High Pressure NMR Reveals Active-Site Hinge Motion of Folate-Bound Escherichia coli Dihydrofolate Reductase

Abstract

A high-pressure ¹⁵N/¹H two-dimensional NMR study has been carried out on folate-bound dihydrofolate reductase (DHFR) from Escherichia coli in the pressure range between 30 and 2000 bar. Several cross-peaks in the ¹⁵N/¹H HSQC spectrum are split into two with increasing pressure, showing the presence of a second conformer in equilibrium with the first. Thermodynamic analysis of the pressure and temperature dependencies indicates that the second conformer is characterized by a smaller partial molar volume (ΔV = −25 mL/mol at 15 °C) and smaller enthalpy and entropy values, suggesting that the second conformer is more open and hydrated than the first. The splittings of the cross-peaks (by ∼1 ppm on ¹⁵N axis at 2000 bar) arise from the hinges of the M20 loop, the C-helix, and the F-helix, all of which constitute the major binding site for the cofactor NADPH, suggesting that major differences in conformation occur in the orientations of the NADPH binding units. The Gibbs free energy of the second, open conformer is 5.2 kJ/mol above that of the first at 1 bar, giving an equilibrium population of about 10%. The second, open conformer is considered to be crucial for NADPH binding, and the NMR line width indicates that the upper limit for the rate of opening is 20 s^-1 at 2000 bar. These experiments show that high pressure NMR is a generally useful tool for detecting and analyzing “open” structures of a protein that may be directly involved in function.