Advanced spectroscopy reveals mechanism of vectorial action in a membrane pump

Judith Herzfeld research imageSome proteins in cell membranes are responsible for actively pumping desired molecules in or unwanted molecules out. Since their discovery, it has been expected that their vectorial action involves the existence of two protein conformations, one in which the active site has a low affinity for substrate and is open to the discharge side of the membrane and the other in which the active site has a high affinity for substrate and is open to the uptake side of the membrane. The driver of the pump is a source of energy that converts the pump from the lower energy state to the higher energy state, from which it can relax back and begin the cycle anew.

However, this model has never fit the longest-studied pump, the light-driven ion pump bacteriorhodopsin. At rest, the active site has a high proton affinity but is open to the discharge side of the membrane. Disruption of the active site by light reduces the proton affinity, but it has been a decades-long mystery how this occurs while maintaining access to the discharge side of the membrane. This mystery has now been solved through advanced spectroscopic studies of photocycle intermediates trapped at low temperatures. Obtained collaboratively by Judith Herzfeld’s group at Brandeis and Robert Griffin’s group at MIT, the spectra trace the establishment of an essential U-shaped pathway to the discharge side of the membrane. The results also explain how this pathway is broken as soon as the proton is released, thereby preventing back flow and enforcing the vectorial action of the pump.

“Primary transfer step in the light-driven ion pump bacteriorhodopsin: an irreversible U-turn revealed by DNP-enhanced MAS NMR.” Qing Zhe Ni, Thach Van Can, Eugenio Daviso, Marina Belenky, Robert G. Griffin, and Judith Herzfeld. J. Am. Chem. Soc., DOI: 10.1021/jacs.8b00022. Publication Date (Web): February 28, 2018

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