Light gathering complexes

The key event in the light phase of photosynthesis, in which the radiation energy is converted into chemical energy, is the process of separation of charges in the reaction centers of photosystems. Charge separation is the process of electron transfer from an excited chlorophyll of reaction centers to a primary acceptor. The separation of charges occurs as a result of the excitation of chlorophyll of the reaction centers upon absorption of a certain energy quantum by it. However, a direct hit of a photon carrying the energy necessary for excitation in the chlorophyll of the reaction center is extremely unlikely. Therefore, effective photosynthesis is possible only with antennas - pigment-protein complexes that capture photons of different wavelengths and direct the excitation energy to the reaction centers. It is known that the vast majority of chlorophyll molecules are part of precisely antenna complexes, and not reaction centers. In higher plants, about 300 chlorophyll antenna molecules are associated with one reaction center ^[1] .

To use the energy of photons that are not absorbed by chlorophyll (the "green dip" region), other pigments are also part of the antennas. In higher plants, these are carotenoids ( carotenes and xanthophylls ), and in a number of algae and some photosynthetic prokaryotes, they are also phycobilins . Chlorophylls and carotenoids bind to proteins non-covalently due to electrostatic interactions, coordination bonds with magnesium and hydrophobic interactions. Phycobilins covalently attach to proteins via thioether and ether bonds ^[2] .

Energy migration in light harvesting complexes always occurs with some energy loss. In this regard, the maximum absorption of the donor pigment is shifted to a shorter wavelength region (compared with the maximum of the acceptor pigment). That is, the excitation energy of the donor pigment is always higher than the excitation energy of the acceptor pigment (part of the energy dissipates into heat) ^[3] . For example, for higher plants, energy migration is typical in the following direction: carotenoids → chlorophyll b → chlorophyll a → chlorophyll a of the reaction center (as a part of the dimer).

The organization of SSCs in different organisms is quite variable (compared with the conservative structure of reaction centers), which reflects the adaptation of phototrophs to different lighting conditions during evolution.

Energy Migration Mechanisms in SSC

Since it was found that efficient energy transfer in antennas occurs at extremely low temperatures (1 ° K = –272 ° C), it was concluded that energy transfer occurs without electron transfer (electron transport at such low temperatures is impossible) ^[4] . The following mechanisms of energy migration are distinguished:

1.  

   The energy of the excited state according to the Förster mechanism is migrated by a singlet-singlet energy transfer. The figure shows the energy level diagram for the corresponding process.
   The inductive resonance mechanism ( Förster energy transfer , or FRET from the English Förster resonance energy transfer ) was proposed in 1948 by T. Förster. This mechanism of energy transfer does not imply electron transfer or photon emission and subsequent absorption, i.e. is nonradiative (despite this, sometimes the abbreviation FRET is incorrectly deciphered as fluorescence resonance energy transfer) ^[5] . Since in an excited state the electron is an oscillating dipole that creates an alternating electric field, then under the conditions of resonance and induction, it can cause similar vibrations of the electron in the neighboring molecule. The resonance condition is the equality of energies between the ground and excited states, i.e. overlap of the absorption and fluorescence spectra of two molecules is necessary. Also, for successful induction, a close arrangement of interacting molecules (not more than 10 nm) is necessary. It is known that the intermolecular distance in the SSC is from 2 to 3 nm; and the existence of a series of different native forms of pigments provides a good overlap of their spectra. All this creates good conditions for the transfer of energy by the inductive resonance mechanism. The rate of energy transfer during the Förster transfer is in the range ^10–9 –10 ⁻¹² s ^[6] , which is associated with the transfer of energy sequentially from the donor pigment to the acceptor pigment ^[7] .
2. The mechanism of exciton migration was proposed by A. Frenkel in 1931. The exciton migration mechanism is also based on the resonant interaction of molecules and is not related to electron transfer, however, it is characteristic of fairly homogeneous, ordered systems that form a crystal lattice zone. By an exciton is meant a quantum of excitation energy (an excited state in which an electron is bound to a nucleus). The exciton mechanism is characterized by the excitation of a whole complex of pigment molecules of the same type in a certain way. In this case, the rate of energy migration in such a homogeneous complex reaches values ​​of the order of ^{10–12 –10–15} s ^{[8] [9]} .
3. Also, provided that electron transitions to an excited level are optically forbidden (typical for the transition of carotenoids S ₀ → S ₁ ) and there is no dipole formation, energy migration is possible by the Terenin-Dexter exchange-resonance mechanism . For the energy migration by the Terenin – Dexter mechanism, an extremely close arrangement of molecules (distance of about 1 nm) and overlapping of external molecular orbitals are required. In this case, the exchange of electrons is possible, both at singlet and triplet levels ^[10] .

These mechanisms of energy transfer are fundamentally different from the mechanisms realized in electron transport chains ( ETCs ), since energy transfer in different parts of the ETC is associated with electron transfer (electron energy migration). The transfer of electrons between cofactors inside the protein complexes of ETC is carried out by 1) semiconductor or 2) resonant (based on the effect of electron tunneling through the energy barrier) mechanisms. Electron transfer in areas with mobile carriers is carried out by the diffuse mechanism ^[11] .

Purple Bacteria

Purple bacteria have one photosystem, in many respects close to photosystem II of cyanobacteria and higher plants . Light collecting complexes are located around this photosystem: LH2 at the periphery and LH1 near the reaction center ^[12] . On the proteins of the complexes are molecules of bacteriochlorophyll and carotenoids . At the same time, shorter wavelength forms of pigments (800 - 850 nm) are characteristic of external LH2 complexes, and longer wavelength forms (about 880 nm) of the internal LH1 complex. Bacteriochlorophyll reaction center (RC) has an even longer wavelength absorption maximum. A similar structure ensures the absorption of photons in LH2 and directed migration through LH1 to the RC. Purple bacteria are characterized by multisubunit SSCs with circular organization. The composition of complexes, as a rule, includes two types of polypeptides : α - and β-subunits . Both subunits are small proteins consisting of hydrophilic sites (cytoplasmic and periplasmic), as well as a transmembrane domain. The organization of proteins and the location of pigments in RC and SSC are studied using the method of x-ray crystallography ^[12] .

For Rhodobacter sphaeroides , a dimeric organization of the complex (LH1 – RC – PufX) _{2 was} shown (with a resolution of 8 Å) ^[13] . The dimer contains two PufX proteins, which form gaps in the LH1 circular antennas, through which reduced ubiquinone comes out from the RC. In addition, this protein is responsible for dimerization. A similar dimeric complex was detected by electron microscopy in the membranes of the bacterium Rhodobaca bogoriensis ^[14] .

In Rhodopseudomonas palustris , the structure of the LH1 – RC – protein W complex (with a resolution of 4.8 Å) was described ^[15] . Protein W, by analogy with PufX, forms a gap in a circular antenna LH1. The gap in LH1 allows access of the mobile carrier of ubiquinone to the RC.

With the highest resolution (3 Å), the structure of the LH1 – RC monomer complex in the thermophilic bacterium Thermochromatium tepidum was described ^[16] . In this case, LH1 completely surrounds the RC and has no gaps; the path for the transport of ubiquinone provides a special channel in the antenna. In addition, from the C-terminus of the LH1 subunits, there are binding sites for calcium cations; Calcium binding is believed to increase the thermal stability of the complex.

Green bacteria

In the chlorosomes of green sulfur bacteria, the light-harvesting complex is located on the cytoplasmic side of the membrane and consists of approximately 10,000 bacteriochlorophyll molecules (mainly bacteriochlorophyll c) bound to proteins. They are surrounded by lipid membranes and their base (bacteriochlorophyll a is at the base of the complexes) are in contact with the light-collecting complex embedded in the membrane surrounding the reaction center. Exciton transfer occurs from bacteriochlorophyll c, which absorbs at a wavelength of about 750 nm (B750) through the bacteriochlorophyll a molecules located at the base (B790), to bacteriochlorophyll a of the light-absorbing complex integrated into the membrane (B804) and, finally, to the reaction center bacteriochlorophyll a ( P840). ^[17]

In higher plants , internal (or core, from the English. Core ) and external light-harvesting complexes are distinguished. Each photosystem (I and II) has both internal and external SSC, i.e. higher plants have 4 types of CCK. External antennas provide photon absorption and migration of excitation energy to internal antennas. Internal antennas are located in the immediate vicinity of the reaction centers; they also absorb light quanta and provide the migration of excitation energy to the reaction centers of the photosystems. Each SSC contains several polypeptides; on each SSC protein there is a strictly defined number of pigments.

CCK photosystem I

FS I External Antenna

The external antenna of PS I includes four Lhca1-4 polypeptides (from the English light harvesting complex), with a molecular weight of about 22 kDa. Each polypeptide carries about 100 molecules of chlorophylls a and b , and xanthophylls (lutein, violoxanthin). The ratio of chlorophyll a / chlorophyll b in the external antenna of PS I is about 3.5. The proteins of the external antenna are organized in the form of a crescent moon around each individual photosystem. Moreover, if PS I forms a trimeric supercomplex, then the crescents of individual PS I close, completely surrounding the trimer. Unlike the mobile trimer of the external antenna CCK II, the external antenna CCK I is constantly connected to PS I and is not capable of diffusion in the membrane. Lhca1-4 proteins are encoded in the nuclear genome.

In tomato , Lhca1 and Lhca4 proteins exist in two isoforms. In the Talus Rezukhovidka , there are two homologous genes encoding Lhca5 and Lhca6 ^{[18] [19]} . It is known that Lhca5 is found in significant quantities in bright light and can form homodimers that bind to Lhca2 and Lhca3. There is evidence that the NADH dehydrogenase complex of chloroplasts , similar to the NADH dehydrogenase complex of mitochondria and homologous to bacterial complex I ^{[20] [21]} , chloroplasts forms a supercomplex with at least two PSIs using proteins Lhca5 and Lhca6. ^[nineteen]

FS I Internal Antenna

The internal FS I antenna is located on two central proteins of the photosystem (proteins A and B), around the P ₇₀₀ reaction center and electron transfer cofactors . The internal antenna contains 95 chlorophyll a molecules, 12-22 β-carotene molecules, 5 of which are in the cis conformation. The internal antenna pigments are arranged in the form of a cylinder surrounding the redox agents of the electron transport chain of FS I. Proteins A and B comprise the nucleus of photosystem I and are encoded in the plastid genome . ^[22]

CCK Photosystem II

FS II External Antenna

The FS II external antenna consists of a mobile antenna and minor antenna proteins. The proteins of the mobile antenna include: Lhcb1-3 (weight about 26 kDa), the minor proteins - Lhcb4-6 (or CP29, CP26, CP23). Lhcb1-3 proteins are encoded in the nuclear genome. ^[23]

Each of the proteins of the mobile antenna contains 7-8 molecules of chlorophyll a, 6 molecules of chlorophyll b , 2 crossed lutein molecules, one molecule of neoxanthin and violoxanthin (or zeaxanthin ). ^[23] The Lhcb2 protein is the main protein of the thylakoid membrane; therefore, it is well studied. Lhcb2 contains an important threonine residue that can undergo phosphorylation, which is important for the transition of chloroplasts from state 1 to state 2. One Lhcb1 protein and two Lhcb2 proteins form a heterotrimer of a mobile antenna - CCK II. The mobile trimer CCK II is capable of diffusion in the thylakoid membrane and can bind to PS I (with the participation of the H subunit), thereby increasing the energy flow to the PS I reaction center and reducing the load on the PS II reaction center.

The minor proteins Lhcb4-6 are located between the mobile antenna and the internal antenna of the PSII complex. Each of these proteins contains 13-15 molecules of chlorophylls and 4-5 molecules of xanthophylls ( lutein , neoxanthin , violo - or zeaxanthin ). The minor proteins of PS II, by virtue of their location, serve as channels for the flow of energy from an external antenna of SSK II to the reaction center of PS II. It is in the SSK II minor proteins that the xanthophyll ( violoxanthine ) cycle takes place, which plays a photoprotective role in case of excessive illumination. ^[23]

FS II Internal Antenna

In contrast to PS I, where the internal antenna is located on the central proteins carrying chlorophylls of the reaction center and electron transfer cofactors, the PS II internal antenna is located on two separate proteins (CP43 and CP47) adjacent to the central PS II proteins (D1 and D2 proteins). Protein CP43 is located near D1, and CP47 is near D2. CP43 carries 13 molecules of chlorophyll a , CP47 - 16, in addition, they contain 3-5 molecules of β-carotene. Proteins CP43 and CP47 are encoded in the plastid genome. ^[24]

Transitional states of chloroplasts

In state 1, the CCKII mobile trimer is associated with PSII. With increasing light intensity, the pool of plastoquinones and cytochromes b ₆ / f complex is re-restored, which activates a special kinase that phosphorylates the mobile trimer. As a result of phosphorylation, the surface of the mobile trimer acquires a negative charge, which leads to its dissociation from PSII. The phosphorylated mobile trimer may be coupled to PSI. The state in which the mobile trimer is associated with PSI is called state 2. During the oxidation of plastoquinones, the dephosphorylation of the mobile antenna occurs with the enzyme protein phosphatase, its return to the region of paired gran membranes and an increase in the energy influx to PSII, which is accompanied by the system switching from 2 to state 1. It is shown that a number of PSI subunits (H, O, L) are necessary for attachment of the CCKII mobile complex and transition to state 2 ^{[25] [26] [27]} . As a result of the transition from state 1 to state 2, the radiation energy is redirected from PSII to PSI, which more efficiently carries out a cyclic electron flow. Switching between state 1 and 2 is an important mechanism for protecting the photosynthetic apparatus from high light intensities. ^[28]

In some cyanobacteria (including prochlorophytes ), glaucocystophytes , cryptophytes, and red algae, the pigments of light-harvesting complexes are represented by tetrapyrroles not locked in the macrocycle — phycobilins . Phycobilins are fixed on proteins by the formation of covalent bonds ( thioether and ether ), while the chromophore molecule takes on the conformation of an open loop. Pigment-protein complexes are hydrophilic and can be extracted by extraction with hot water. Hydrolysis of the covalent bond between the pigment and apoprotein requires treatment with hydrochloric acid when heated. Intensive fluorescence is characteristic of phycobiliproteins, however, when protein is denatured , phycobiliproteins lose this ability.

Several classes of phycobilins are distinguished, with different spectral characteristics:

1. phycoerythrins - red (maximum absorption from 540 to 570 nm, absent in glaucocystophytes);
2. phycocyanins - blue (maximum absorption from 615 to 630 nm);
3. allophycocyanins are blue-green (the absorption maximum is about 620-670 nm, absent in cryptophytes).

In algae cells, phycobiliproteins are organized into light-harvesting complexes (phycobilisomes), which are located on the surface of tylocoid membranes. Phycobilisomes can be hemispherical or hemispherical. Also, the composition of phycobilis includes special proteins responsible for the aggregation of phycobilin pigments and the assembly of phycobilis. The organization of phycobilis is such that phycobilins with shorter wavelength absorption maxima are located on the periphery, and the shorter ones are near the reaction centers. Energy migration in phycobilisomes occurs with the dissipation of part of the excitation energy into heat and follows a general rule: from shorter wavelength pigments to longer wavelength ones (phycoerythrins → phycocyanins → allophycocyanins) ^[29] .

In cryptoft phycobiliproteins, they are located in the lumen of thylakoid and standard phycobilisomes are absent ^[30] .

The ratio of phycobilin pigments in different species of algae is determined by the spectral composition of the light they use. At large depths of the water column, mainly short-wave blue light penetrates. In this regard, phycoerythrins that effectively absorb high-energy quanta accumulate in red algae , which usually lives at great depths. And the cyanobacteria that inhabit fresh water bodies and the upper layers of the water column of the oceans mainly accumulate phycocyanins and allophycocyanins. Кроме того у водорослей одного вида соотношение пигментов также не постоянен и модифицируется в зависимости от глубины обитания (явление хроматической адаптации ) ^[31] .

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