Crowding Milleu stabilizes apo-myoglobin against chemical-induced denaturation: Dominance of hardcore repulsions in the heme devoid protein

Macromolecular crowding can have significant consequences on the structure and dynamics of a protein. The size and shape of a co-solute molecule and the nature of protein contribute significantly in macromolecular crowding, which results in different outcomes in similar conditions. The structure of apo-myoglobin (apo-Mb) both in the absence and presence of denaturants (GdmCl and urea) was investigated in crowded conditions at pH 7.0, with a comparable size of crowders (~70 kDa) but of different shapes (ficoll and dextran) at various concentrations using spectroscopic techniques like absorption and circular dichroism to monitor changes in secondary and tertiary structure, respectively. The crowders in the absence of denaturants showed structural stabilization of the tertiary structure while no significant change in the secondary structure was observed. The effect of crowders on the stability of the protein was also investigated using probes such as Δε₂₉₁ and θ₂₂₂ using chemical denaturants. The analysis of chemical-induced denaturation curves showed that both the crowders stabilize apo-Mb by increasing the values of the midpoint of transition (C_m) and change in free energy in the absence of denaturant (∆G_D°), and it was observed that dextran 70 shows more stabilization than ficoll 70 under similar conditions. In this study apo-Mb showed stabilization under crowded conditions, which is a deviation from earlier work from our group where holo form of the same protein was destabilized. This study emphasizes that volume exclusion is a dominant force in a simple protein while soft interactions may play important role in the proteins that are possessing prosthetic group. Hence, the effect of crowders is protein-dependent, and excluded volume plays a great role in the stabilization of apo-Mb, which does not interact with the crowders.