MIT 7 61 - Chemistry of ion coordination and hydration (6 pages)

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Chemistry of ion coordination and hydration



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Chemistry of ion coordination and hydration

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Pages:
6
School:
Massachusetts Institute of Technology
Course:
7 61 - Eukaryotic Cell Biology: Principles and Practice
Eukaryotic Cell Biology: Principles and Practice Documents

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articles Chemistry of ion coordination and hydration revealed by a K channel Fab complex at 2 0 A resolution Yufeng Zhou Joa o H Morais Cabral Amelia Kaufman Roderick MacKinnon Howard Hughes Medical Institute Laboratory of Molecular Neurobiology and Biophysics Rockefeller University 1230 York Avenue New York New York 10021 USA Ion transport proteins must remove an ion s hydration shell to coordinate the ion selectively on the basis of its size and charge To discover how the K channel solves this fundamental aspect of ion conduction we solved the structure of the KcsA K channel in complex with a monoclonal Fab antibody fragment at 2 0 A resolution Here we show how the K channel displaces water molecules around an ion at its extracellular entryway and how it holds a K ion in a square antiprism of water molecules in a cavity near its intracellular entryway Carbonyl oxygen atoms within the selectivity lter form a very similar square antiprism around each K binding site as if to mimic the waters of hydration The selectivity lter changes its ion coordination structure in low K solutions This structural change is crucial to the operation of the selectivity lter in the cellular context where the K ion concentration near the selectivity lter varies in response to channel gating Potassium channels control the electric potential across cell membranes by catalysing the rapid selective diffusion of K ions down their electrochemical gradient1 The structure of the K channel has provided a rm basis for understanding the mechanisms of rapid K ion transport underlying electrical signalling in cells2 Through the interactions of dehydrated K ions within the channel s selectivity lter high conduction rates are achieved in the setting of exquisite ion selectivity3 Two fundamental questions surrounding this process are addressed in the present study The rst is how the K channel mediates the transfer of a K ion from its hydrated state in solution to its dehydrated state in the selectivity



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