Solution Ionic Strength Can Modulate Functional Loop Conformations in E. coli Dihydrofolate Reductase.

Area of Science:

• Biochemistry
• Structural Biology
• Enzyme Kinetics

Background:

• The M20 loop in E. coli dihydrofolate reductase (ecDHFR) exists in multiple conformations, influencing enzyme catalysis.
• The transition between closed and occluded M20 loop states was hypothesized to facilitate product release.

Purpose of the Study:

• To investigate the effect of solution ionic strength on ecDHFR M20 loop conformations, independent of ligand binding.
• To understand how environmental factors influence enzyme structure relevant to catalysis and drug design.

Main Methods:

• Molecular dynamics simulations of ecDHFR in model CaCl2 solutions.
• Analysis of free energy barriers between M20 loop conformations at varying ionic strengths (IM).

Main Results:

• A significant free energy barrier forms between occluded and closed M20 loop states at ionic strengths above the E. coli physiological threshold (~0.24 M).
• At high ionic strengths (>0.3 M), the occluded conformation is stabilized, consistent with crystal structures.
• At lower ionic strengths (≤0.15 M), the M20 loop favors open/partially closed conformations, also aligning with experimental structures.

Conclusions:

• Solution ionic strength is a critical factor influencing ecDHFR M20 loop conformations.
• Enzyme structures obtained at non-physiological ionic strengths may not accurately represent catalytic mechanisms or be suitable for structure-based drug design.