Fig. 4
From: Cryo-EM structure of the RC-LH core complex from an early branching photosynthetic prokaryote
Novel architecture of the RC–LH complex of R. castenholzii and its quinone shuttling channel. a Ribbon representation and comparison of the transmembrane architecture of the core complex from R. castenholzii (purple) with that of T. tepidum (blue, accession code 3WMM) and Rhodops. palustris (cyan, accession code 1PYH). The BChl pigments in LH are shown in sticks. The transmembrane helices of the Cyt c subunit, H subunit, protein W, and subunit X are labeled as C-TM, H, W, and X, respectively. b The side and bottom-up view of the proposed quinone channel of rcRC–LH complex. The BChls and keto-γ-carotene are shown as spheres. The gap between the C-TM and the 15th LHαβ is proposed to be the quinone escape channel. The quinone-binding sites are highlighted by red and orange circles, and the possible quinone shuttling path is shown as red arrows. c Schematic model of the energy and electron transfer in rcRC–LH complex. The model shows one cross-section that is perpendicular to the membrane. The B800, keto-γ-carotene, and B880 are highly conjugated and the energy from sunlight can be harvested and transferred efficiently among them (red arrows). The energy from the excited B880s also can transfer to the special-pair BChls (P), and facilitate the charge separation. The electron can then transfer to QB through BChl, BPheo, QA, and iron atom sequentially (blue arrows). The P+ receives one electron from heme of RC-attached tetra-heme Cyt c and the electron donor of heme is the blue copper protein auracyanin (Au), which is reduced by alternative complex III (ACIII). This diagram was created by Abode Illustrator. d The cross-section parallel to the membrane is shown as a schematic model for the quinone transfer. The LH ring barrier possesses one gate between C-TM and the 15th LHαβ for quinone shuttling, which is flanked by subunit X. Fully reduced quinone (hydroquinone) diffuses out of the RC and is replaced by a new quinone. The hydroquinone can transfer electrons to ACIII and then reduce the Au. The color code of all panels is same as Fig. 1
