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Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two
photosystems
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 ...
in the
photosynthetic light reactions of
algae
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mic ...
,
plant
Plants are predominantly photosynthetic eukaryotes of the kingdom Plantae. Historically, the plant kingdom encompassed all living things that were not animals, and included algae and fungi; however, all current definitions of Plantae exclud ...
s, and
cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
.
Photosystem
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 ...
I
is an
integral membrane protein
An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a signi ...
complex
Complex commonly refers to:
* Complexity, the behaviour of a system whose components interact in multiple ways so possible interactions are difficult to describe
** Complex system, a system composed of many components which may interact with each ...
that uses
light
Light or visible light is electromagnetic radiation that can be perceived by the human eye. Visible light is usually defined as having wavelengths in the range of 400–700 nanometres (nm), corresponding to frequencies of 750–420 tera ...
energy
In physics, energy (from Ancient Greek: ἐνέργεια, ''enérgeia'', “activity”) is the quantitative property that is transferred to a body or to a physical system, recognizable in the performance of work and in the form of heat a ...
to catalyze the
transfer of electrons across the
thylakoid
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 thyl ...
membrane from
plastocyanin
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 m ...
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
Chemiosmosis is the movement of ions across a semipermeable membrane bound structure, down their electrochemical gradient. An important example is the formation of adenosine triphosphate (ATP) by the movement of hydrogen ions (H+) across a membra ...
that is used to generate
ATP. PSI is composed of more than 110
cofactors, significantly more than
Photosystem II
Photosystem II (or water-plastoquinone oxidoreductase) is the first protein complex in the light-dependent reactions of oxygenic photosynthesis. It is located in the thylakoid membrane of plants, algae, and cyanobacteria. Within the photosystem ...
.
History
This photosystem is known as PSI because it was discovered before Photosystem II, although future experiments showed that Photosystem II is actually the first enzyme of the photosynthetic electron transport chain. Aspects of PSI were discovered in the 1950s, but the significance of these discoveries was not yet recognized at the time.
Louis Duysens first proposed the concepts of Photosystems I and II in 1960, and, in the same year, a proposal by Fay Bendall and Robert Hill assembled earlier discoveries into a coherent theory of serial photosynthetic reactions.
Hill and Bendall's hypothesis was later confirmed in experiments conducted in 1961 by the Duysens and Witt groups.
Components and action
Two main subunits of PSI, PsaA and PsaB, are closely related proteins involved in the
binding of the vital electron transfer cofactors P, Acc, A, A, and F. PsaA and PsaB are both
integral membrane proteins
An integral, or intrinsic, membrane protein (IMP) is a type of membrane protein that is permanently attached to the biological membrane. All ''transmembrane proteins'' are IMPs, but not all IMPs are transmembrane proteins. IMPs comprise a signi ...
of 730 to 750
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although hundreds of amino acids exist in nature, by far the most important are the alpha-amino acids, which comprise proteins. Only 22 alpha am ...
s that contain 11
transmembrane
A transmembrane protein (TP) is a type of integral membrane protein that spans the entirety of the cell membrane. Many transmembrane proteins function as gateways to permit the transport of specific substances across the membrane. They frequentl ...
segments. A
iron-sulfur cluster">Fe-4Siron-sulfur cluster called F is
coordinated by four
cysteine
Cysteine (symbol Cys or C; ) is a semiessential proteinogenic amino acid with the formula . The thiol side chain in cysteine often participates in enzymatic reactions as a nucleophile.
When present as a deprotonated catalytic residue, sometime ...
s; two cysteines are provided each by PsaA and PsaB. The two cysteines in each are proximal and located in a
loop
Loop or LOOP may refer to:
Brands and enterprises
* Loop (mobile), a Bulgarian virtual network operator and co-founder of Loop Live
* Loop, clothing, a company founded by Carlos Vasquez in the 1990s and worn by Digable Planets
* Loop Mobile, an ...
between the ninth and tenth transmembrane segments. A
leucine zipper
A leucine zipper (or leucine scissors) is a common three-dimensional structural motif in proteins. They were first described by Landschulz and collaborators in 1988 when they found that an enhancer binding protein had a very characteristic 30-amin ...
motif seems to be present
downstream
Downstream may refer to:
* Downstream (bioprocess)
* Downstream (manufacturing)
* Downstream (networking)
* Downstream (software development)
* Downstream (petroleum industry)
* Upstream and downstream (DNA), determining relative positions on DNA ...
of the cysteines and could contribute to dimerisation of PsaA/PsaB. The terminal electron acceptors F and F, also
Fe-4Siron-sulfur clusters, are located in a 9-kDa protein called PsaC that binds to the PsaA/PsaB core near F.
Photon
Photoexcitation
Photoexcitation is the production of an excited state of a quantum system by photon absorption. The excited state originates from the interaction between a photon and the quantum system. Photons carry energy that is determined by the wavelength ...
of the pigment molecules in the antenna complex induces electron and energy transfer.
Antenna complex
The antenna complex is composed of molecules of
chlorophyll
Chlorophyll (also chlorophyl) 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 , ("pale green") and , ("leaf"). Chlorophyll allow plants to a ...
and
carotenoids mounted on two proteins.
[
] These pigment molecules transmit the
resonance energy from photons when they become photoexcited. Antenna molecules can absorb all
wavelength
In physics, the wavelength is the spatial period of a periodic wave—the distance over which the wave's shape repeats.
It is the distance between consecutive corresponding points of the same phase on the wave, such as two adjacent crests, tro ...
s of light within the
visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visual perception, visible to the human eye. Electromagnetic radiation in this range of wavelengths is called ''visible light'' or simply light. A typical human eye wil ...
.
The number of these pigment molecules varies from organism to organism. For instance, the
cyanobacterium
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blue ...
''Synechococcus elongatus'' (''Thermosynechococcus elongatus'') has about 100 chlorophylls and 20 carotenoids, whereas
spinach
Spinach (''Spinacia oleracea'') is a leafy green flowering plant native to central and western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common edible vegetable consumed either f ...
chloroplasts have around 200 chlorophylls and 50 carotenoids.
Located within the antenna complex of PSI are molecules of chlorophyll called
P700
P700, or photosystem I primary donor, is the reaction-center chlorophyll ''a'' molecular dimer associated with photosystem I in plants, algae, and cyanobacteria.
Etymology
Its name is derived from the word “pigment” (P) and the presence ...
reaction centers. The energy passed around by antenna molecules is directed to the reaction center. There may be as many as 120 or as few as 25 chlorophyll molecules per P700.
P700 reaction center
The P700 reaction center is composed of modified
chlorophyll ''a'' that best absorbs light at a wavelength of 700
nm.
P700 receives energy from antenna molecules and uses the energy from each photon to raise an electron to a higher energy level (P700*). These electrons are moved in pairs in an
oxidation/reduction process from P700* to electron acceptors, leaving behind P700. The pair of P700* - P700 has an
electric potential
The electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as the amount of work energy needed to move a unit of electric charge from a reference point to the specific point in ...
of about −1.2
volt
The volt (symbol: V) is the unit of electric potential, electric potential difference (voltage), and electromotive force in the International System of Units (SI). It is named after the Italian physicist Alessandro Volta (1745–1827).
Defi ...
s. The reaction center is made of two chlorophyll molecules and is therefore referred to as a
dimer
Dimer may refer to:
* Dimer (chemistry), a chemical structure formed from two similar sub-units
** Protein dimer, a protein quaternary structure
** d-dimer
* Dimer model, an item in statistical mechanics, based on ''domino tiling''
* Julius Dimer ...
.
The dimer is thought to be composed of one chlorophyll ''a'' molecule and one chlorophyll ''a''′ molecule. However, if P700 forms a complex with other antenna molecules, it can no longer be a dimer.
Modified chlorophyll A and A
The two modified chlorophyll molecules are early electron acceptors in PSI. They are present one per PsaA/PsaB side, forming two branches electrons can take to reach F. A accepts electrons from P700*, passes it to A of the same side, which then passes the electron to the quinone on the same side. Different species seems to have different preferences for either A/B branch.
Phylloquinone
A
phylloquinone
Phytomenadione, also known as vitamin K1 or phylloquinone, is a vitamin found in food and used as a dietary supplement. It is on the World Health Organization's List of Essential Medicines.
As a supplement it is used to treat certain bleeding ...
, sometimes called vitamin K,
is the next early electron acceptor in PSI. It oxidizes A in order to receive the electron and in turn is re-oxidized by F, from which the electron is passed to F and F.
The reduction of F
x appears to be the rate-limiting step.
[
]
Iron–sulfur complex
Three proteinaceous iron–sulfur reaction centers are found in PSI. Labeled F, F, and F, they serve as electron relays. F and F are bound to protein subunit
In structural biology, a protein subunit is a polypeptide chain or single protein molecule that assembles (or "''coassembles''") with others to form a protein complex.
Large assemblies of proteins such as viruses often use a small number of ty ...
s of the PSI complex and F is tied to the PSI complex. Various experiments have shown some disparity between theories of iron–sulfur cofactor orientation and operation order. In one model, F passes an electron to F, which passes it on to F to reach the ferredoxin.[
]
Ferredoxin
Ferredoxin (Fd) is a soluble
In chemistry, solubility is the ability of a substance, the solute, to form a solution with another substance, the solvent. Insolubility is the opposite property, the inability of the solute to form such a solution.
The extent of the solubil ...
protein that facilitates reduction of to NADPH. Fd moves to carry an electron either to a lone thylakoid or to an enzyme
Enzymes () are proteins that act as biological catalysts by accelerating chemical reactions. The molecules upon which enzymes may act are called substrates, and the enzyme converts the substrates into different molecules known as products. A ...
that reduces . Thylakoid membranes have one binding site for each function of Fd. The main function of Fd is to carry an electron from the iron-sulfur complex to the enzyme ferredoxin– reductase.
Ferredoxin– reductase (FNR)
This enzyme transfers the electron from reduced ferredoxin to to complete the reduction to NADPH. FNR may also accept an electron from NADPH by binding to it.
Plastocyanin
Plastocyanin
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 m ...
is an electron carrier that transfers the electron from cytochrome b6f to the P700 cofactor of PSI in its ionized state P700.
Ycf4 protein domain
The Ycf4 protein domain found on the thylakoid membrane is vital to photosystem I. This thylakoid transmembrane protein helps assemble the components of photosystem I. Without it, photosynthesis would be inefficient.
Evolution
Molecular data show that PSI likely evolved from the photosystems of green sulfur bacteria
The green sulfur bacteria are a phylum of obligately anaerobic photoautotrophic bacteria that metabolize sulfur.
Green sulfur bacteria are nonmotile (except ''Chloroherpeton thalassium'', which may glide) and capable of anoxygenic photosynthe ...
. The photosystems of green sulfur bacteria and those of cyanobacteria
Cyanobacteria (), also known as Cyanophyta, are a phylum of gram-negative bacteria that obtain energy via photosynthesis. The name ''cyanobacteria'' refers to their color (), which similarly forms the basis of cyanobacteria's common name, blu ...
, alga
Algae (; singular alga ) is an informal term for a large and diverse group of photosynthetic eukaryotic organisms. It is a polyphyletic grouping that includes species from multiple distinct clades. Included organisms range from unicellular mi ...
e, and higher plants are not the same, but there are many analogous functions and similar structures. Three main features are similar between the different photosystems. First, redox potential is negative enough to reduce ferredoxin. Next, the electron-accepting reaction centers include iron–sulfur proteins. Last, redox centres in complexes of both photosystems are constructed upon a protein subunit dimer. The photosystem of green sulfur bacteria even contains all of the same cofactors of the electron transport chain
An electron transport chain (ETC) is a series of protein complexes and other molecules that transfer electrons from electron donors to electron acceptors via redox reactions (both reduction and oxidation occurring simultaneously) and couples th ...
in PSI. The number and degree of similarities between the two photosystems strongly indicates that PSI and the analogous photosystem of green sulfur bacteria evolved from a common ancestral photosystem.
See also
* Biohybrid solar cell
References
External links
Photosystem I: Molecule of the Month in the Protein Data Bank
Photosystem I in A Companion to Plant Physiology
James Barber FRS Photosystems I & II
{{Multienzyme complexes
Photosynthesis
Light reactions
EC 1.97.1
Protein complexes