A comparative look at structural variation among RC–LH1 ‘Core’ complexes present in anoxygenic phototrophic bacteria

Abstract

All purple photosynthetic bacteria contain RC-LH1 'Core' complexes. The structure of this complex from Rhodobacter sphaeroides, Rhodopseudomonas palustris and Thermochromatium tepidum has been solved using X-ray crystallography. Recently, the application of single particle cryo-EM has revolutionised structural biology and the structure of the RC-LH1 'Core' complex from Blastochloris viridis has been solved using this technique, as well as the complex from the non-purple Chloroflexi species, Roseiflexus castenholzii. It is apparent that these structures are variations on a theme, although with a greater degree of structural diversity within them than previously thought. Furthermore, it has recently been discovered that the only phototrophic representative from the phylum Gemmatimonadetes, Gemmatimonas phototrophica, also contains a RC-LH1 'Core' complex. At present only a low-resolution EM-projection map exists but this shows that the Gemmatimonas phototrophica complex contains a double LH1 ring. This short review compares these different structures and looks at the functional significance of these variations from two main standpoints: energy transfer and quinone exchange.

Structures of Rhodopseudomonas palustris RC-LH1 complexes with open or closed quinone channels

“…2B ) suggesting that, despite apparent differences between open and closed rings, the local environments of the BChls are very similar. The absorption redshift may be a result of reduced thermal motion and increased stability upon ring closure ( 18 , 19 ), alterations to pigment coupling caused by ring closure ( 20 , 21 ), or a combination of these two effects ( 11 ).…”

Cryo-EM structure of the Rhodospirillum rubrum RC–LH1 complex at 2.5 Å

“…Only a few degrees of conical angle are required to account for this level of curvature, and we conclude that in the native membrane a ring of lipid molecules binds to the outer face of the LH1-β polypeptide on the cytoplasmic side of the complex. Inspection of the density map shows disordered detergent molecules in the position ; we suggest that this point of contact is a potential site for initiating the assembly of a curved array of LH1 αβ subunits that culminates in a fully encircled RC, so it is assigned as αβ (1). Each of the 16 pairs of transmembrane α and β polypeptides, numbered in Figure 1D, binds two BChl a GG molecules creating a ring of 32 closely spaced and paired BChl a pigments.…”

“…Reaction centre-light harvesting complex 1 (RC-LH1) complexes are the central functional units of photosynthesis in purple phototrophic bacteria. Solar energy absorbed by a circular LH1 assembly migrates to an enclosed membrane-bound reaction centre (RC) [1], where a succession of charge separation and protonation events produces a quinol that leaves the RC-LH1 complex, carrying protons and electrons to a cytochrome (cyt) bc complex [2,3]. The LH1 complex is formed from an oligomeric assembly of transmembrane αβ heterodimers, which bind bacteriochlorophyll a (BChl a) and carotenoid pigments and curve round the central RC complex.…”

Redesigning the photosynthetic light reactions to enhance photosynthesis – the PhotoRedesign consortium

“…Green algae and plants such as Arabidopsis contain membrane‐embedded antennas, the light‐harvesting complexes I and II (LHCI/II; for review, see Cao et al., 2018; Caspy and Nelson, 2018) that absorb light mostly in the 400–510‐nm and 620–700‐nm regions (Figure 1b). The light‐harvesting systems of Rhodobacter also consist of intrinsic membrane complexes, the light‐harvesting complex 1 (LH1) and 2 (LH2) antenna, which harvest light in the 400–550‐nm and 800–900‐nm regions of the solar spectrum (for review, see Gardiner et al., 2020; Niederman, 2016; Saer and Blankenship, 2017; Figure 1b).…”

“…The PSI and PSII cores in Synechocystis and Arabidopsis bind chlorophyll (Chl) a and β‐carotene, whereas the Rhodobacter RC binds bacteriochlorophyll (BChl) a and spheroidene/spheroidenone. Phycobilisomes contain different types of linear tetrapyrroles called bilins, whereas purple bacterial LH1 and LH2 antennas contain near‐infrared (NIR)‐absorbing BChls and also bind a variety of carotenoids that provide the main pigmentation in the visible region of the spectrum (for review, see Gardiner et al., 2020; Leupold et al., 2000). In plant and algal LHCs, the spectrum of cofactors, and in particular carotenoids, is rather diverse.…”