Molecular and macromolecular crowding-induced stabilization of proteins: Effect of dextran and its building block alone and their mixtures on stability and structure of lysozyme.

Dextran 70 and its building block (glucose) has been used as macromolecular crowder and osmolyte, respectively. The difference in size and structure of both made us inquisitive to measure stability of lysozyme in the presence of their mixture. The effects of mixture of a fixed dextran 70 concentration (300 mg/ml) containing different concentrations of glucose and vice versa, were studied. It was observed that Tm (the midpoint of denaturation curve) and △GDo (standard Gibbs free energy change at 25 °C) of lysozyme increase with the increasing concentration of dextran 70 and glucose alone and their mixture. We asked a question whether the effect of synthetic crowding agent on the stability of protein is due to the property of its monomer or due to the crowder. In this study, we observed that both dextran and its monomer stabilize the protein by same mechanism which is volume exclusion. The stabilization arises due to change in in the entropy of unfolding, while there is insignificant change in enthalpy of the protein with increasing concentration of the co-solute. Furthermore, the efficacy of stabilization by glucose increases in the crowded environment, when the concentration of dextran 70 is more than 200 mg/ml in the mixture.

Carbohydrate-Based Macromolecular Crowding-Induced Stabilization of Proteins: Towards Understanding the Significance of the Size of the Crowder.

There are a large number of biomolecules that are accountable for the extremely crowded intracellular environment, which is totally different from the dilute solutions, i.e., the idealized conditions. Such crowded environment due to the presence of macromolecules of different sizes, shapes, and composition governs the level of crowding inside a cell. Thus, we investigated the effect of different sizes and shapes of crowders (ficoll 70, dextran 70, and dextran 40), which are polysaccharide in nature, on the thermodynamic stability, structure, and functional activity of two model proteins using UV-Vis spectroscopy and circular dichroism techniques. We observed that (a) the extent of stabilization of α-lactalbumin and lysozyme increases with the increasing concentration of the crowding agents due to the excluded volume effect and the small-sized and rod-shaped crowder, i.e., dextran 40 resulted in greater stabilization of both proteins than dextran 70 and ficoll 70; (b) structure of both the proteins remains unperturbed; and (c) enzymatic activity of lysozyme decreases with the increasing concentration of the crowder.

Mixture of Macromolecular Crowding Agents Has a Non-additive Effect on the Stability of Proteins.

The folding and unfolding of proteins inside a cell take place in the presence of macromolecules of various shapes and sizes. Such crowded conditions can significantly affect folding, stability, and biophysical properties of proteins. Thus, to logically mimic the intracellular environment, the thermodynamic stability of two different proteins (lysozyme and α-lactalbumin) was investigated in the presence of mixtures of three crowding agents (ficoll 70, dextran 70, and dextran 40) at different pH values. These crowders possess different shapes and sizes. It was observed that the stabilizing effect of mixtures of crowders is more than the sum effects of the individual crowder, i.e., the stabilizing effect is non-additive in nature. Moreover, dextran 40 (in the mixture) has been found to exhibit the greatest stabilization when compared with other crowders in the mixture. In other words, the small size of the crowder has been observed to be a dominant factor in stabilization of the proteins. Graphical Abstract.

Characterization of intermediate state of myoglobin in the presence of PEG 10 under physiological conditions.

The macromolecular crowding progressively has been gaining prominence in recent years as it acts as a sword with double-edge on protein stability and folding, i.e., showing assorted results of having both stabilizing and destabilizing effects. We studied the effects of different concentrations of polyethylene glycol (PEG-10) on structure and stability of myoglobin. The tertiary structure of myoglobin was found to be perturbed in the presence of polyethylene glycol, however there was insignificant change in the secondary structure. It was observed that polyethylene glycol induces molten globule state in myoglobin, where the intermediate state holds hydrophobic patches and larger hydrodynamic volume as compared to the native protein. In addition, isothermal titration calorimetry (ITC) showed strong binding between myoglobin and polyethylene glycol, at the physiological pH. We hypothesize that polyethylene glycol induces molten globule conformation in myoglobin by interacting with heme group of myoglobin. We caution that the binding of protein with crowder and other soft interactions need to be gravely well thought-out when studying macromolecular crowding. In our case, destabilizing protein-crowder interactions could compete and overcome the stabilizing exclusion volume effect.

Size-dependent studies of macromolecular crowding on the thermodynamic stability, structure and functional activity of proteins: in vitro and in silico approaches.

BACKGROUND: The environment inside cells in which proteins fold and function are quite different from that of the dilute buffer solutions often used during in vitro experiments. The presence of large amounts of macromolecules of varying shapes, sizes and compositions makes the intracellular milieu extremely crowded. SCOPE OF REVIEW: The overall concentration of macromolecules ranges from 50 to 400gl-1, and they occupy 10-40% of the total cellular volume. These differences in solvent conditions and the level of crowdedness resulting in excluded volume effects can have significant consequences on proteins' biophysical properties. A question that arises is: how important is it to examine the roles of shape, size and composition of macromolecular crowders in altering the biological properties of proteins? This review article aims at focusing, gathering and summarizing all of the research investigations done by means of in vitro and in silico approaches taking into account the size-dependent influence of the crowders on proteins' properties. MAJOR CONCLUSIONS: Altogether, the internal architecture of macromolecular crowding environment including size, shape and concentration of crowders, appears to be playing an extremely important role in causing changes in the biological processes. Most often the small sized crowders have been found more effective crowding agents. However, thermodynamic stability, structure and functional activity of proteins have been governed by volume exclusion as well as soft (chemical) interactions. GENERAL SIGNIFICANCE: The article provides an understanding of importance of internal architecture of the cellular environment in altering the biophysical properties of proteins.

Relationship between protein stability and functional activity in the presence of macromolecular crowding agents alone and in mixture: An insight into stability-activity trade-off.

The cellular environment is crowded with different kinds of molecules with varying sizes, shapes and compositions. Most of the experiments studying the nature and behaviour of a protein have been done on the isolated protein in dilute buffer solutions which actually do not imitate the in vivo situation. To understand the consequences of such crowded environment, we investigated the effect of macromolecular crowding on the stability and activity of hen egg white lysozyme. Two crowding agents, dextran 70 and ficoll 70 which have different shapes and composition, have been employed in this study. To mimic the cellular condition from physiological point of view, the effect of mixtures of both the crowding agents has been also studied. The results indicate that owing to volume exclusion, lysozyme is stabilized while its activity decays with the increasing concentration of both the crowders elucidating the hypothesis of stability-activity trade-off. Mixed macromolecular crowding exerts greater effect than the sum of constituent crowding agents (dextran 70 and ficoll 70).