-1-Tat-based candidate vaccines and to investigate their efficacies in mice. To this end, we added CpG-ODNs, Chitosan and Alum as adjuvants to the Tat-based candidate vaccine formulations. Then, we compared frequency and magnitude of both humoral and cellular immune responses from mice immunized with the adjuvant-formulated Tat candidate vaccines against those obtained from mice immunized with recombinant Tat protein alone. Mice were subcutaneously immunized three times at 2-week intervals with the candidate vaccines. Measurements of anti-Tat immune responses showed that all vaccinated groups had a good immunity compared to the control groups and developed high levels of both humoral and cellular responses. However, immunized mice with CpG-ODNs, and Chitosan-adjuvanted Tat vaccines elicited stronger T-cell responses (both humoral and cellular immunity) compared to the others. These data suggest that co-administration of recombinant Tat protein with CpG-ODNs and Chitosan may serve as a potential formulation for enhancing of the Tat vaccine-induced immunity and might have effects on shaping Th polarization induced by HIV1-Tat protein vaccines.

Hyperactive Arg39Lys mutated mnemiopsin: implication of positively charged residue in chromophore binding cavity.

Mnemiopsin, a Ca(2+)-regulated photoprotein isolated from Mnemiopsis leidyi, belongs to the family of ctenophore photoproteins. These proteins emit blue light from a chromophore, which is tightly but non-covalently bound in their central hydrophobic core that contains 21 conserved residues. In an effort to investigate the role of Arg39 (the sole charged residue in coelenterazine binding cavity of ctenophore photoproteins) in bioluminescence properties of these photoproteins, three mutated forms of mnemiopsin 1 (R39E, R39K and R39M) were constructed and characterized. The results indicate that while the luminescence activity of R39K mutated mnemiopsin has increased about nine fold compared to the wild type, R39M and R39E mutated mnemiopsins have entirely lost their activities. The most distinguished properties of R39K mutated photoprotein are its high activity, slow rate of luminescence decay and broad pH profile compared to the wild type. The complete loss of bioluminescence activity in mutated photoproteins with negatively charged and aliphatic residues (R39E and R39M, respectively) shows that the presence of a positively charged residue at this position is necessary. The results of spectroscopic studies, including CD, intrinsic and extrinsic fluorescence measurements and acrylamide quenching studies show that, while the substitutions lead to structural rigidity in R39E and R39M mutated mnemiopsins, structural flexibility is obvious in R39K mutated mnemiopsin. The presence of a more localized positive charge on ε-amino group of Lys compared to guanidinium group of Arg residue in close proximity to the choromophre might affect its fixation in the binding cavity and result in increased bioluminescence activity in this mutated photoprotein. It appears that the polarity and flexibility of positively charged residue at this position finely tunes the luminescence properties of ctenophore photoproteins.

Identification and partial characterization of midgut proteases in the lesser mulberry pyralid, Glyphodes pyloalis.

Proteolytic activities in digestive system extracts from the larval midgut of the lesser mulberry pyralid, Glyphodes pyloalis Walker (Lepidoptera: Pyralidae), were analyzed using different specific peptide substrates and proteinase inhibitors. High proteolytic activities were found at pH 10.0 and a temperature of 50° C using azocasein as substrate. The trypsin was active in the pH range of 9.5- 12.0, with its maximum activity at pH 11.5. Ethylene diamine tetraacetic acid had the most inhibitory effect, and 44% inhibition was detected in the presence of this inhibitor. Phenyl methane sulfonyl floride and N-tosyl-L-phe chloromethyl ketone also showed considerable inhibition of larval azocaseinolytic activity, with 40.2 and 35.1% inhibition respectively. These data suggest that the midgut of larvae contains mainly metalloproteases and serine proteases, mainly chymotrypsin. The effect of several metal ions on the activity of proteases showed that NaCl, CaCl2, CoCl2 (5 and 10 mM), and MnCl2 (5mM) reduced the protease activity. The kinetic parameters of trypsin-like proteases using N-benzoyl-L-arg-p-nitroanilide as substrate indicated that the Km and Vmax values of trypsin in the alimentary canal were 50.5 ± 2.0 µM and 116.06 ± 1.96 nmol min(-1) mg(-1) protein, respectively. Inhibition assays showed only small amounts of cysteine proteases were present in the G. pyloalis digestive system. The midgut digestive protease system of G. pyloalis is as diverse as that of any of the other polyphagous lepidopteran insect species, and the midgut of larvae contains mainly metalloproteases. Moreover, serine proteases and chymotrypsin also play main roles in protein digestion. Characterization of the proteolytic properties of the digestive enzymes of G. pyloalis offers an opportunity for developing appropriate and effective pest management strategies via metalloproteases and chymotrypsin inhibitors.

Site-directed mutagenesis of photoprotein mnemiopsin: implication of some conserved residues in bioluminescence properties.

Mnemiopsin is a Ca(2+)-binding photoprotein from Mnemiopsis leidyi that emits a flash of blue light upon reacting with coelenterazine and Ca(2+). The light emission is a result of an intramolecular oxidation reaction. Similar to the other Ca(2+)-binding photoproteins, mnemiopsin is composed of apophotoprotein (206 amino acid residues), the imidazopyrazine chromophore, coelenterazine, and molecular oxygen. The biochemical properties of this photoprotein have been recently characterized but so far there has been no individual study on the role of critical residues. In this study, we introduced some mutations in the mnemiopsin structure for investigation of the roles of some critical residues in the substrate binding cavity, and neighboring residues in the mechanism of the reaction and the bioluminescence properties of the photoprotein. Mutants of mnemiopsin were produced by substitution of residues M77, W101 and M151. Three mutants (W101F, W101Y and M151Y mutants) had significantly reduced luminescence activity and altered bioluminescent properties (such as decay rate, Ca(2+) sensitivity, etc.), whereas the fourth (M77H mutant) lost its luminescence activity completely. Our experimental and theoretical studies suggest that residue M77 probably has structural importance and participates in stabilization of active site residues, whereas residue M151 is one of the critical mechanistic residues in ctenophore photoproteins.

An analysis of temperature adaptation in cold active, mesophilic and thermophilic Bacillus α-amylases.

A comparative biochemical and structural study was performed on a cold active α-amylase from Bacillus cereus (BCA) and two well-known homologous mesophilic and thermophilic α-amylases from Bacillus amyloliquefaciens (BAA) and Bacillus licheniformis (BLA). In spite of a high degree of sequence and structural similarity, drastic variations were found for T(opt) as 50, 70 and 90°C for BCA, BAA and BLA, respectively. The half-lives of thermoinactivation were 1 and 9 min for BCA and BAA at 80°C respectively, whilst there was no inactivation for BLA at this temperature. Thermodynamic studies on inactivation process suggested that lower thermostability of BCA is due to lower inactivation slope of the Arrhenius plots and subsequently, lower E(a) and ΔH(#). Increased K(m) and accessible surface area for catalytic residues along with a decreased number of internal interactions in this region in BCA compared to BLA suggest that BCA substrate-binding site might be temperature sensitive and is probably more flexible. On the other hand, fewer ion pairs, destructive substitutions and disruption of aromatic interaction networks in structurally critical regions of Bacillus α-amylases result in a severe decrease in BCA thermostability compared to its mesophilic and thermophilic homologues.