Thermodynamic Basis for Conformational Coupling in an ATP-Binding Cassette Exporter.

ATP-binding cassette (ABC) transporters constitute one of the largest protein superfamilies, and they mediate the transport of diverse substrates across the membrane. The molecular mechanism for transducing the energy from ATP binding and hydrolysis into the conformational changes remains elusive. Here, we determined the thermodynamics underlying the ATP-induced global conformational switching for the ABC exporter TmrAB using temperature-resolved pulsed electron-electron double resonance (PELDOR or DEER) spectroscopy. We show that a strong entropy-enthalpy compensation mechanism enables the closure of the nucleotide-binding domains (NBDs) over a wide temperature range. This is mechanically coupled with an outward opening of the transmembrane domains (TMDs) accompanied by an entropy gain. The conserved catalytic glutamate plays a key role in the overall energetics. Our results reveal the thermodynamic basis for the chemomechanical energy coupling in an ABC exporter and present a new strategy to explore the energetics of similar membrane protein complexes.