View source for Light-harvesting complex

{{Short description|Protein-pigment complex}}
In [[biology]], a '''light-harvesting complex''' or LHC is an aggregate consisting of [[protein]]s bound with [[chromophores]] ([[chlorophyll]]s and [[carotenoid]]s) that play a key role in [[photosynthesis]]. LHCs are arrayed around [[photosynthetic reaction center]]s in both [[plant]]s and [[anoxygenic photosynthesis|photosynthetic bacteria]] and collect more of the incoming light than would be captured by the reaction centers alone. The light captured by the chromophores excites molecules from their ground states to (short-lived) higher-energy states, known as the excited states.<ref name=Fassioli2014 /> This energy is then focused toward the reaction centers by [[Förster resonance energy transfer]].

Light-harvesting complexes are found in a wide variety among the different photosynthetic species, with no homology among the major groups.<ref>{{cite journal |last1=Kühlbrandt |first1=Werner |title=Structure and function of bacterial light-harvesting complexes |journal=Structure |date=June 1995 |volume=3 |issue=6 |pages=521–525 |doi=10.1016/S0969-2126(01)00184-8 |doi-access=free|pmid=8590011 }}</ref> 

==Function==
<!-- Image with unknown copyright status removed: [[Image:LHC in plants.jpg|thumb|300px| The arrangement of light-harvesting complexes around a photosynthetic reaction center.]] -->
Photosynthesis is a process where light is absorbed or harvested by pigment protein complexes which are able to turn sunlight into chemical energy.<ref name=Fassioli2014>{{Cite journal|last1=Fassioli|first1=Francesca|last2=Dinshaw|first2=Rayomond|last3=Arpin|first3=Paul C.|last4=Scholes|first4=Gregory D.|date=2014-03-06|title=Photosynthetic light harvesting: excitons and coherence|url= |journal=Journal of the Royal Society Interface|volume=11|issue=92|pages=20130901|doi=10.1098/rsif.2013.0901|pmc=3899860|pmid=24352671}}</ref> In this process, a molecule of the pigment protein absorbs a [[photon]] of sunlight, leading to electronic excitation delivered to the reaction centre where the process of charge separation can take place<ref name=Fassioli2014 /> if the energy of the absorbed photon matches that of an electronic transition. The result of such excitation can be a return to the ground state or to another electronic state of the same molecule. When the excited molecule has a nearby neighbour molecule, the excitation energy may also be transferred, through electromagnetic interactions, from one molecule to another. This process is called [[resonance energy transfer]], and the rate depends strongly on the distance between the energy donor and energy acceptor molecules. Before an excited molecule can transition back to its ground state, energy needs to be harvested.{{clarify|date=May 2025}}{{dubious|date=May 2025}} This excitation is transferred among chromophores where it is delivered to the reaction centre.<ref name=Fassioli2014 /> Light-harvesting complexes have their pigments specifically positioned to optimize these rates.

==In purple bacteria==
{{Main|Bacterial antenna complex}}
[[Purple bacteria]] are a type of photosynthetic organism with a light harvesting complex consisting of two pigment protein complexes, referred to as LH1 and LH2.<ref name=Ramos2019>{{Cite journal|last1=Ramos|first1=Felipe Cardoso|last2=Nottoli|first2=Michele|last3=Cupellini|first3=Lorenzo|last4=Mennucci|first4=Benedetta|date=2019-10-30|title=The molecular mechanisms of light adaption in light-harvesting complexes of purple bacteria revealed by a multiscale modeling|journal=Chemical Science|language=en|volume=10|issue=42|pages=9650–9662|doi=10.1039/C9SC02886B|pmid=32055335|pmc=6988754|issn=2041-6539|doi-access=free}}</ref> Within the photosynthetic membrane, these two complexes differ in their arrangement.<ref name=Ramos2019/> The LH1 complexes surround the reaction centre, while the LH2 complexes are arranged peripherally around the LH1 complexes and the reaction centre.<ref name=Ramos2019 /> Purple bacteria use [[bacteriochlorophyll]] and carotenoids to gather light energy. These proteins are arranged in a ring-like fashion, creating a cylinder that spans the membrane.<ref name="PUB00001417">{{cite journal |vauthors=Wagner-Huber R, Brunisholz RA, Bissig I, Frank G, Suter F, Zuber H |title=The primary structure of the antenna polypeptides of Ectothiorhodospira halochloris and Ectothiorhodospira halophila. Four core-type antenna polypeptides in E. halochloris and E. halophila |journal=Eur. J. Biochem. |volume=205 |issue=3 |pages=917–925 |year=1992 |pmid=1577009 |doi=10.1111/j.1432-1033.1992.tb16858.x|doi-access=free }}</ref><ref name="PUB00003468">{{cite journal |vauthors=Brunisholz RA, Zuber H |title=Structure, function and organization of antenna polypeptides and antenna complexes from the three families of Rhodospirillaneae |journal=J. Photochem. Photobiol. B |volume=15 |issue=1 |pages=113–140 |year=1992 |pmid=1460542 |doi=10.1016/1011-1344(92)87010-7|bibcode=1992JPPB...15..113B }}</ref>

==In green bacteria==
{{main|Chlorosome}}
The main light harvesting complex in Green bacteria is known as the chlorosome. The chlorosome is equipped with rod-like BChl c aggregates with protein embedded lipids surrounding it. Chlorosomes are found outside of the membrane which covers the reaction centre. [[Green sulphur bacteria]] and some [[Chloroflexia]] use ellipsoidal complexes known as the chlorosome to capture light. Their form of bacteriochlorophyll is green.<ref name=Hu1998>{{Cite journal|last1=Hu|first1=Xiche|last2=Damjanović|first2=Ana|last3=Ritz|first3=Thorsten|last4=Schulten|first4=Klaus|date=1998-05-26|title=Architecture and mechanism of the light-harvesting apparatus of purple bacteria|journal=Proceedings of the National Academy of Sciences|language=en|volume=95|issue=11|pages=5935–5941|doi=10.1073/pnas.95.11.5935|issn=0027-8424|pmid=9600895|pmc=34498|bibcode=1998PNAS...95.5935H|doi-access=free}}</ref>

==In cyanobacteria and plants==
{{Main|Light-harvesting complexes of green plants}}

[[Chlorophyll]]s and [[carotenoids]] are important in light-harvesting complexes present in plants. [[Chlorophyll b]] is almost identical to [[chlorophyll a]], except it has a [[formyl group]] in place of a [[methyl group]]. This small difference makes chlorophyll b absorb [[light]] with [[wavelengths]] between 400 and 500&nbsp;nm more efficiently. Carotenoids are long linear [[organic chemistry|organic molecules]] that have alternating single and double bonds along their length. Such molecules are called [[polyenes]]. Two examples of carotenoids are [[lycopene]] and [[carotene|β-carotene]]. These molecules also absorb light most efficiently in the 400 – 500&nbsp;nm range.
Due to their absorption region, carotenoids appear red and yellow and provide most of the red and yellow colours present in [[fruits]] and [[flowers]].

The carotenoid molecules also serve a safeguarding function. Carotenoid molecules suppress damaging photochemical reactions, in particular those including [[oxygen]], which exposure to sunlight can cause. Plants that lack carotenoid molecules quickly die upon exposure to oxygen and light.

==Phycobilisome==
{{main|Phycobilisome}}
[[Image:Phycobilisome structure.jpg|thumb|350px|Schematic layout of protein subunits in a phycobilisome.]]
The antenna-shaped light harvesting complex of [[cyanobacteria]], [[glaucocystophyta]], and [[red algae]] is known as the phycobilisome; it is composed of linear tetrapyrrole pigments. Pigment-protein complexes, referred to as R-phycoerythrin, are rod-like in shape and make up the rods and core of the phycobilisome.<ref name=Hu1998 /> Little light reaches algae that reside at a depth of one meter or more in seawater, as light is absorbed by seawater. The pigments, such as [[phycocyanobilin]] and [[phycoerythrobilin]], are the chromophores that bind through a covalent [[thioether]] bond to their [[apoprotein]]s{{Fix|text=Needs disambiguation}} at cystein residues. The apoprotein with its chromophore is called phycocyanin, phycoerythrin, and allophycocyanin, respectively.{{clarify|date=May 2025}}{{Fix|text="respectively" means they need distinguishing in some way}} They often occur as [[hexamer]]s of α and β subunits (α<sub>3</sub>β<sub>3</sub>)<sub>2</sub>. They enhance the amount and spectral window of light absorption and fill the "green gap", which occurs in higher plants.<ref>{{cite journal |last1=Singh |first1=NK |last2=Sonani |first2=RR |last3=Rastogi |first3=RP |last4=Madamwar |first4=D |title=The phycobilisomes: an early requisite for efficient photosynthesis in cyanobacteria. |journal=EXCLI Journal |date=2015 |volume=14 |pages=268–89 |doi=10.17179/excli2014-723 |pmid=26417362|pmc=4553884 }}</ref>

The geometrical arrangement of a phycobilisome is very elegant and results in 95% efficiency of energy transfer. There is a central core of [[allophycocyanin]], which sits above a photosynthetic reaction center. There are [[phycocyanin]] and [[phycoerythrin]] subunits that radiate out from this center like thin tubes. This increases the surface area of the absorbing section and helps focus and concentrate light energy down into the reaction center to form chlorophyll. The energy transfer from excited electrons absorbed by pigments in the phycoerythrin subunits at the periphery of these antennas appears at the reaction center in less than 100&nbsp;ps.<ref>Light Harvesting by Phycobilisomes Annual Review of Biophysics and Biophysical Chemistry Vol. 14: 47-77 (Volume publication date June 1985)</ref>

==See also==
* [[Photosynthesis]]
* [[Photosynthetic reaction center]]
* [[Photosystem II light-harvesting protein]]
* [[Light harvesting pigment]]

==References==
{{Reflist}}

==Further reading==
* Caffarri (2009)Functional architecture of higher plantphotosystem II supercomplexes. ''The EMBO Journal'' 28: 3052–3063
* Govindjee & Shevela (2011) Adventures with cyanobacteria: a personal perspective. ''Frontiers in Plant Science''.
* Liu et al. (2004) Crystal structure of spinach major light-harvesting complex at 2.72A° resolution. ''Nature'' 428: 287–292.
* Lokstein (1994)The role of light-harvesting complex II energy dissipation: an in-vivo fluorescence in excess excitation study on the origin of high-energy quenching. Journal of Photochemistry and Photobiology 26: 175-184
* MacColl (1998) Cyanobacterial Phycobilisomes. JOURNAL OF STRUCTURAL BIOLOGY 124(2-3): 311-34.

==External links==
* {{MeshName|Light-harvesting+protein+complexes}}
* [http://www.life.illinois.edu/govindjee/photoweb Photosynthesis and all sub categories]

{{Carrier proteins}}

[[Category:Photosynthesis]]
[[Category:Transmembrane proteins]]