L3 loop-mediated mechanisms of pore closing in porin: a molecular dynamics perturbation approach.

L3 loop-mediated mechanisms for pore closing in porin are investigated with molecular dynamics simulation, using an approach that can be related to the phenomenon of voltage gating. Voltage gating is seen as a perturbation of the electrostatic screening inside the porin pore where, by the influence of the potential gradient, water and counter-ion distribution can be slightly displaced from their equilibrium distribution. This is simulated by perturbing the screening electrostatics of ionizable groups inside the pore. Under these conditions, a localized conformational change takes place, involving 12 (Ile102-Ala113) out of the 44 residues of the loop. The pore is reduced to a sixth of its open state size. The conformational change can be achieved with a small perturbation and it is reversible once the perturbation is switched off (relaxation process). Other types of behaviour predominating at higher simulation temperatures are found for the loop, involving an extra conformational change in the Thr92-Asp96 loop segment. This conformational change completely closes the pore, but is not reversible under the simulation conditions. Both zones involved in the conformational changes contain or overlap the zones which were described previously, using other techniques, to be the most flexible zones of the loop.