
Photosystem I (PSI, or plastocyanin–ferredoxin oxidoreductase) is one of two
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 ...
s in the
photosynthetic light reactions of
algae
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthesis, photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular ...
,
plant
Plants are the eukaryotes that form the Kingdom (biology), kingdom Plantae; they are predominantly Photosynthesis, photosynthetic. This means that they obtain their energy from sunlight, using chloroplasts derived from endosymbiosis with c ...
s, and
cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
.
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 can be classified as IMPs, but not all IMPs are transmembrane proteins. IMPs comp ...
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, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...
energy
Energy () is the physical quantity, quantitative physical property, property that is transferred to a physical body, body or to a physical system, recognizable in the performance of Work (thermodynamics), work and in the form of heat and l ...
to catalyze the
transfer of electrons across the
thylakoid
Thylakoids are membrane-bound compartments inside chloroplasts and cyanobacterium, cyanobacteria. They are the site of the light-dependent reactions of photosynthesis. Thylakoids consist of a #Membrane, thylakoid membrane surrounding a #Lumen, ...
membrane from
plastocyanin to
ferredoxin
Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...
. Ultimately, the electrons that are transferred by Photosystem I are used to produce the moderate-energy hydrogen carrier
NADPH
Nicotinamide adenine dinucleotide phosphate, abbreviated NADP or, in older notation, TPN (triphosphopyridine nucleotide), is a cofactor used in anabolic reactions, such as the Calvin cycle and lipid and nucleic acid syntheses, which require N ...
. 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 of 730 to 750
amino acid
Amino acids are organic compounds that contain both amino and carboxylic acid functional groups. Although over 500 amino acids exist in nature, by far the most important are the 22 α-amino acids incorporated into proteins. Only these 22 a ...
s that contain 11
transmembrane
A transmembrane protein 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 frequently u ...
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 chemical formula, formula . The thiol side chain in cysteine enables the formation of Disulfide, disulfide bonds, and often participates in enzymatic reactions as ...
s; two cysteines are provided each by PsaA and PsaB. The two cysteines in each are proximal and located in a
loop 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 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 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 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 allows plants to absorb energy ...
and
carotenoid
Carotenoids () are yellow, orange, and red organic pigments that are produced by plants and algae, as well as several bacteria, archaea, and fungi. Carotenoids give the characteristic color to pumpkins, carrots, parsnips, corn, tomatoes, cana ...
s 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 and mathematics, wavelength or spatial period of a wave or periodic function is the distance over which the wave's shape repeats.
In other words, it is the distance between consecutive corresponding points of the same ''phase (waves ...
s of light within the
visible spectrum
The visible spectrum is the spectral band, band 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).
The optica ...
.
The number of these pigment molecules varies from organism to organism. For instance, the
cyanobacterium ''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 Asia, Central and Western Asia. It is of the order Caryophyllales, family Amaranthaceae, subfamily Chenopodioideae. Its leaves are a common vegetable consumed eit ...
chloroplasts have around 200 chlorophylls and 50 carotenoids.
Located within the antenna complex of PSI are molecules of chlorophyll called
P700 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
Electric potential (also called the ''electric field potential'', potential drop, the electrostatic potential) is defined as electric potential energy per unit of electric charge. More precisely, electric potential is the amount of work (physic ...
of about −1.2
volt
The volt (symbol: V) is the unit of electric potential, Voltage#Galvani potential vs. electrochemical potential, electric potential difference (voltage), and electromotive force in the International System of Units, International System of Uni ...
s. The reaction center is made of two chlorophyll molecules and is therefore referred to as a
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, 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 t ...
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
Ferredoxins (from Latin ''ferrum'': iron + redox, often abbreviated "fd") are iron–sulfur proteins that mediate electron transfer in a range of metabolic reactions. The term "ferredoxin" was coined by D.C. Wharton of the DuPont Co. and applied t ...
(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 solubi ...
protein that facilitates reduction of to NADPH. Fd moves to carry an electron either to a lone thylakoid or to an enzyme
An enzyme () is a protein that acts as a biological catalyst by accelerating chemical reactions. The molecules upon which enzymes may act are called substrate (chemistry), substrates, and the enzyme converts the substrates into different mol ...
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 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 photosystems of green sulfur bacteria and those of cyanobacteria
Cyanobacteria ( ) are a group of autotrophic gram-negative bacteria that can obtain biological energy via oxygenic photosynthesis. The name "cyanobacteria" () refers to their bluish green (cyan) color, which forms the basis of cyanobacteri ...
, alga
Algae ( , ; : alga ) is an informal term for any organisms of a large and diverse group of photosynthetic organisms that are not plants, and includes species from multiple distinct clades. Such organisms range from unicellular microalgae, suc ...
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 which 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