at ionic strengths below the intracellular ion concentration-derived ionic strength in Escherichia coli ( at or below 0.237 M), the DHFR M20 loop tends to adopt open/closed conformations, and rarely an occluded loop state. As the ionic strength exceeds the physiological ionic strength of 0.237 M, the loop tends to adopt a closed/occluded conformation. the solution ionic strength affects the M20 loop stability differently depending on its conformation: High ionic strengths stabilize the occluded conformation more than low ionic strengths, as Ca2+ ions can approach E17 of the M20 loop and stabilize its orientation, whereas they are distal from the E17 at low ionic strengths. Both low and high ionic strengths can stabilize the closed conformation, |
Escherichia coli |