Equilibrium and kinetic studies on folding of canine milk lysozyme

Determination of the structural ensemble of the molten globule state of a protein by computer simulations

We have used computer simulations to investigate the structural nature of the molten globule (MG) state of canine milk lysozyme. To sample the conformational space efficiently, we performed replica-exchange umbrella sampling simulations with the radius of gyration as a reaction coordinate. We applied the Weighted Histogram Analysis Method to the trajectory of the simulations to obtain the potential of mean force, from which we identified representative structures corresponding to local minima in the free energy surface. The representative structures obtained in this way are in accord with the characteristics of the MG state reported previously by experimental studies. We conjecture that the MG state comprises a series of partially structured states undergoing relatively fast conformational interchange.

Monitoring thermal and chemical unfolding of Brugia malayi calreticulin using fluorescence and Circular Dichroism spectroscopy

Calreticulin of Brugia malayi (BmCRT) play very important role in host-parasite interaction. In previous study it was found that BmCRT is responsible for prevention of host classical complement pathway activation via its interaction with first component C1q of the human host. Therefore, BmCRT is an essential protein for parasite survival and an important drug target to fend filariasis. In the present study, we have carried out a systamatic biophysical characterization of BmCRT protein. Unfolding of BmCRT was found to be non-cooperative two-state process in the presence of both denaturant GdmCl and urea. The results also illustrated that protein lost its 50% activity at 1.5M GdmCl and 3M Urea. Partially unfolded and molten-globule like intermediate state was observed at 0.8 to 1.2M GdmCl while Urea unfolding showed intermediate state at 1.2 to 1.6M. Unfolding pathway monitored with the help of apolar quencher, favor above observations. All of these findings support the presence of detectable intermediate state during unfolding pathway of BmCRT. Furthermore, this study indicates that BmCRT is more stable toward temperature (Tm=65°C), pH and trypsin digestion. These differences in properties as compared to host can be fruitfully utilized for synthesis of compounds effective against the parasite.

Compact, cost-efficient microfluidics-based stopped-flow device

Stopped-flow technology is frequently used to monitor rapid (bio)chemical reactions with high temporal resolution, e.g., in dynamic investigations of enzyme reactions, protein interactions, or molecular transport mechanisms. However, conventional stopped-flow devices are often overly complex, voluminous, or costly. Moreover, excessive amounts of sample are often wasted owing to inefficient designs. To address these shortcomings, we propose a stopped-flow system based on microfluidic design principles. Our simple and cost-efficient approach offers distinct advantages over existing technology. In particular, the use of injection-molded disposable microfluidic chips minimizes required sample volumes and associated costs, simplifies handling, and prevents adverse cross-contamination effects. The cost of the system developed is reduced by an order of magnitude compared with the cost of commercial systems. The system contains a high-precision valve system for fluid control and features automated data acquisition capability with high temporal resolution. Analyses with two well-established reaction kinetics yielded a dead time of approximately 8-9 ms.

Methanol-Induced Tertiary and Secondary Structure Changes of Granulocyte-Colony Stimulating Factor

We have studied methanol-induced conformational changes in rmethuG-CSF at pH 2.5 by means of circular dichroism (CD), fluorescence and infrared (IR) spectroscopy, and 8-anilino-1-naphthalene sulfonic acid (ANS) binding. Methanol has little effect on the secondary and tertiary structures of rmethuG-CSF when its concentration is in the range of 0 to 20% (v/v). At 30% (v/v) methanol, rmethuG-CSF has ANS binding ability. In the methanol concentration range of 30 to 70% (v/v) the amount of alpha-helix decreases a little, and the tertiary structure decreases significantly. At methanol concentrations above 70% (v/v), a transition to a more helical state occurs, while there is little change in the tertiary structure, and no ANS binding ability. Thermal denaturation studies involving CD have demonstrated that as the methanol concentration increases the melting temperature and the cooperativity of transition decrease, and the transition covers a much wider range of temperature. It seems that the decreased cooperativity means an increase in the concentration of partially folded intermediate states during the unfolding of rmethuG-CSF.

Kinetics of folding and unfolding of goat alpha-lactalbumin

The equilibrium unfolding and the kinetic folding and unfolding of goat alpha-lactalbumin (GLA) were studied by near- and far-ultraviolet circular dichroism (CD) and by stopped-flow fluorescence spectroscopy. Specifically, the influence of environmental conditions such as pH and Ca2+ binding was examined. Compared to the apo-form, the Ca2+-bound form was found to be strongly stabilized in equilibrium conditions at pH 7.5 and 25 degrees C. The kinetics of the refolding of apo-GLA show a major change of fluorescence intensity during the experimental dead-time, but this unresolved effect is strongly diminished in holo-GLA. In both cases, however, the chevron plots can adequately be fitted to a three-state model. Moreover, double-mix stopped-flow experiments showed that the native state (N) is reached through one major pathway without the occurrence of alternative tracks. In contrast to the homologous bovine alpha-lactalbumin (BLA), the compactness of GLA is strongly influenced by the presence of Ca2+ ions. Unlike the two-state transition observed in guanidine hydrochloride (GdnHCl)-induced equilibrium denaturation experiments at higher pH, an equilibrium intermediate state (I) is involved in denaturation at pH 4.5. In the latter case, analysis of the kinetic data makes clear that the intermediate and the unfolded states (U) show practically no Gibbs free energy difference and that they are in rapid equilibrium with each other. A possible explanation for these variations in stability and in folding characteristics with pH could be the degree of protonation of His107 that directly influences non-native interactions. Variation of environmental conditions and even small differences in sequence, therefore, can result in important effects on thermodynamic and folding parameters.