CHAPTER 5: Chemical Interactions of Betaine with Folate and Other Biomolecules

Glycine betaine is an osmoprotectant, or small molecule produced by the cell to respond to osmotic stress. The ability of betaine to help proteins fold arises from its inability to act as a hydrogen-bond donor, thus it is highly excluded from the surface of proteins. Small molecule vapor pressure osmometry studies find betaine is highly excluded from phosphates and carboxylate oxygens. In contrast, betaine interacts with aromatic carbons and cationic and amide nitrogens. As a solute for the latter atoms, betaine is preferred over water by approximately 1.3– to 1.7–fold. These preferential interactions allow binding of betaine to synthesis and transport proteins. In particular, “aromatic boxes” use several cation–π interactions to bind betaine. This type of interaction also appears to play a role in solvating folate as folate possesses aromatic p-aminobenzoate and pterin rings. While these interactions are weak, they are harder to break than the folate–water pairs, thus high concentrations of betaine impede binding of folate to the chromosomal dihydrofolate reductase from E. coli by a factor of 3.6. In vivo osmotic stress effects are additionally observed on dihydrofolate reductase function, suggesting osmolarity is a key environmental factor that affects enzyme fitness.