An Outer Membrane Protein Undergoes Enthalpy- and Entropy-Driven Transitions

摘要: β-Barrel membrane proteins often fluctuate among various open substates, yet the nature of these transitions is not fully understood. Using temperature-dependent, single-molecule electrophysiology analysis, along with rational protein design, we show that OccK1, a member outer carboxylate channel from Pseudomonas aeruginosa, features discrete gating dynamics comprising both enthalpy-driven and entropy-driven current transitions. OccK1 was chosen for analysis transitions, because it monomeric transmembrane β-barrel known high-resolution crystal structure displays three distinguishable, time-resolvable substates. Native loop-deletion showed substantial changes in activation enthalpies entropies but modest alterations equilibrium free energies, confirming system never departs equilibrium. Moreover, some fluctuations indicated counterintuitive, negative enthalpy, which compensated by significant decrease entropy. Temperature scanning single-channel properties exhibited thermally induced switch energetically most favorable substate at lowest examined temperature 4 °C. Therefore, such semiquantitative assessment fluctuation only demonstrates complexity also reveals distinct functional traits under different circumstances.