Binding residues and catalytic domain of soluble Saccharomyces cerevisiae processing alpha-glucosidase I

Synthesis of new clioquinol derivatives as potent α-glucosidase inhibitors; molecular docking, kinetic and structure-activity relationship studies

Diabetes mellitus is a chronic metabolic disorder with increasing prevalence and long-term complications. The aim of this study was to identify α-glucosidase inhibitory compounds with potential anti-hyperglycemic activity. For this purpose, a series of new clioquinol derivatives 2a-11a was synthesized, and characterized by various spectroscopic techniques. The enzyme inhibitory activities of the resulting derivatives were assessed using an in-vitro mechanism-based assay. All the tested compounds 2a-11a of the series showed a significant α-glucosidase inhibition with IC₅₀ values 43.86-325.81 µM, as compared to the standard drug acarbose 1C₅₀: 875.75 ± 2.08 µM. Among them, compounds 4a, 5a, 10a, and 11a showed IC₅₀ values of 105.51 ± 2.41, 119.24 ± 2.37, 99.15 ± 2.06, and 43.86 ± 2.71 µM, respectively. Kinetic study of the active analogues showed competitive, non-competitive, and mixed-type inhibitions. Furthermore, the molecular docking study was performed to elucidate the binding interactions of most active analogues with the various sites of α-glucosidase enzyme. The results indicate that these compounds have the potential to be further studied as new anti-diabetic agents.

Nanocapsules containing Saussurea lappa essential oil: Formulation, characterization, antidiabetic, anti-cholinesterase and anti-inflammatory potentials

Plant-based remedies have been widely used for the management of variable diseases due to their safety and less side effects. In the present study, we investigated Saussurea lappa CB. Clarke. (SL) given its largely reported medicinal effects. Specifically, our objective was to provide an insight into a new polymethyl methacrylate based nanocapsules as carriers of SL essential oil and characterize their biologic functions. The nanoparticles were prepared by nanoprecipitation technique, characterized and analyzed for their cytotoxicity, anti-inflammatory, anti-Alzheimer and antidiabetic effects. The results revealed that the developed nanoparticles had a diameter around 145 nm, a polydispersity index of 0.18 and a zeta potential equal to +45 mV and they did not show any cytotoxicity at 25 μg·mL^-1. The results also showed an anti-inflammatory activity (reduction in metalloprotease MMP-9 enzyme activity and RNA expression of inflammatory cytokines: TNF-α, GM-CSF and IL1β), a high anti-Alzheimer's effect (IC50 around 25.0 and 14.9 μg·mL^-1 against acetylcholinesterase and butyrylcholinesterase, respectively), and a strong antidiabetic effect (IC50 were equal to 22.9 and 75.8 μg·mL^-1 against α-amylase and α-glucosidase, respectively). Further studies are required including the in vivo studies (e.g., preclinical), the pharmacokinetic properties, the bioavailability and the underlying associated metabolic pathways.

Specificity of Processing α-glucosidase I is guided by the substrate conformation: crystallographic and in silico studies

BACKGROUND

The enzyme “GluI” is key to the synthesis of critical glycoproteins in the cell.

RESULTS

We have determined the structure of GluI, and modeled binding with its unique sugar substrate.

CONCLUSION

The specificity of this interaction derives from a unique conformation of the substrate.

SIGNIFICANCE

Understanding the mechanism of the enzyme is of basic importance and relevant to potential development of antiviral inhibitors. Processing α-glucosidase I (GluI) is a key member of the eukaryotic N-glycosylation processing pathway, selectively catalyzing the first glycoprotein trimming step in the endoplasmic reticulum. Inhibition of GluI activity impacts the infectivity of enveloped viruses; however, despite interest in this protein from a structural, enzymatic, and therapeutic standpoint, little is known about its structure and enzymatic mechanism in catalysis of the unique glycan substrate Glc3Man9GlcNAc2. The first structural model of eukaryotic GluI is here presented at 2-Å resolution. Two catalytic residues are proposed, mutations of which result in catalytically inactive, properly folded protein. Using Autodocking methods with the known substrate and inhibitors as ligands, including a novel inhibitor characterized in this work, the active site of GluI was mapped. From these results, a model of substrate binding has been formulated, which is most likely conserved in mammalian GluI.

Production and crystallization of processing α-glucosidase I: Pichia pastoris expression and a two-step purification toward structural determination

Eukaryotic N-glycoprotein processing in the endoplasmic reticulum begins with the catalytic action of processing α-glucosidase I (αGlu). αGlu trims the terminal glucose from nascent glycoproteins in an inverting-mechanism glycoside hydrolysis reaction. αGlu has been studied in terms of kinetic parameters and potential key residues; however, the active site is unknown. A structural model would yield important insights into the reaction mechanism. A model would also be useful in developing specific therapeutics, as αGlu is a viable drug target against viruses with glycosylated envelope proteins. However, due to lack of a high-yielding overexpression and purification scheme, no eukaryotic structural model of αGlu has been determined. To address this issue, we overexpressed the Saccharomyces cerevisiae soluble αGlu, Cwht1p, in the host Pichia pastoris. It was purified in a simple two-step protocol, with a final yield of 4.2mg Cwht1p per liter of growth culture. To test catalytic activity, we developed a modified synthesis of a tetrasaccharide substrate, Glc(3)ManOMe. Cwht1p with Glc(3)ManOMe shows a K(m) of 1.26 mM. Cwht1p crystals were grown and subjected to X-ray irradiation, giving a complete diffraction dataset to 2.04 Å resolution. Work is ongoing to obtain phases so that we may further understand this fundamental member of the N-glycosylation pathway through the discovery of its molecular structure.

Protein glycosylation in Candida

Candidiasis is a significant cause of invasive human mycosis with associated mortality rates that are equivalent to, or worse than, those cited for most cases of bacterial septicemia. As a result, considerable efforts are being made to understand how the fungus invades host cells and to identify new targets for fungal chemotherapy. This has led to an increasing interest in Candida glycobiology, with an emphasis on the identification of enzymes essential for glycoprotein and adhesion metabolism, and the role of N- and O-linked glycans in host recognition and virulence. Here, we refer to studies dealing with the identification and characterization of enzymes such as dolichol phosphate mannose synthase, dolichol phosphate glucose synthase and processing glycosidases and synthesis, structure and recognition of mannans and discuss recent findings in the context of Candida albicans pathogenesis.

Identifying residues in antigenic determinants by chemical modification

Chemical modification of the side chains of amino acid residues was one of the first methods developed to investigate epitopes in protein antigens. The principle of the method is that alteration of the structure of a key residue of an epitope by a chemical modification will alter reactivity with antibody by affecting either specificity or avidity or both. Chemical modification has the advantage that it can be applied to discontinuous as well as continuous epitopes and may be of value in identifying cryptic epitopes. We consider here the several recent studies that have applied site-specific chemical modification to the identification of epitopes on antigens, including the use of formaldehyde, glutaraldehyde, and acid anhydrides, to produce allergoids where determinants important to reaction with IgE are modified but the ability to elicit an IgG response is retained. It is noteworthy that modification of amino groups with charge reversal appears to be the most useful approach. The approach to the use of site-specific chemical modification as a tool for the study of protein function is discussed, and emphasis is placed on the necessity to (1) validate the specificity of modification and (2) assess potential conformational change that may occur secondary to modification. Finally, a list of chemical reagents used for protein modification is presented, together with properties and references to use.

Conversion of α1,2-mannosidase E-I from Candida albicans to α1,2-mannosidase E-II by limited proteolysis

Previous studies demonstrated the presence in Candida albicans ATCC 26555 of two soluble alpha1,2-mannosidases: E-I and E-II. In contrast, in the C. albicans CAI-4 mutant only E-I was detected and it could be processed by a membrane-bound proteolytic activity from the ATCC 26555 strain, generating an active 43 kDa polypeptide. Here, alpha1,2-mannosidase E-I from strain ATCC 26555 was purified by conventional methods of protein isolation and affinity chromatography in Concanavalin A-Sepharose 4B. Analytical electrophoresis of the purified enzyme revealed two polypeptides of 52 and 23 kDa, the former being responsible for enzyme activity as revealed by zymogram analysis. Time course proteolysis with an aspartyl protease from Aspergillus saitoi, converted alpha1,2-mannosidase E-I into an active polypeptide of 43 kDa which trimmed Man(9)GlcNAc(2), generating Man(8)GlcNAc(2) isomer B and mannose. Trimming was inhibited preferentially by 1-deoxymannojirimycin. Both, the molecular mass and the enzyme properties of the proteolytic product were identical to those described for alpha1,2-mannosidase E-II therefore supporting the notion that E-I is the precursor of E-II.

Truncations and functional carboxylic acid residues of yeast processing alpha-glucosidase I

Yeast alpha-glucosidase I (Cwh41p) encoded by CWH41 is an endoplasmic reticulum (ER) membrane-bound glycoprotein (833 residues), which plays an important role in the early steps of the N-glycosylation pathway. In this study functional expression of three truncated fragments of Cwh41p, all containing the catalytic region, was investigated. Cwht1p (E35-F833), with deletion of the N-terminus and transmembrane domain, was expressed as a catalytically active fragment while R320-F833(Cwht2p) and M526-F833 (Cwht3p) were not detected. Significantly higher glucosidase I activity was found in a soluble extract from yeast overexpressing CWHT1 (1,400 U/g biomass) than yeast overexpressing CWH41 (300 U/g biomass). Cwht1p was purified as a soluble 94 kDa non-glycosylated protein with a specific activity (3,600 U/mg protein) comparable to that of the soluble alpha-glucosidase I (3000 U/mg protein). These findings indicate that the active conformation of the enzyme is not dependent on protein glycosylation and suggest that the M1-I28 region of Cwh41p carries an ER-targeting signal sequence. In addition, two highly conserved carboxylic acid residues, E580 and D584 of Cwht1p (corresponding to E613 and D617 of Cwh41p), located within the catalytic domain of yeast enzyme were subjected to mutation. Substitution of each residue with Ala resulted in low expression and undetectable glucosidase I activity. These findings indicate that E613 and D617 play a crucial role in maintaining alpha-glucosidase I activity.