Probing the photointermediates of light-driven sodium ion pump KR2 by DNP-enhanced solid-state NMR

Abstract

The functional mechanism of the light-driven sodium pump Krokinobacter eikastus rhodopsin 2 (KR2) raises fundamental questions since the transfer of large cations must differ from the better-known principles of rhodopsin-based proton pumps. Addressing these questions must involve a better understanding of the KR2 photointermediates. Here, DNP-enhanced solid-state NMR spectroscopy on cryo-trapped photointermediates shows that the K-state with 13-cis retinal directly interconverts into the subsequent L-state with distinct retinal carbon chemical shift differences and an increased out-of-plane twist around the C14-C15 bond. The retinal converts back into an all-trans conformation in the O-intermediate, which is the key state for sodium transport. However, retinal carbon and Schiff base nitrogen chemical shifts differ from those observed in the KR2 dark state all-trans conformation indicating a perturbation through the nearby bound sodium ion. Our findings are supplemented by optical as well as IR spectroscopy and are discussed in the context of known 3D structures.

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Identifiers

ORCID for Alexander J Leeder: orcid.org/0000-0001-9271-6038

ORCID for Lynda J. Brown: orcid.org/0000-0002-5678-0814

ORCID for Richard C.D. Brown: orcid.org/0000-0003-0156-7087

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