Alagiri Srinivasan

Inhibition of protein aggregation: supramolecular assemblies of arginine hold the key.

Background Aggregation of unfolded proteins occurs mainly through the exposed hydrophobic surfaces. Any mechanism of inhibition of this aggregation should explain the prevention of these hydrophobic interactions. Though arginine is prevalently used as an aggregation suppressor, its mechanism of action is not clearly understood. We propose a mechanism based on the hydrophobic interactions of arginine. Methodology We have analyzed arginine solution for its hydrotropic effect by pyrene solubility and the presence of hydrophobic environment by 1-anilino-8-naphthalene sulfonic acid fluorescence. Mass spectroscopic analyses show that arginine forms molecular clusters in the gas phase and the cluster composition is dependent on the solution conditions. Light scattering studies indicate that arginine exists as clusters in solution. In the presence of arginine, the reverse phase chromatographic elution profile of Alzheimers amyloid beta 1-42 (Aβ1-42) peptide is modified. Changes in the hydrodynamic volume of Aβ1-42 in the presence of arginine measured by size exclusion chromatography show that arginine binds to Aβ1-42. Arginine increases the solubility of Aβ1-42 peptide in aqueous medium. It decreases the aggregation of Aβ1-42 as observed by atomic force microscopy. Conclusions Based on our experimental results we propose that molecular clusters of arginine in aqueous solutions display a hydrophobic surface by the alignment of its three methylene groups. The hydrophobic surfaces present on the proteins interact with the hydrophobic surface presented by the arginine clusters. The masking of hydrophobic surface inhibits protein-protein aggregation. This mechanism is also responsible for the hydrotropic effect of arginine on various compounds. It is also explained why other amino acids fail to inhibit the protein aggregation.

First Structural Evidence of a Specific Inhibition of Phospholipase A2 by alpha-Tocopherol (Vitamin E) and its Implications in Inflammation: Crystal Structure of the Complex Formed Between Phospholipase A2 and alpha-Tocopherol at 1.8 A Resolution

This is the first structural evidence of alpha-tocopherol (alpha-TP) as a possible candidate against inflammation, as it inhibits phospholipase A2 specifically and effectively. The crystal structure of the complex formed between Vipera russelli phospholipase A2 and alpha-tocopherol has been determined and refined to a resolution of 1.8 A. The structure contains two molecules, A and B, of phospholipase A2 in the asymmetric unit, together with one alpha-tocopherol molecule, which is bound specifically to one of them. The phospholipase A2 molecules interact extensively with each other in the crystalline state. The two molecules were found in a stable association in the solution state as well, thus indicating their inherent tendency to remain together as a structural unit, leading to significant functional implications. In the crystal structure, the most important difference between the conformations of two molecules as a result of their association pertains to the orientation of Trp31. It may be noted that Trp31 is located at the mouth of the hydrophobic channel that forms the binding domain of the enzyme. The values of torsion angles (phi, psi, chi(1) and chi(2)) for both the backbone as well as for the side-chain of Trp31 in molecules A and B are -94 degrees, -30 degrees, -66 degrees, 116 degrees and -128 degrees, 170 degrees, -63 degrees, -81 degrees, respectively. The conformation of Trp31 in molecule A is suitable for binding, while that in B hinders the passage of the ligand to the binding site. Consequently, alpha-tocopherol is able to bind to molecule A only, while the binding site of molecule B contains three water molecules. In the complex, the aromatic moiety of alpha-tocopherol is placed in the large space at the active site of the enzyme, while the long hydrophobic channel in the enzyme is filled by hydrocarbon chain of alpha-tocopherol. The critical interactions between the enzyme and alpha-tocopherol are generated between the hydroxyl group of the six-membered ring of alpha-tocopherol and His48 N(delta1) and Asp49 O(delta1) as characteristic hydrogen bonds. The remaining part of alpha-tocopherol interacts extensively with the residues of the hydrophobic channel of the enzyme, giving rise to a number of hydrophobic interactions, resulting in the formation of a stable complex.

Crystal Structure of Lactoperoxidase at 2.4 Å Resolution

Lactoperoxidase (LPO) is a member of the mammalian peroxidase superfamily. It catalyzes the oxidation of thiocyanate and halides. Freshly isolated and purified samples of caprine LPO were saturated with ammonium iodide and crystallized using 20% polyethylene glycol 3350 in a hanging drop vapor diffusion setup. The structure has been determined using X-ray crystallographic method and refined to R(cryst) and R(free) factors of 0.196 and 0.203, respectively. The structure determination revealed an unexpected phosphorylation of Ser198 in LPO, which is also confirmed by anti-phosphoserine antibody binding studies. The structure is also notable for observing densities for glycan chains at all the four potential glycosylation sites. Caprine LPO consists of a single polypeptide chain of 595 amino acid residues and folds into an oval-shaped structure. The structure contains 20 well-defined alpha-helices of varying lengths including a helix, H(2a), unique to LPO, and two short antiparallel beta-strands. The structure confirms that the heme group is covalently linked to the protein through two ester linkages involving carboxylic groups of Glu258 and Asp108 and modified methyl groups of pyrrole rings A and C, respectively. The heme moiety is slightly distorted from planarity, but pyrrole ring B is distorted considerably. However, an iron atom is displaced only by 0.1 A from the plane of the heme group toward the proximal site. The substrate diffusing channel in LPO is cylindrical in shape with a diameter of approximately 6 A. Two histidine residues and six buried water molecules are connected through a hydrogen-bonded chain from the distal heme cavity to the surface of protein molecule and seemingly form the basis of proton relay for catalytic action. Ten iodide ions have been observed in the structure. Out of these, only one iodide ion is located in the distal heme cavity and is hydrogen bonded to the water molecule W1. W1 is also hydrogen bonded to the heme iron as well as to distal His109. The structure contains a calcium ion that is coordinated to seven oxygen atoms and forms a typical pentagonal bipyramidal coordination geometry.

Inhibition of Lactoperoxidase by Its Own Catalytic Product: Crystal Structure of the Hypothiocyanate-Inhibited Bovine Lactoperoxidase at 2.3-Å Resolution

To the best of our knowledge, this is the first report on the structure of product-inhibited mammalian peroxidase. Lactoperoxidase is a heme containing an enzyme that catalyzes the inactivation of a wide range of microorganisms. In the presence of hydrogen peroxide, it preferentially converts thiocyanate ion into a toxic hypothiocyanate ion. Samples of bovine lactoperoxidase containing thiocyanate (SCN(-)) and hypothiocyanate (OSCN(-)) ions were purified and crystallized. The structure was determined at 2.3-A resolution and refined to R(cryst) and R(free) factors of 0.184 and 0.221, respectively. The determination of structure revealed the presence of an OSCN(-) ion at the distal heme cavity. The presence of OSCN(-) ions in crystal samples was also confirmed by chemical and spectroscopic analysis. The OSCN(-) ion interacts with the heme iron, Gln-105 N(epsilon1), His-109 N(epsilon2), and a water molecule W96. The sulfur atom of the OSCN(-) ion forms a hypervalent bond with a nitrogen atom of the pyrrole ring D of the heme moiety at an S-N distance of 2.8 A. The heme group is covalently bound to the protein through two ester linkages involving carboxylic groups of Glu-258 and Asp-108 and the modified methyl groups of pyrrole rings A and C, respectively. The heme moiety is significantly distorted from planarity, whereas pyrrole rings A, B, C, and D are essentially planar. The iron atom is displaced by approximately 0.2 A from the plane of the heme group toward the proximal site. The substrate channel resembles a long tunnel whose inner walls contain predominantly aromatic residues such as Phe-113, Phe-239, Phe-254, Phe-380, Phe-381, Phe-422, and Pro-424. A phosphorylated Ser-198 was evident at the surface, in the proximity of the calcium-binding channel.

Structure of buffalo lactoferrin at 2.5 Å resolution using crystals grown at 303 K shows different orientations of the N and C lobes

The structure of buffalo lactoferrin has been determined at 303 K. The crystals belong to orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 77.5, b = 91.0, c = 131.5 A and Z = 4. The structure has been refined to an R factor of 0.187. The overall structure of the protein is similar to its structure determined at 277 K in a different crystal form. However, the lobe orientations in the two structures differ by 9.0 degrees, suggesting significant inter-lobe flexibility in this family of proteins. The inter-lobe interactions are predominantly hydrophobic and could act as a cushion for a change in orientation under the influence of external conditions. On the other hand, the domain arrangements are found to be similar in 277 and 303 K crystal structures, with orientations differing by 1.5 and 1.0 degrees in the N and C lobes, respectively. The results of these investigations suggest that the increase in temperature helps in the production of better quality crystals.

MALDI-TOF mass spectrometry for rapid identification of clinical fungal isolates based on ribosomal protein biomarkers

This study aimed to evaluate the identification of clinical fungal isolates (yeast and molds) by protein profiling using Matrix-assisted laser desorption ionization-time of flight mass spectrometry (MALDI-TOF/MS). A total of 125 clinical fungal culture isolates (yeast and filamentous fungi) were collected. The test set included 88 yeast isolates (Candida albicans, Candida glabrata, Candida guilliermondii, Candida kefyr, Candida krusei, Candida parapsilosis, Candida rugosa, Candida tropicalis and Cryptococcus neoformans) and 37 isolates of molds (Alternaria spp., Aspergillus flavus, Aspergillus fumigatus, Aspergillus niger, Cunninghamella spp., Histoplasma capsulatum, Microsporum gypseum, Microsporum nanum, Rhizomucor spp. and Trichophyton spp.). The correlation between MALDI TOF MS and conventional identification for all these 125 fungal isolates included in the study was 87.2% at the species level and 90.4% at the genus level. MALDI TOF MS results revealed that the correlation in yeast (n=88) identification was 100% both at the genus and species levels whereas, the correlation in mold (n=37) identification was more heterogeneous i.e. 10.81% isolates had correct identification up to the genus level, 56.7% isolates had correct identification both at the genus and species levels, whereas 32.42% isolates were deemed Not Reliable Identification (NRI). But, with the modification in sample preparation protocol for molds, there was a significant improvement in identification. 86.4% isolates had correct identification till the genus and species levels whereas, only 2.7% isolates had Not Reliable Identification. In conclusion, this study demonstrates that MALDI-TOF MS could be a possible alternative to conventional techniques both for the identification and differentiation of clinical fungal isolates. However, the main limitation of this technique is that MS identification could be more precise only if the reference spectrum of the fungal species is available in the database.

Crystal Structure of a Complex Formed between a Snake Venom Phospholipase A2 and a Potent Peptide Inhibitor Phe-Leu-Ser-Tyr-Lys at 1.8 Å Resolution

Phospholipase A2is an important enzyme involved in the production of prostaglandins and their related compounds causing inflammatory disorders. Among the several peptides tested, the peptide Phe-Leu-Ser-Tyr-Lys (FLSYK) showed the highest inhibition. The dissociation constant (K d ) for this peptide was calculated to be 3.57 ± 0.05 × 10−9 m. In order to further improve the degree of inhibition of phospholipase A2, a complex between Russells viper snake venom phospholipase A2 and a peptide inhibitor FLSYK was crystallized, and its structure was determined by crystallographic methods and refined to an R-factor of 0.205 at 1.8 Å resolution. The structure contains two crystallographically independent molecules of phospholipase A2 (molecules A and B) and a peptide molecule specifically bound to molecule A only. The two molecules formed an asymmetric dimer. The dimerization caused a modification in the binding site of molecule A. The overall conformations of molecules A and B were found to be generally similar except three regions i.e. the Trp-31-containing loop (residues 25–34), the β-wing consisting of two antiparallel β-strands (residues 74–85) and the C-terminal region (residues 119–133). Out of the above three, the most striking difference pertains to the conformation of Trp-31 in the two molecules. The orientation of Trp-31 in molecule A was suitable for the binding of FLSYK, while it disallowed the binding of peptide to molecule B. The structure of the complex clearly shows that the peptide is so placed in the binding site of molecule A that the side chain of its lysine residue interacted extensively with the enzyme and formed several hydrogen bonds in addition to a strong electrostatic interaction with critical Asp-49. The C-terminal carboxylic group of the peptide interacted with the catalytic residue His-48.

Structure of the bifunctional inhibitor of trypsin and α-amylase from ragi seeds at 2.2 Å resolution

The crystal structure of a bifunctional inhibitor of alpha-amylase and trypsin (RATI) from ragi seeds (Indian finger millet, Eleusine coracana Gaertneri) has been determined by X-ray diffraction at 2.2 A resolution. The inhibitor consists of 122 amino acids, with five disulfide bridges, and belongs to the plant alpha-amylase/trypsin inhibitor family. The crystals were grown by the microdialysis method using ammonium sulfate as a precipitating agent. The structure was determined by the molecular-replacement method using as models the structures of Corn Hageman factor inhibitor (CHFI) and of RATI at 2.9 A resolution determined previously. It has been refined to an R factor of 21.9%. The structure shows an r.m.s. deviation for C(alpha) atoms of 2.0 A compared with its own NMR structure, whereas the corresponding value compared with CHFI is found to be 1.4 A. The r.m.s. difference for C(alpha) atoms when compared with the same protein in the structure of the complex with alpha-amylase is 0.7 A. The conformations of trypsin-binding loop and the alpha-amylase-binding N-terminal region were also found to be similar in the crystal structures of native RATI and its complex with alpha-amylase. These regions differed considerably in the NMR structure.

Structural Evidence of Substrate Specificity in Mammalian Peroxidases: STRUCTURE OF THE THIOCYANATE COMPLEX WITH LACTOPEROXIDASE AND ITS INTERACTIONS AT 2.4 A RESOLUTION

The crystal structure of the complex of lactoperoxidase (LPO) with its physiological substrate thiocyanate (SCN–) has been determined at 2.4Å resolution. It revealed that the SCN– ion is bound to LPO in the distal heme cavity. The observed orientation of the SCN– ion shows that the sulfur atom is closer to the heme iron than the nitrogen atom. The nitrogen atom of SCN– forms a hydrogen bond with a water (Wat) molecule at position 6′. This water molecule is stabilized by two hydrogen bonds with Gln423 Nϵ2 and Phe422 oxygen. In contrast, the placement of the SCN– ion in the structure of myeloperoxidase (MPO) occurs with an opposite orientation, in which the nitrogen atom is closer to the heme iron than the sulfur atom. The site corresponding to the positions of Gln423, Phe422 oxygen, and Wat6′ in LPO is occupied primarily by the side chain of Phe407 in MPO due to an entirely different conformation of the loop corresponding to the segment Arg418–Phe431 of LPO. This arrangement in MPO does not favor a similar orientation of the SCN– ion. The orientation of the catalytic product OSCN– as reported in the structure of LPO·OSCN– is similar to the orientation of SCN– in the structure of LPO·SCN–. Similarly, in the structure of LPO·SCN–·CN–, in which CN– binds at Wat1, the position and orientation of the SCN– ion are also identical to that observed in the structure of LPO·SCN.

Physicochemical characterisation of β-chitosan from Sepioteuthis lessoniana gladius

β-Chitin and its chitosan from the gladius of Sepioteuthis lessoniana have been isolated, purified, characterised and compared with the commercial chitosan. Ash, moisture, mineral, metal and elemental content were analyzed using standard techniques. The optical activity of chitin was found to be levorotatory. The degree of deacetylation was calculated by potentiometric titration and (1)H NMR. Viscosity average molecular weight of β-chitosan was calculated by viscometry and size average molecular weight by GPC. The structure of β-chitosan was elucidated with FT-IR and NMR. Thermal nature, crystalline structure and morphology of β-chitosan were characterised through DSC, XRD and SEM, respectively. The water and fat binding capacity of β-chitosan presently studied was significantly higher than that of the commercial chitosan. The result of the present study adds that S. lessoniana gladius is also an additional source of β-chitin and chitosan of higher yield, lower molecular weight and higher degree of deacetylation.