Cryo-EM structure of a photosystem I variant containing an unusual plastoquinone derivative in its electron transfer chain.

Photosystem I (PS I) is a light-driven oxidoreductase responsible for converting photons into chemical bond energy. Its application for renewable energy was revolutionized by the creation of the MenB deletion (Δ) variant in the cyanobacterium sp. PCC 6803, in which phylloquinone is replaced by plastoquinone-9 with a low binding affinity. This permits its exchange with exogenous quinones covalently coupled to dihydrogen catalysts that bind with high affinity, thereby converting PS I into a stable solar fuel catalyst. Here, we reveal the 2.03-Å-resolution cryo-EM structure of a recent MenB variant of PS I. The quinones and their binding environment are analyzed in the context of previous biophysical data, thereby enabling a protocol to solve future PS I hybrids and constructs from this genetically tractable cyanobacterium.