Carotenoids as accessory and protective pigments in photosynthesis

Fotosinteza je neophodan i izuzetno složen biokemijski proces u biljkama. Odvija se u reakcijskim centrima smještenim na tilakoidnim membranama kloroplasta. Razlikujemo dva reakcijska centra biljaka – PSII i PSI. Na PSII se stvara protonski gradijent, formira se O2 i oslobađa se e- koji se potom prenosi do PSI. Mehanizmi zaštite fotosustava omogućuju jednolik prijenos energije na reakcijske centre i optimalan tijek elektrona među njima. Zbog svoje građe PSII je podložan oksidaciji koja rezultira gubitkom funkcije, u čijem sprečavanju vrlo važnu ulogu imaju pomoćni i zaštitni pigmenti poput karotenoida. Karotenoidi se javljaju već kod prvih bakterija i prisutni su u svim carstvima, u fotosintetskim i u nefotosintetskim organizmima. Karotenoidi, klorofili i proteini zajedno tvore antenske komplekse koji služe za sakupljanje svjetlosne energije i njezin prijenos na reakcijske centre. Kao dio antenskih kompleksa, karotenoidi imaju ulogu pomoćnih pigmenta za sakupljanje svjetlosne energije i njezin prijenos do klorofila a te ulogu zaštitnih pigmenata koji sprečavaju oksidaciju reakcijskog centra. Naime, zbog građe u obliku linearnih organskih molekule s višestruko konjugiranim dvostrukim vezama, pi konjugirane strukture, mogu prenositi energiju koju apsorbiraju do molekula klorofila a u reakcijskom centru ili preuzeti višak energije s reakcijskog centra, te zahvaljujući antioksidativnom djelovanju mogu spriječiti oštećenja uzrokovana reaktivnim oblicima kisika na reakcijskom centru i tilakoidnim membranama. Važnu ulogu u zaštiti od oštećenja fotosistema oksidacijom ima ksantofilski ciklus čije su komponente poznate ali nedostaje cjelovit biofizički mehanizam.Photosynthesis is a vital and complex biochemical process that occurs in the reaction centers of the chloroplast thylakoid membranes in plants. There are two reaction centers – PSII and PSI. PSII establishes proton gradient, creates oxygen and releases electrons that are subsequently transferred to PSI. Photosystem protection mechanisms allow a balanced energy transfer between reaction centers and an optimal electron flow. Due to its built PSII is susceptible to oxidation that leads to a loss of function. Accessory and protective pigments such as carotenoids have a great contribution in stopping oxidation. Carotenoids were first developed in early bacteria and now they are present in all domains, both in photosynthetic and non-photosynthetic organisms. Carotenoids, chlorophylls and proteins together form antenna complexes that are used to harvest light and transfer its energy on the reaction centers. The role of carotenoids in antenna complexes is that of accessory pigments harvesting energy and transferring it on chlorophyll a molecules. Besides being accessory pigments, as protective pigments they have a role in protecting reaction centers from oxidation. Due to their linear molecular structure that contains multiple conjugated double bonds, called piconjugated system, they can either transfer absorbed energy toward chlorophyll a reaction center molecule or accept excess reaction center energy. They can also stop the damage from reactive oxygen species on reaction centers and thylakoid membranes due to their antioxidative abilities. Protection from oxidative damage of photosystem is mainly provided by xanthophyll cycle. Main components of the cycle are quite known but the actual biophysicalmechanism is not yet derived