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P700, or photosystem I primary donor, is a molecular dimer of chlorophyll a associated with the reaction-center of photosystem I in plants, algae, and cyanobacteria. [1] [2] [3] [4]
Its name is derived from the word “pigment” (P) and the presence of a major bleaching band centered around 695-700 nm in the flash-induced absorbance difference spectra of P700/ P700+•. [5]
The structure of P700 consists of a heterodimer with two distinct chlorophyll molecules, most notably chlorophyll a and chlorophyll a’, giving it an additional name of “special pair”. [6] Inevitably, however, the special pair of P700 behaves as if it were just one unit. This species is vital due to its ability to absorb light energy with a wavelength approximately between 430 nm-700 nm, and transfer high-energy electrons to a series of acceptors that are situated near it, like Fe-S complex, Ferridoxyn(FD), which have a higher redox potential i.e. greater affinity to electron. [7]
Photosystem I operates with the functions of producing NADPH, the reduced form of NADP+(Fd2-red + NADH + 2 NADP+ + H+ = Fdox + NAD+ + 2 NADPH.), at the end of the photosynthetic reaction through electron transfer, and of providing energy to a proton pump and eventually ATP, for instance in cyclic electron transport.
When photosystem I absorbs light, an electron is excited to a higher energy level in the P700 chlorophyll. The resulting P700 with an excited electron is designated as P700*, which is a strong reducing agent due to its very negative redox potential of -1.2V . [8]
Following the excitation of P700, one of its electrons is passed on to an electron acceptor, Ao, triggering charge separation producing an anionic Ao− and cationic P700+. Subsequently, electron transfer continues from Ao to a phylloquinone molecule known as A1, and then to three iron-sulfur clusters. [9]
Type I photosystems use iron-sulfur cluster proteins as terminal electron acceptors. Thus, the electron is transferred from Fx to another iron sulfur cluster, FA, and then passed on to the last iron-sulfur cluster serving as an electron acceptor, FB. Eventually, the electron is transferred to the protein ferredoxin, causing it to transform into its reduced form, which subsequently finalizes the process by reducing NADP+ to NADPH.
The rate of electrons being passed from P700* to the subsequent electron acceptors is high, preventing the electron from being transferred back to P700+. [10] Consequently, in most cases, the electrons transferring within photosystem I follow a linear pathway, from the excitation of the P700 special pair to the production of NADPH.
In certain situations, it is vital for the photosynthetic organism to recycle the electrons being transferred, resulting in the electron from the terminal iron-sulfur cluster FB transferring back to the cytochrome b6f complex (adaptor between photosystems II and I). [11] Utilizing the energy of P700+, the cyclic pathway creates a proton gradient useful for the production of ATP, while no NADPH is produced, since the protein ferredoxin does not become reduced. [12]
P700+ recovers its lost electron by oxidizing plastocyanin, which regenerates P700.
Chlorophyll is any of several related green pigments found in cyanobacteria and in the chloroplasts of algae and plants. Its name is derived from the Greek words χλωρός, khloros and φύλλον, phyllon ("leaf"). Chlorophyll allows plants to absorb energy from light.
Respiratory complex I, EC 7.1.1.2 is the first large protein complex of the respiratory chains of many organisms from bacteria to humans. It catalyzes the transfer of electrons from NADH to coenzyme Q10 (CoQ10) and translocates protons across the inner mitochondrial membrane in eukaryotes or the plasma membrane of bacteria.
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a thylakoid membrane surrounding a thylakoid lumen. Chloroplast thylakoids frequently form stacks of disks referred to as grana. Grana are connected by intergranal or stromal thylakoids, which join granum stacks together as a single functional compartment.
Plastocyanin is a copper-containing protein that mediates electron-transfer. It is found in a variety of plants, where it participates in photosynthesis. The protein is a prototype of the blue copper proteins, a family of intensely blue-colored metalloproteins. Specifically, it falls into the group of small type I blue copper proteins called "cupredoxins".
Photosystems are functional and structural units of protein complexes involved in photosynthesis. Together they carry out the primary photochemistry of photosynthesis: the absorption of light and the transfer of energy and electrons. Photosystems are found in the thylakoid membranes of plants, algae, and cyanobacteria. These membranes are located inside the chloroplasts of plants and algae, and in the cytoplasmic membrane of photosynthetic bacteria. There are two kinds of photosystems: PSI and PSII.
Photosystem I is one of two photosystems in the photosynthetic light reactions of algae, plants, and cyanobacteria. Photosystem I is an integral membrane protein complex that uses light energy to catalyze the transfer of electrons across the thylakoid membrane from plastocyanin to ferredoxin. Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier NADPH. The photon energy absorbed by Photosystem I also produces a proton-motive force that is used to generate ATP. PSI is composed of more than 110 cofactors, significantly more than Photosystem II.
In the process of photosynthesis, the phosphorylation of ADP to form ATP using the energy of sunlight is called photophosphorylation. Cyclic photophosphorylation occurs in both aerobic and anaerobic conditions, driven by the main primary source of energy available to living organisms, which is sunlight. All organisms produce a phosphate compound, ATP, which is the universal energy currency of life. In photophosphorylation, light energy is used to pump protons across a biological membrane, mediated by flow of electrons through an electron transport chain. This stores energy in a proton gradient. As the protons flow back through an enzyme called ATP synthase, ATP is generated from ADP and inorganic phosphate. ATP is essential in the Calvin cycle to assist in the synthesis of carbohydrates from carbon dioxide and NADPH.
The cytochrome b6f complex (plastoquinol/plastocyanin reductase or plastoquinol/plastocyanin oxidoreductase; EC 7.1.1.6) is an enzyme found in the thylakoid membrane in chloroplasts of plants, cyanobacteria, and green algae, that catalyzes the transfer of electrons from plastoquinol to plastocyanin:
A photosynthetic reaction center is a complex of several proteins, pigments, and other co-factors that together execute the primary energy conversion reactions of photosynthesis. Molecular excitations, either originating directly from sunlight or transferred as excitation energy via light-harvesting antenna systems, give rise to electron transfer reactions along the path of a series of protein-bound co-factors. These co-factors are light-absorbing molecules (also named chromophores or pigments) such as chlorophyll and pheophytin, as well as quinones. The energy of the photon is used to excite an electron of a pigment. The free energy created is then used, via a chain of nearby electron acceptors, for a transfer of hydrogen atoms (as protons and electrons) from H2O or hydrogen sulfide towards carbon dioxide, eventually producing glucose. These electron transfer steps ultimately result in the conversion of the energy of photons to chemical energy.
In biophysics, the Kautsky effect is a phenomenon consisting of a typical variation in the behavior of a plant fluorescence when exposed to light. It was discovered in 1931 by H. Kautsky and A. Hirsch.
DCMU is an algicide and herbicide of the arylurea class that inhibits photosynthesis. It was introduced by Bayer in 1954 under the trade name of Diuron.
Photoinhibition is light-induced reduction in the photosynthetic capacity of a plant, alga, or cyanobacterium. Photosystem II (PSII) is more sensitive to light than the rest of the photosynthetic machinery, and most researchers define the term as light-induced damage to PSII. In living organisms, photoinhibited PSII centres are continuously repaired via degradation and synthesis of the D1 protein of the photosynthetic reaction center of PSII. Photoinhibition is also used in a wider sense, as dynamic photoinhibition, to describe all reactions that decrease the efficiency of photosynthesis when plants are exposed to light.
Quinone-interacting membrane-bound oxidoreductase is a membrane-bound protein complex present in the electron transport chain of sulfate reducers and some sulfur oxidizers.
Nitrate reductase (NADPH) (EC 1.7.1.3, assimilatory nitrate reductase, assimilatory reduced nicotinamide adenine dinucleotide phosphate-nitrate reductase, NADPH-nitrate reductase, assimilatory NADPH-nitrate reductase, triphosphopyridine nucleotide-nitrate reductase, NADPH:nitrate reductase, nitrate reductase (NADPH2), NADPH2:nitrate oxidoreductase) is an enzyme with systematic name nitrite:NADP+ oxidoreductase. This enzyme catalises the following chemical reaction
Cytochrome b559 is an important component of Photosystem II (PSII) is a multisubunit protein-pigment complex containing polypeptides both intrinsic and extrinsic to the photosynthetic membrane. Within the core of the complex, the chlorophyll and beta-carotene pigments are mainly bound to the antenna proteins CP43 (PsbC) and CP47 (PsbB), which pass the excitation energy on to chlorophylls in the reaction centre proteins D1 and D2 that bind all the redox-active cofactors involved in the energy conversion process. The PSII oxygen-evolving complex (OEC) provides electrons to re-reduce the PSII reaction center, and oxidizes 2 water molecules to recover its reduced initial state. It consists of OEE1 (PsbO), OEE2 (PsbP) and OEE3 (PsbQ). The remaining subunits in PSII are of low molecular weight, and are involved in PSII assembly, stabilisation, dimerization, and photoprotection.
NADH dehydrogenase [ubiquinone] iron-sulfur protein 7, mitochondrial, also knowns as NADH-ubiquinone oxidoreductase 20 kDa subunit, Complex I-20kD (CI-20kD), or PSST subunit is an enzyme that in humans is encoded by the NDUFS7 gene. The NDUFS7 protein is a subunit of NADH dehydrogenase (ubiquinone) also known as Complex I, which is located in the mitochondrial inner membrane and is the largest of the five complexes of the electron transport chain.
Light-dependent reactions are certain photochemical reactions involved in photosynthesis, the main process by which plants acquire energy. There are two light dependent reactions: the first occurs at photosystem II (PSII) and the second occurs at photosystem I (PSI).
Chlorophyll fluorescence is light re-emitted by chlorophyll molecules during return from excited to non-excited states. It is used as an indicator of photosynthetic energy conversion in plants, algae and bacteria. Excited chlorophyll dissipates the absorbed light energy by driving photosynthesis, as heat in non-photochemical quenching or by emission as fluorescence radiation. As these processes are complementary processes, the analysis of chlorophyll fluorescence is an important tool in plant research with a wide spectrum of applications.
Fumarate reductase (quinol) (EC 1.3.5.4, QFR,FRD, menaquinol-fumarate oxidoreductase, quinol:fumarate reductase) is an enzyme with systematic name succinate:quinone oxidoreductase. This enzyme catalyzes the following chemical reaction:
In photosynthesis, state transitions are rearrangements of the photosynthetic apparatus which occur on short time-scales. The effect is prominent in cyanobacteria, whereby the phycobilisome light-harvesting antenna complexes alter their preference for transfer of excitation energy between the two reaction centers, PS I and PS II. This shift helps to minimize photodamage caused by reactive oxygen species (ROS) under stressful conditions such as high light, but may also be used to offset imbalances between the rates of generating reductant and ATP.