SDS modulates amyloid fibril formation and conformational change in succinyl-ConA at low pH

SDBS induces multiple catalase conformations in a dose-dependent manner

Amyloid fibrils have been linked to several incurable diseases. They are long and thin fibrous proteins that self-assemble into fibrils. Small molecules can stimulate amyloid fibrillation, but the mechanism by which this happens is not well understood. This study examined how a negatively charged benzene ring containing surfactant, sodium dodecylbenzene sulphonate (SDBS), affects the fibrillation of bovine liver catalase (BLC). After SDBS treatment, BLC conformational changes were examined in vitro using turbidity, RLS kinetics, intrinsic fluorescence, ThT fluorescence, far-UV CD, and TEM. BLC in the native state was alpha-helical at pH 7.4, while it was converted to a random coil structure at pH 2.0. Far-UV CD and intrinsic fluorescence data showed that at concentrations <0.1 mM of SDBS, randomly coiled BLC assumed a native-like alpha-helical structure. However, between 0.1 and 1.0 mM SDBS, BLC was aggregated. ThT fluorescence and far-UV CD measurements showed the amyloid-like structures in the aggregated BLC. At higher SDBS concentrations (>1.0 mM) at pH 2.0, BLC again attains a native-like alpha-helical structure. It is essential for therapeutic purposes to clearly understand the process underlying surfactant- or lipid-induced fibrillation.

Exploring the Structural Importance of the C3=C4 Double Bond in Plant Alkaloids Harmine and Harmaline on Their Binding Interactions with Hemoglobin

Harmine and harmaline are two structurally similar heterocyclic β-carboline plant alkaloids with various therapeutic properties, having a slight structural difference in the C3=C4 double bond. In the present study, we have reported the nature of the interaction between hemoglobin (Hb) with harmine and harmaline by employing several multispectroscopic, calorimetric, and molecular docking approaches. Fluorescence spectroscopic studies have shown stronger interaction of harmine with Hb compared to that of almost structurally similar harmaline. Steady-state anisotropy experiments further show that the motional restriction of harmine in the presence of Hb is substantially higher than that of the harmaline-Hb complex. Circular dichroism (CD) study demonstrates no conformational change of Hb in the presence of both alkaloids, but CD study in 1-cm cuvette path length also demonstrates stronger affinity of harmine toward Hb compared to harmaline. From the thermal melting study, it has been found that both harmine and harmaline slightly affect the stability of Hb. From isothermal titration calorimetry (ITC), we have found that the binding process is exothermic and enthalpy driven. Molecular docking studies indicated that both harmine and harmaline prefer identical binding sites in Hb. This study helps us to understand that slight structural differences in harmine and harmaline can alter the interaction properties significantly, and this key information may help in the drug discovery processes.

SDS-induced hexameric oligomerization of myotoxin-II from Bothrops asper assessed by sedimentation velocity and nuclear magnetic resonance

We report the solution behavior, oligomerization state, and structural details of myotoxin-II purified from the venom of Bothrops asper in the presence and absence of sodium dodecyl sulfate (SDS) and multiple lipids, as examined by analytical ultracentrifugation and nuclear magnetic resonance. Molecular functional and structural details of the myotoxic mechanism of group II Lys-49 phospholipase A2 homologues have been only partially elucidated so far, and conflicting observations have been reported in the literature regarding the monomeric vs. oligomeric state of these toxins in solution. We observed the formation of a stable and discrete, hexameric form of myotoxin-II, but only in the presence of small amounts of SDS. In SDS-free medium, myotoxin-II was insensitive to mass action and remained monomeric at all concentrations examined (up to 3 mg/ml, 218.2 μM). At SDS concentrations above the critical micelle concentration, only dimers and trimers were observed, and at intermediate SDS concentrations, aggregates larger than hexamers were observed. We found that the amount of SDS required to form a stable hexamer varies with protein concentration, suggesting the need for a precise stoichiometry of free SDS molecules. The discovery of a stable hexameric species in the presence of a phospholipid mimetic suggests a possible physiological role for this oligomeric form, and may shed light on the poorly understood membrane-disrupting mechanism of this myotoxic protein class.

Agitation does not induce fibrillation in reduced hen egg-white lysozyme at physiological temperature and pH

Several proteins and peptides tend to form an amyloid fibril, causing a range of unrelated diseases, from neurodegenerative to certain types of cancer. In the native state, these proteins are folded and soluble. However, these proteins acquired β-sheet amyloid fibril due to unfolding and aggregation. The conversion mechanism from well-folded soluble into amorphous or amyloid fibril is not well understood yet. Here, we induced unfolding and aggregation of hen egg-white lysozyme (HEWL) by reducing agent dithiothreitol and applied mechanical sheering force by constant shaking (1000 rpm) on the thermostat for 7 days. Our turbidity results showed that reduced HEWL rapidly formed aggregates, and a plateau was attained in nearly 5 h of incubation in both shaking and non-shaking conditions. The turbidity was lower in the shaking condition than in the non-shaking condition. The thioflavin T binding and transmission electron micrographs showed that reduced HEWL formed amorphous aggregates in both conditions. Far-UV circular dichroism results showed that reduced HEWL lost nearly all alpha-helical structure, and β-sheet secondary structure was not formed in both conditions. All the spectroscopic and microscopic results showed that reduced HEWL formed amorphous aggregates under both conditions.