Mechanisms of photoprotection and nonphotochemical quenching in pea light-harvesting complex at 2.5 Å resolution

Abstract

The plant light‐harvesting complex of photosystem II (LHC‐II) collects and transmits solar energy for photosynthesis in chloroplast membranes and has essential roles in regulation of photosynthesis and in photoprotection. The 2.5 Å structure of pea LHC‐II determined by X‐ray crystallography of stacked two‐dimensional crystals shows how membranes interact to form chloroplast grana, and reveals the mutual arrangement of 42 chlorophylls a and b , 12 carotenoids and six lipids in the LHC‐II trimer. Spectral assignment of individual chlorophylls indicates the flow of energy in the complex and the mechanism of photoprotection in two close chlorophyll a –lutein pairs. We propose a simple mechanism for the xanthophyll‐related, slow component of nonphotochemical quenching in LHC‐II, by which excess energy is transferred to a zeaxanthin replacing violaxanthin in its binding site, and dissipated as heat. Our structure shows the complex in a quenched state, which may be relevant for the rapid, pH‐induced component of nonphotochemical quenching.