Partial specific volumes and interactions with solvent components of α-globulin fromSesamum indicum L. in urea and guanidine hydrochloride

Oligomeric states of the Shigella translocator protein IpaB provide structural insights into formation of the type III secretion translocon

The Shigella flexneri Type III secretion system (T3SS) senses contact with human intestinal cells and injects effector proteins that promote pathogen entry as the first step in causing life threatening bacillary dysentery (shigellosis). The Shigella Type III secretion apparatus (T3SA) consists of an anchoring basal body, an exposed needle, and a temporally assembled tip complex. Exposure to environmental small molecules recruits IpaB, the first hydrophobic translocator protein, to the maturing tip complex. IpaB then senses contact with a host cell membrane, forming the translocon pore through which effectors are delivered to the host cytoplasm. Within the bacterium, IpaB exists as a heterodimer with its chaperone IpgC; however, IpaB's structural state following secretion is unknown due to difficulties isolating stable protein. We have overcome this by coexpressing the IpaB/IpgC heterodimer and isolating IpaB by incubating the complex in mild detergents. Interestingly, preparation of IpaB with n-octyl-oligo-oxyethylene (OPOE) results in the assembly of discrete oligomers while purification in N,N-dimethyldodecylamine N-oxide (LDAO) maintains IpaB as a monomer. In this study, we demonstrate that IpaB tetramers penetrate phospholipid membranes to allow a size-dependent release of small molecules, suggesting the formation of discrete pores. Monomeric IpaB also interacts with liposomes but fails to disrupt them. From these and additional findings, we propose that IpaB can exist as a tetramer having inherent flexibility, which allows it to cooperatively interact with and insert into host cell membranes. This event may then lay the foundation for formation of the Shigella T3SS translocon pore.

Preferential interaction of denaturants with rice bran lipase

The catalytic stability of rice bran lipase has been determined in the presence of three denaturants viz, urea, GuHCl and GuHSCN. The enzyme is completely inactive above 7 M urea, 4 M GuHCl and 2 M GuHSCN concentration. The extent of denaturant interaction has been determined by the partial specific volume measurements of the enzyme. The preferential interaction parameter (xi 3) has values of 0.042, 0.064 and 0.075 g/g, and the denaturation volume changes are -180, -240 and -270 ml/mol in presence of 8 M urea, 6 M GuHC1; and 3 M GuHSCN, respectively. The experimental values of number of denaturant molecules bound (A3) are 0.418, 0.566 and 0.320 g/g and the calculated values are 0.321, 0.511 and 0.632 g/g in presence of 8 M urea, 6 M GuHCl and 3 M GuHSCN, respectively. Fluorescence emission measurements indicated a decrease in the fluorescence emission intensity and a red shift in the emission maximum as the denaturant concentration is increased indicating the gradual exposure of aromatic chromophores. The instability of the enzyme in the presence of these denaturants has been indicated by a decreased value of apparent thermal denaturation temperature (Tm) of the enzyme from a control value of 67 degrees C. The results obtained in the present study explain the extent of inactivation/stability of rice bran lipase in presence of these denaturants at different concentrations.

Mechanism of solvent-induced thermal stabilization of alpha-amylase from Bacillus amyloliquefaciens

The transition temperature of irreversible thermal inactivation of alpha-amylase from Bacillus amyloliquefaciens was estimated to be 60 degrees C. At this temperature, the enzyme inactivation followed first-order kinetics, having a half-life (t 1/2) of 12 min with a rate constant (k) of 0.06 min-1. Conformational change was a prerequisite for this thermal inactivation. This is governed by stepwise temperature-dependent phenomena. Among the solvent stabilizers tested, the enzyme was thermally stable in presence of DMSO and PEG 300 and the stabilizing efficiency of these cosolvents was concentration-dependent. The enzyme was partially stabilized by 5.0 M DMSO and 1.9 M PEG 300 up to 78 degrees C. However, above 78 degrees C the enzyme was inactivated in these cosolvents also. The mechanism of stabilization has been explained by preferential hydration of the enzyme in these structure stabilizing solvents by exclusion from the protein surface and interface by measurement of partial specific volume in these cosolvents. The data suggest a high value of preferential interaction parameter, (delta g3/delta g2)tau, mu 1, mu 3 being -0.606/g/g g/g in 40% DMSO and a low value of -0.025 g/g in 5% glycerol. The preferential interaction parameters in sucrose and glycerol suggests that (delta g3/delta g2)tau, mu 1, mu 3m is highest of -0.420 g/g in 10% glycerol than any other cosolvent.

Structural stability of lipase from wheat germ

Purified lipase from wheat germ was used for the determination of preferential interaction parameters under different stabilizing cosolvent conditions. The partial specific volume of the enzyme was measured under both isomolal and isopotential conditions in phosphate buffer at pH 7.0, 0.02 M, and the value was found to be 0.730 +/- 0.001 and 0.731 +/- 0.002 mL/g, respectively. The partial specific volume measurements with different cosolvents indicated that the enzyme has a (delta g3/delta g2)T,mu1,mu3 values of -0.119 +/- 0.012, -0.073 +/- 0.009 and -0.141 +/- 0.020 g/g, respectively, in 25% glucose, 25% sucrose and 25% DMSO. The (delta g3/delta g2)T,mu1,mu3 values in 10 and 20% glycerol were -0.054 +/- 0.012 and -0.073 +/- 0.016 g/g, respectively. Based on these values it is clear that the enzyme is stabilized in the presence of these cosolvents by increasing its hydration, of which DMSO is stabilizing to the maximum extent. The stabilization of the enzyme was also confirmed by the thermal denaturation measurements in the presence of these cosolvents which indicated a shift in the apparent thermal denaturation temperature of the enzyme towards higher temperatures. The data are supported further by the ultraviolet difference spectral as well as fluorescence measurements in the presence of these cosolvents. The stabilization has been attributed to the preferential hydration of the enzyme in the presence of these cosolvents.

Association of proteins in acidic solutions--a case study with beta-globulin

The investigation of the effect of acid pH on the structure of beta-globulin indicated several transitions as a function of pH. Upon reducing the pH from 7.0, the beta-globulin molecule underwent an expansion due to hydration up to pH 5.0, and a further increase in H+ concentration resulted in unfolding. This is a single step cooperative denaturation as indicated by the viscosity profile. At extreme acid pH values (below pH 2.0) the protein associates or folds to a different conformational motif as shown by blue shift of ultraviolet fluorescence emission maximum and decrease in reduced viscosity values by more than 30% due to an entropically driven hydrophobic interaction. The conformational analysis of beta-globulin showed a decrease up to pH 3.0, followed by an increase in the ordered structure at low pH values indicating that the low pH values stabilized this new conformation. These results are discussed in view of the molten globule structure of proteins.

Structural stability of lipase from wheat germ in alkaline pH

The present investigation shows the effect of alkaline pH on the structure-function relationship of lipase from wheat germ. There is a 70% decrease in lipase activity at pH 10.0, which decreases to 93% at pH 12.0 as compared to neutral pH activity (Rajendran et al. 1990). This change is shown to be as a result of loss of alpha-helical structure with a concomitant increase in aperiodic structure. The results with fluorescence spectra and tyrosyl ionization indicate gradual exposure of aromatic side chains of tyrosine and tryptophan to the bulk solvent along with the structural changes. The enzyme is in an extended form at alkaline pH with a volume change of - 1300 ml mol as also indicated by increase in reduced viscosity to 12.5 ml g and significant decrease in sedimentation coefficient. The kinetics of the reaction points to a cooperative pseudo first-order reaction as determined by stopped-flow kinetic analysis in the ultraviolet region. The inactivation mechanism appears to follow a two-step mechanism of a fast and a slow reaction.

Effect of acidic pH on the protein carmin from safflower seed (Carthamus tinctorius)

The effect of a decrease in pH on the structural integrity of carmin has been monitored by a variety of biophysical techniques. The protein undergoes initial dissociation up to pH 3.5-4.0 without any significant denaturation. Below this pH the process of dissociation and denaturation appears to be simultaneous. Further, in the pH range of 2.5-1.6 the protein reassociates to probably a different polymer resulting from possibly, an entropically driven hydrophobic interaction. The process of dissociation appears to be reversible to a large extent. The process of denaturation appears to be governed by the kinetic path that the denatured protein molecule follows either by a sudden decrease in pH or through a gradual decrease in pH. These results are interpreted while keeping in view the oligomeric and globular structure of carmin at neutral pH. The results would help in understanding of structure-function relationship of the protein and its role in hydrogen ion binding in vivo.

Association-dissociation and denaturation-renaturation of high-molecular-weight protein: carmin from safflower seed (Carthamus tinctorius L.) in alkaline solution

The effect of alkaline pH on the association, dissociation, and denaturation of carmin, the high-molecular-weight protein from safflower seed was investigated in the pH range 7-12, using various biophysical techniques. The results indicate that the multimeric protein carmin dissociates at pH 8.0 where denaturation has not set in. The association-dissociation of the protein can be represented schematically as 11S in equilibrium 7S in equilibrium 4S----2S. Above pH 10, the protein undergoes simultaneous dissociation and denaturation. The denaturation process appears to be complete at approximately pH 12.5. The protein undergoes conformational change and covalent modifications and cleavage during the denaturation process. A reversibility study shows that the process of dissociation is reversible to a large extent, whereas denaturation appears to be irreversible. These results are discussed in terms of association-dissociation, denaturation and alkaline-catalyzed covalent modifications and cleavage of seed proteins.

Physicochemical properties of oilseed proteins

This review attempts to consolidate the information on the physicochemical properties of proteins from groundnut, soybean, sesame seed, mustard seed, rapeseed, sunflower seed, cottonseed, and other oilseeds. It deals with the extraction and characterization of the oilseed proteins and describes the methods for isolation of the various protein fractions and determination of their physicochemical characteristics. Also discussed are the subunit composition of the oligomeric proteins, their hydrodynamic properties, and the effect of denaturants on these proteins. The similarity in the properties of the proteins from various oilseed materials is discussed. The review article aims to indicate the gaps in our knowledge about the physicochemical properties of oilseed proteins and suggests areas for future investigation.