MECHANISMS OF ELECTRON TRANSPORT IN PHOTOSYNTHESIS CSIR-NET/JRF
MECHANISMS
OF ELECTRON TRANSPORT
This blog deals with
the role of light in photosynthesis, the structure of the photosynthetic
apparatus, and the processes that begin with the excitation of chlorophyll by
light and culminate in the synthesis of ATP and NADPH.
Here we will consider in detail the
chemical reactions involved in electron transfer during photosynthesis. We will
discuss the excitation of chlorophyll by light and the reduction of the first electron
acceptor, the flow of electrons through photosystems II and I, the oxidation of
water as the primary source of electrons, and the reduction of the final electron
acceptor (NADP+).
Almost all the chemical processes that make up
the light reactions of photosynthesis are carried out by four major protein
complexes: photosystem II, the
cytochrome b6f complex, photosystem I, and the ATP
synthase. These four integral membrane complexes are vectorally
oriented in the thylakoid membrane to function as follows.
• Photosystem II oxidizes water to O2 in the thylakoid lumen
and in the process releases protons into the lumen.
• Cytochrome b6 f receives electrons
from PSII and delivers them to PSI. It also transports additional protons into
the lumen from the stroma.
• Photosystem I reduces NADP+ to NADPH in the stroma by
the action of ferredoxin (Fd) and the flavoprotein ferredoxin–NADP reductase
(FNR).
• ATP synthase produces ATP as protons diffuse back through it from
the lumen into the stroma.
The transfer of electrons and
protons in the thylakoid membrane is carried out vectorially by four
proteincomplexes. Water is oxidized and protons are released in the lumen by
PSII. PSI reduces NADP+ to NADPH in the stroma, via the action of ferredoxin
(Fd) and the flavoprotein ferredoxin–NADP reductase (FNR). Protons are also transported
into the lumen by the action of the cytochrome b6 f complex and
contribute to the electrochemical proton gradient. These protons must then
diffuse to the ATP synthase enzyme, where their diffusion down the
electrochemical potential gradient is used to synthesize ATP in the stroma.
Reduced plastoquinone (PQH2) and plastocyanin transfer electrons to
cytochrome b6 f and to PSI, respectively. Dashed lines
represent electron transfer; solid lines represent proton movement.
Energy Is Captured When an Excited
Chlorophyll Reduces an Electron Acceptor Molecule
The first reaction that converts
electron energy into chemical energy that is, the primary photochemical event is
the transfer of an electron from the
excited state of a chlorophyll in the
reaction center to an acceptor molecule. An equivalent way to view this
process is that the absorbed photon causes an electron rearrangement in the reaction
center chlorophyll, followed by an electron transfer process in which part of
the energy in the photon is captured in the form of redox energy. Immediately
after the photochemical event, the reaction center chlorophyll is in an
oxidized state (electron deficient, or positively charged) and the nearby
electron acceptor molecule is reduced (electron rich, or negatively charged).
The system is now at a critical juncture. The lower-energy orbital of the
positively charged oxidized reaction center chlorophyll has a vacancy and can
accept an electron. If the acceptor molecule donates its electron back to the
reaction center chlorophyll, the system will be returned to the state that
existed before the light excitation, and all the absorbed energy will be
converted into heat. This wasteful recombination process, however, does
not appear to occur to any substantial degree in functioning reaction centers.
Instead, the acceptor transfers its extra electron to a secondary acceptor and
so on down the electron transport chain. The oxidized reaction center of the chlorophyll
that had donated an electron is re-reduced by a secondary donor, which in turn
is reduced by a tertiary donor. In plants, the ultimate electron
donor is H2O, and the ultimate electron acceptor is NADP+.
The essence of photosynthetic
energy storage is thus the initial transfer of an electron from an excited
chlorophyll to an acceptor molecule, followed by a very rapid series of secondary
chemical reactions that separate the positive and negative charges. These
secondary reactions separate the charges to opposite sides of the thylakoid
membrane in approximately 200 picoseconds (1 picosecond = 10–12 s). With
the charges thus separated, the reversal reaction is many orders of magnitude
slower, and the energy has been captured. Each of the secondary electron
transfers is accompanied by a loss of some energy, thus making the process effectively
irreversible. The quantum yield for the production of stable products in
purified reaction centers from photosynthetic bacteria has been measured as
1.0; that is, every photon produces stable products, and no reversal reactions
occur.
Although these types of
measurements have not been made on purified reaction centers from higher
plants, the measured quantum requirements for O2 production under optimal
conditions (low-intensity light) indicate that the values for the primary
photochemical events are very close to 1.0. The structure of the reaction
center appears to be extremely fine-tuned for maximal rates of productive
reactions and minimal rates of energy-wasting reactions.
The Reaction Center Chlorophylls of
the Two Photosystems Absorb at Different Wavelengths
PSI and PSII have distinct absorption
characteristics. Precise measurements of absorption maxima were made possible
by optical changes in the reaction center chlorophylls in the reduced and oxidized
states. The reaction center chlorophyll is transiently in an oxidized state
after losing an electron and before being re-reduced by its electron donor.
In the oxidized state, the strong
light absorbance in the red region of the spectrum that is characteristic of
chlorophylls is lost, or bleached. It is therefore possible to monitor the
redox state of these chlorophylls by time-resolved optical absorbance
measurements in which this bleaching is monitored directly . Using such
techniques, Bessel Kok found that the reaction center chlorophyll of
photosystem I absorbs maximally at 700 nm in its reduced state.
Accordingly, this chlorophyll is named P700 (the P stands for pigment).
H. T. Witt and coworkers found the analogous optical transient of photosystem
II at 680 nm, so its reaction center chlorophyll is known as P680.
And the reaction center bacteriochlorophyll from purple photosynthetic bacteria
as P870.
The X-ray structure of the
bacterial reaction center clearly indicates that P870 is a closely coupled pair
or dimer of bacteriochlorophylls, rather than a single molecule. The primary donor
of photosystem I, P700, is a dimer of chlorophyll a molecules.
Photosystem II also contains a dimer of chlorophylls, although the primary
donor, P680, may not reside entirely on these pigments. In the oxidized state,
reaction center chlorophylls contain an unpaired electron.
The Photosystem II Reaction Center
Is a Multisubunit Pigment–Protein Complex
Photosystem II is contained in a
multisubunit protein supercomplex . In higher plants, the multisubunit protein
supercomplex has two complete reaction centers and some antenna
complexes.
The core of the reaction center
consists of two membrane proteins known as D1 and D2, as well as
other proteins, .The primary donor chlorophyll (P680), additional chlorophylls,
carotenoids, pheophytins, and plastoquinones (two electron acceptors described
in the following section) are bound to the membrane proteins D1 and D2. These
proteins have some sequence similarity to the L and M peptides of purple
bacteria. Other proteins serve as antenna complexes or are involved in
oxygen evolution. Some, such as cytochrome b559, have no known function
but may be involved in a protective cycle around photosystem II.
Water Is Oxidized to Oxygen by
Photosystem II
Water is oxidized according to
the following chemical reaction (Hoganson and Babcock 1997):
2 H2O → O2 +
4 H+ + 4 e–
This equation indicates that four
electrons are removed from two water molecules, generating an oxygen molecule
and four hydrogen ions.
Water is a very stable molecule.
Oxidation of water to form molecular oxygen is very difficult, and the
photosynthetic oxygen-evolving complex is the only known biochemical system
that carries out this reaction. Photosynthetic oxygen evolution is also the
source of almost all the oxygen in Earth’s atmosphere.
The protons produced by water
oxidation are released into the lumen of the thylakoid, not directly into the
stromal compartment . They are released into the lumen because of the vectorial
nature of the membrane and the fact that the oxygen-evolving complex is
localized on the interior surface of the thylakoid. These protons are
eventually transferred from the lumen to the stroma by translocation through
ATP synthase. In this way the protons released during water oxidation
contribute to the electrochemical potential driving ATP formation.
It has been known for many years
that manganese (Mn) is an essential cofactor in the water-oxidizing process,
and a classic hypothesis in photosynthesis research postulates that Mn ions
undergo a series of oxidations— which are known as S states, and are
labeled S0, S1, S2, S3, and S4 that are
perhaps linked to H2O oxidation and the generation of O2.
This hypothesis has received strong
support from a variety of experiments, most notably X-ray absorption and ESR
studies, both of which detect the manganese directly. Analytical experiments
indicate that four Mn ions are associated with each oxygen-evolving complex.
Other experiments have shown that Cl– and Ca2+ ions are
essential for O2 evolution.
One electron carrier, generally
identified as Yz, functions between the oxygen-evolving complex and P680 . To
function in this region, Yz needs to have a very strong tendency to retain its
electrons. This species has been identified as a radical formed from a tyrosine
residue in the D1 protein of the PSII reaction center.
Pheophytin and Two Quinones Accept
Electrons from Photosystem II
Evidence from spectral and
ESR studies indicates that pheophytin acts
as an early acceptor in photosystem II, followed by a complex of two
plastoquinones in close proximity to an iron atom. Pheophytin is a
chlorophyll in which the central magnesium atom has been replaced by
two hydrogen atoms. This chemical change gives pheophytin chemical and
spectral properties that are slightly different from those of chlorophyll. The
precise arrangement of the carriers in the electron acceptor complex is not
known, but it is probably very similar to that of the reaction center of purple
bacteria.
Two plastoquinones (QA
and QB) are bound to the reaction center and receive electrons from
pheophytin in a sequential fashion . Transfer of the two electrons to QB
reduces it to QB 2–, and the reduced QB 2– takes two
protons from the stroma side of the medium, yielding a fully reduced plastohydroquinone
(QH2). The plastohydroquinone then dissociates from the reaction
center complex and enters the hydrocarbon portion of the membrane, where it in
turn transfers its electrons to the cytochrome b6 f complex.
Unlike the large protein complexes of the thylakoid membrane, hydroquinone is a
small, nonpolar molecule that diffuses readily in the nonpolar core of the
membrane bilayer.
Electron Flow through the
Cytochrome b6f Complex Also Transports Protons
The cytochrome b6
f complex is a large multisubunit protein with several prosthetic
groups .It contains two b-type hemes and one c-type heme (cytochrome
f ). In c-type cytochromes the heme is covalently attached to
the peptide; in b-type cytochromes the chemically similar protoheme
group is not covalently attached. In addition, the complex contains a Rieske
iron–sulfur protein (named for the scientist who discovered it), in which
two iron atoms are bridged by two sulfur atoms. The structures of cytochrome f
and the related cytochrome bc1 complex have been determined and
suggest a mechanism for electron and proton flow. The precise way by which
electrons and protons flow through the cytochrome b6 f complex is
not yet fully understood, but a mechanism known as the Q cycle accounts
for most of the observations. In this mechanism, plastohydroquinone (QH2)
is oxidized, and one of the two electrons is passed along a linear electron
transport chain toward photosystem I, while the other electron goes through a
cyclic process that increases the number of protons pumped across the membrane.
In the linear electron transport
chain, the oxidized Rieske protein (FeSR) accepts an electron from
plastohydroquinone (QH2) and transfers it to cytochrome f . Cytochrome
f then transfers an electron to the blue-colored copper protein
plastocyanin (PC), which in turn reduces oxidized P700 of PSI. In the cyclic
part of the process, the plastosemiquinone transfers its other electron to one
of the b-type hemes, releasing both of its protons to the lumenal side of the
membrane. The b-type heme transfers its electron through the second b-type
heme to an oxidized quinone molecule, reducing it to the semiquinone form near
the stromal surface of the complex. Another similar sequence of electron flow fully
reduces the plastoquinone, which picks up protons from the stromal side of the
membrane and is released from the b6 f complex as
plastohydroquinone.
The net result of two turnovers of
the complex is that two electrons are transferred to P700, two
plastohydroquinones are oxidized to the quinone form, and one oxidized plastoquinone
is reduced to the hydroquinone form. In addition, four protons are transferred
from the stromal to the lumenal side of the membrane.
By this mechanism, electron flow
connecting the acceptor side of the PSII reaction center to the donor side of
the PSI reaction center also gives rise to an electrochemical potential across
the membrane, due in part to H+ concentration differences on the two
sides of the membrane. This electrochemical potential is used to power the
synthesis of ATP. The cyclic electron flow through the cytochrome b and
plastoquinone increases the number of protons pumped per electron beyond what
could be achieved in a strictly linear sequence.
Plastoquinone and Plastocyanin Carry Electrons between Photosystems
II and I
The location of the two
photosystems at different sites on the thylakoid membranes requires that at least
one component be capable of moving along or within the membrane in order to
deliver electrons produced by photosystem II to photosystem I. The cytochrome b6
f complex is distributed equally between the grana and the stroma
regions of the membranes, but its large size makes it unlikely that it is the
mobile carrier. Instead, plastoquinone or plastocyanin or possibly both are
thought to serve as mobile carriers to connect the two photosystems. Plastocyanin
is a small (10.5 kDa), water-soluble, copper- containing protein that
transfers electrons between the cytochrome b6 f complex and P700.
This protein is found in the lumenal space . In certain green algae and
cyanobacteria, a c-type cytochrome is sometimes found instead of
plastocyanin; which of these two proteins is synthesized depends on the amount
of copper available to the organism.
The Photosystem I Reaction Center Reduces
NADP+
The PSI reaction center complex is
a large multisubunit complex (Figure 7.30) (Jordan et al. 2001). In contrast to
PSII, a core antenna consisting of about 100 chlorophylls is a part of the PSI
reaction center, P700. The core antenna and P700 are bound to two proteins,
PsaA and PsaB, with molecular masses in the range of 66 to 70 . The antenna
pigments form a bowl surrounding the electron transfer cofactors, which are in
the center of the complex. In their reduced form, the electron carriers that
function in the acceptor region of photosystem I are all extremely strong reducing
agents. These reduced species are very unstable and thus difficult to identify.
Evidence indicates that one of these early acceptors is a chlorophyll molecule, and another is a quinone
species, phylloquinone, also known as vitamin K1. Additional electron acceptors
include a series of three membrane-associated iron–sulfur proteins, or bound
ferredoxins, also known as Fe–S centers FeSX, FeSA, and FeSB
. Fe–S center X is part of the P700-binding protein; centers A and B reside on
an 8 kDa protein that is part of the PSI reaction center complex. Electrons are
transferred through centers Aand B to ferredoxin (Fd), a small, water-soluble
iron–sulfur protein.
The membrane-associated flavoprotein ferredoxin–NADP reductase
(FNR) reduces NADP+ to NADPH, thus completing the
sequence of noncyclic electron transport that begins with the oxidation
of water. In addition to the reduction of NADP+, reduced
ferredoxin produced by photosystem I has several other functions in
the chloroplast, such as the supply of reductants to reduce nitrate and
the regulation of some of the carbon fixation enzymes.
Cyclic Electron Flow Generates ATP
but no NADPH
Some of the cytochrome b6 f
complexes are found in the stroma region of the membrane, where photosystem
I is located. Under certain conditions cyclic electron flow from the
reducing side of photosystem I, through the b6 f complex
and back to P700, is known to occur. This cyclic electron flow is coupled to
proton pumping into the lumen, which can be utilized for ATP synthesis but does
not oxidize water or reduce NADP+. Cyclic electron flow is
especially important as an ATP source in the bundle sheath chloroplasts of some
plants that carry out C4 carbon fixation.
Some Herbicides Block Electron Flow
The use of herbicides to kill unwanted plants is widespread in
modern agriculture. Many different classes of herbicides have been developed,
and they act by blocking amino acid, carotenoid, or lipid biosynthesis or by
disrupting cell division. Other herbicides, such as DCMU
(dichlorophenyldimethylurea)
and paraquat, block photosynthetic electron flow . DCMU
is also known as diuron.
Paraquat has acquired public notoriety because of its use on marijuana crops. Many
herbicides, DCMU among them, act by blocking electron flow at the quinone
acceptors of photosystem II, by competing for the binding site of plastoquinone
that is normally occupied by QB. Other herbicides, such as paraquat, act by
accepting electrons from the early acceptors of photosystem I and then reacting
with oxygen to form superoxide, O2 –, a species that is very
damaging to chloroplast components, especially lipids.
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