Effect of urea on protein-ligand association

Modulation of protein-saccharide interactions by deep-sea osmolytes under high pressure stress

Deep-sea organisms must cope with high hydrostatic pressures (HHP) up to the kbar regime to control their biomolecular processes. To alleviate the adverse effects of HHP on protein stability most organisms use high amounts of osmolytes. Little is known about the effects of these high concentrations on ligand binding. We studied the effect of the deep-sea osmolytes trimethylamine-N-oxide, glycine, and glycine betaine on the binding between lysozyme and the tri-saccharide NAG3, employing experimental and theoretical tools to reveal the combined effect of osmolytes and HHP on the conformational dynamics, hydration changes, and thermodynamics of the binding process. Due to their different chemical makeup, these cosolutes modulate the protein-sugar interaction in different ways, leading to significant changes in the binding constant and its pressure dependence. These findings suggest that deep-sea organisms may down- and up-regulate reactions in response to HHP stress by altering the concentration and type of the intracellular osmolyte.

Harnessing Pressure Modulation for Exploring Ligand Binding Reactions in Cosolvent Solutions

A comprehensive understanding of ligand-protein interactions requires information about all thermodynamic parameters that describe the complexation reaction, and they should be able to provide the necessary information to understand the molecular forces that drive complex formation. Usually, binding studies are performed at ambient pressure conditions. However, in addition to using temperature variation to reveal enthalpic and entropic contributions to ligand binding, complementary pressure-dependent studies providing volumetric properties of the reaction can be beneficial. Changes in partial molar volume can inform about changes in packing and hydration upon ligand binding. Here, after a general discussion of pressure effects on ligand binding reactions, we present a comprehensive study of the effect of pressure and a widely used organic cosolvent, dimethyl sulfoxide (DMSO), on the binding of a small aromatic ligand, proflavine, to the enzyme α-chymotrypsin. We found that DMSO, which acts as a competitive inhibitor for proflavine, has a strong impact on the interaction process, resulting in a decrease of the binding constant. While the reaction performed in neat buffer is basically pressure insensitive, the partial molar volume of the complex in the presence of DMSO is larger compared with the uncomplexed state, rendering the binding constant markedly smaller upon pressurization. We also show that the magnitude and sign of the binding volume provide valuable information about the interaction mechanism and hydration changes, which is of particular interest when cosolvents are present.

Effects of Osmolytes on Ligand Binding to Dihydropteroate Synthase from Bacillus anthracis

Osmolyte interactions with ligands can affect their affinity for proteins and are dependent upon the cosolute and the functional groups of the ligand. Here, we explored ligand binding to Bacillus anthracis dihydropteroate synthase (BaDHPS) under osmotic stress conditions. Osmolyte effects were specific to the cosolute and ligand, suggesting interaction of the osmolytes with the free ligands in solution. The association rates of pterin pyrophosphate were mostly unaffected by the osmolytes, except for a 2-fold decrease in the presence of 1 M trehalose, while the dissociation rates decreased in most osmolyte solutions. The viscosity and dielectric constant of the solution did not correlate with the effects of the osmolytes. Experimental results were compared with predicted preferential interaction coefficients (Δμ₂₃/RT) between the osmolytes and ligands. The Δμ₂₃/RT were able to predict the experimental data for most of the osmolytes. Trehalose and proline effects did not correlate with the predicted values, indicating that these two osmolytes may affect binding in more complex ways than simple preferential interactions. Additionally, osmolytes weakly interacted with the sulfa drug sulfathiazole, which altered its affinity for BaDHPS, suggesting that these types of weak interactions can also impact drug binding. As osmolytes affect ligands binding to two different folate cycle enzymes (DHFRs and DHPS), we predicted how ligand binding to other folate cycle enzymes will be altered by the presence of osmolytes.