Effect of trifluoroethanol on the conformational stability of a hyperthermophilic esterase: a CD study

Unfolding action of alcohols on a highly negatively charged state of cytochrome C

It is well-known that hydrophobic effect play a major role in alcohol-protein interactions leading to structure unfolding. Studies with extremely alkaline cytochrome c (U(B) state, pH 13) in the presence of the first four alkyl alcohols suggests that the hydrophobic effect persistently overrides even though the protein carries a net charge of -17 under these conditions. Equilibrium unfolding of the U(B) state is accompanied by an unusual expansion of the chain involving an intermediate, I(alc), from which water is preferentially excluded, the extent of water exclusion being greater with the hydrocarbon content of the alcohol. The mobility and environmental averaging of side chains in the I(alc) state are generally constrained relative to those in the U(B) state. A few nuclear magnetic resonance-detected tertiary interactions are also found in the I(alc) state. The fact that the I(alc) state populates at low concentrations of methanol and ethanol and the fact that the extent of chain expansion in this state approaches that of the U(B) state indicate a definite influence of electrostatic repulsion severed by the low dielectric of the water/alcohol mixture. Interestingly, the U(B) ⇌ I(alc) segment of the U(B) ⇌ I(alc) ⇌ U equilibrium, where U is the unfolded state, accounts for roughly 85% of the total number of water molecules preferentially excluded in unfolding. Stopped-flow refolding results report on a submillisecond hydrophobic collapse during which almost the entire buried surface area associated with the U(B) state is recovered, suggesting the overwhelming influence of hydrophobic interaction over electrostatic repulsions.

Spatial structure peculiarities of influenza A virus matrix M1 protein in an acidic solution that simulates the internal lysosomal medium

The structure of the C-terminal domain of the influenza virus A matrix M1 protein, for which X-ray diffraction data were still missing, was studied in acidic solution. Matrix M1 protein was bombarded with thermally-activated tritium atoms, and the resulting intramolecular distribution of the tritium label was analyzed to assess the steric accessibility of the amino acid residues in this protein. This technique revealed that interdomain loops and the C-terminal domain of the protein are the most accessible to labeling with tritium atoms. A model of the spatial arrangement of the C-terminal domain of matrix M1 protein was generated using rosetta software adjusted to the data obtained by tritium planigraphy experiments. This model suggests that the C-terminal domain is an almost flat layer with a three-α-helical structure. To explain the high level of tritium label incorporation into the C-terminal domain of the M1 protein in an acidic solution, we also used independent experimental approaches (CD spectroscopy, limited proteolysis and MALDI-TOF MS analysis of the proteolysis products, dynamic light scattering and analytical ultracentrifugation), as well as multiple computational algorithms, to analyse the intrinsic protein disorder. Taken together, the results obtained in the present study indicate that the C-terminal domain is weakly structured. We hypothesize that the specific 3D structural peculiarities of the M1 protein revealed in acidic pH solution allow the protein greater structural flexibility and enable it to interact effectively with the components of the host cell.

Aggregation of amyloid Abeta((1-40)) peptide in perdeuterated 2,2,2-trifluoroethanol caused by ultrasound sonication

Ultrasound sonication of protein and peptide solutions is routinely used in biochemical, biophysical, pharmaceutical and medical sciences to facilitate and accelerate dissolution of macromolecules in both aqueous and organic solvents. However, the impact of ultrasound waves on folding/unfolding of treated proteins, in particular, on aggregation kinetics of amyloidogenic peptides and proteins is not understood. In this work, effects of ultrasound sonication on the misfolding and aggregation behavior of the Alzheimer's Abeta((1-40))-peptide is studied by pulsed-field gradient (PFG) spin-echo diffusion NMR and UV circular dichroism (CD) spectroscopy. Upon simple dissolution of Abeta((1-40)) in perdeuterated trifluoroethanol, CF(3)-CD(2)-OD (TFE-d(3)), the peptide is present in the solution as a stable monomer adopting alpha-helical secondary structural motifs. The self-diffusion coefficient of Abeta((1-40)) monomers in TFE-d(3) was measured as 1.35 x 10(-10) m(2) s(-1), reflecting its monomeric character. However, upon ultrasonic sonication for less than 5 min, considerable populations of Abeta molecules (ca 40%) form large aggregates as reflected in diffusion coefficients smaller than 4.0 x 10(-13) m(2) s(-1). Sonication for longer times (up to 40 min in total) effectively reduces the fraction of these aggregates in (1)H PFG NMR spectra to ca 25%. Additionally, absorption below 230 nm increased significantly upon sonication treatment, an observation, which also clearly confirms the ongoing aggregation process of Abeta((1-40)) in TFE-d(3). Surprisingly, upon ultrasound sonication only small changes in the peptide secondary structure were detected by CD: the peptide molecules mainly adopt alpha-helical motifs in both monomers and aggregates formed upon sonication.

Hydrophobic core of the steroidogenic acute regulatory protein for cholesterol transport

The steroidogenic acute regulatory protein (StAR), the first family member of START (StAR-related lipid transport) proteins, plays an essential role by facilitating the movement of cholesterol from the outer to inner mitochondrial membrane. Wild-type and mutant StAR binds cholesterol with similar intensity, but only wild-type StAR can transport it to mitochondria. Here, we report that the hydrophobic core is crucial for biological activity of proteins with START domains. Wild-type StAR increased steroidogenic activity by 7-9-fold compared to mutant R182L StAR, but both of them showed similar near-UV CD spectra. The fluorescence maximum of wild-type StAR is red shifted in comparison to mutant StAR under identical urea concentration. TFE increased the alpha-helical contribution of wild-type StAR more than the mutant protein. Acrylamide quenching for the wild-type protein (K(SV) = 12.0 +/- 0.2-11.2 +/- 0.5 M(-1)) exceeded that of the mutant protein (K(SV) = 4 +/- 0.2 M(-1)). Consistent with these findings, the hydrophobic probe ANS bound wild-type StAR (K(app) = 8.1 x 10(5) M(-1)) to a greater degree than mutant StAR (K(app) = 3.75 x 10(5) M(-1)). Partial proteolysis examined by mass spectrometry suggests that only wild-type StAR has a protease-sensitive C-terminus, but not the mutant. Stopped-flow CD revealed that the time of unfolding of mutant StAR was 0.017 s. In contrast, the wild-type StAR protein is unfolded in 16.3 s. In summary, these results demonstrate that wild-type StAR adopts a very flexible form due to the accommodation of more water molecules, while mutant StAR is generated by an alternate folding pathway making it inactive.

Carboxylic ester hydrolases from hyperthermophiles

Carboxylic ester hydrolyzing enzymes constitute a large group of enzymes that are able to catalyze the hydrolysis, synthesis or transesterification of an ester bond. They can be found in all three domains of life, including the group of hyperthermophilic bacteria and archaea. Esterases from the latter group often exhibit a high intrinsic stability, which makes them of interest them for various biotechnological applications. In this review, we aim to give an overview of all characterized carboxylic ester hydrolases from hyperthermophilic microorganisms and provide details on their substrate specificity, kinetics, optimal catalytic conditions, and stability. Approaches for the discovery of new carboxylic ester hydrolases are described. Special attention is given to the currently characterized hyperthermophilic enzymes with respect to their biochemical properties, 3D structure, and classification.

Protein stability modulated by a conformational effector: effects of trifluoroethanol on bovine serum albumin

The link between the thermodynamic properties of a solution and the conformational space explored by a protein is of fundamental importance to understand and control solubility, misfolding and aggregation processes. Here, we study the thermodynamic and conformational stability of a model protein, bovine serum albumin (BSA), by addition of trifluoroethanol (TFE), which is known to affect both the solvent properties and the protein structure. The solvent-mediated pair-wise interactions are investigated by static and dynamic light scattering, and by small angle X-ray scattering. The protein conformational details are studied by far- and near-UV circular dichroism (CD), and steady state fluorescence from tryptophan and from 1-anilino-8-naphthalene sulfonate (ANS). At low TFE concentrations, our results show that protein-protein interaction is dominated by steric repulsion accompanied by a consistent protein solvation. Minor local conformational changes also occur, but they do not affect the stability of BSA. At TFE concentrations above the threshold of 16% v/v, attractive interactions become prevalent, along with conformational changes related to a loosening of BSA tertiary structure. The onset of thermodynamic instability is triggered by the enhancement of hydrophobic attraction over repulsion, due to minor local changes of protein conformation and hydration. In the present context, TFE acts as a conformational effector, since it affects the intermolecular interaction and the activity of the proteins in solution through a direct mechanism.

Characterization of the non-native trifluoroethanol-induced intermediate conformational state of the Shiga toxin B-subunit

The effect of increasing concentrations of 2,2,2-trifluoroethanol (TFE) on the conformational stability of the Shiga toxin B-subunit (STxB), a bacterial homopentameric protein involved in cell-surface binding and intracellular transport, has been studied by far-, near-UV circular dichroism (CD), intrinsic fluorescence, analytical ultracentrifugation, and differential scanning calorimetry (DSC) under equilibrium conditions. Our data show that the native structure of STxB is highly perturbed by the presence of TFE. In fact, at concentrations of TFE above 20% (v/v), the native pentameric conformation of the protein is cooperatively transformed into a helix-rich monomeric and partially folded conformational state with no significant tertiary structure. Additionally, no cooperative transition was detected upon a further increase in the TFE concentration (above 40% (v/v)). The thermal stability of STxB was investigated at several different TFE concentrations using DSC and CD spectroscopy. Thermal transitions at TFE concentrations of up to 20% (v/v) were successfully fitted to the two-state folding/unfolding coupled to oligomerization model consistent with the transition between a pentameric folded conformation to a monomeric state of the protein, which the presence of TFE stabilizes as a partially folded conformation.