Photophosphorylation

The production of ATP using the energy of sunlight is called photophosphorylation. Only two sources of energy are available to living organisms: sunlight and oxidation-reduction (redox) reactions. All organisms produce ATP, which is the universal energy currency of life.

In photophosphorylation, light energy is used to create a high-energy electron donor and a lower-energy electron acceptor. Electrons then move spontaneously from donor to acceptor through an electron transport chain.

Background
ATP is made by an enzyme called ATP synthase. The structure of this enzyme and its underlying gene is remarkably similar in all known forms of life. ATP synthase is powered by a transmembrane electrochemical potential gradient, usually in the form of a proton gradient. The function of the electron transport chain is to produce this gradient. In all living organisms, a series of redox reactions is used to produce a transmembrane electrochemical potential gradient, or a so called proton motive force (pmf).

Redox reactions are chemical reactions in which electrons are transferred from a donor molecule to an acceptor molecule. The underlying force driving these reactions is the Gibbs free energy of the reactants and products. The Gibbs free energy is the energy available (“free”) to do work. Any reaction that decreases the overall Gibbs free energy of a system will proceed spontaneously.

The transfer of electrons from a high-energy molecule (the donor) to a lower-energy molecule (the acceptor) can be spatially separated into a series of intermediate redox reactions. This is an electron transport chain.

The fact that a reaction is thermodynamically possible does not mean that it will actually occur. A mixture of hydrogen gas and oxygen gas does not spontaneously ignite. It is necessary either to supply an activation energy, or to lower the intrinsic activation energy of the system, in order to make most biochemical reactions proceed at a useful rate. Living systems use complex macromolecular structures (enzymes) to lower the activation energies of biochemical reactions.

It is possible to couple a thermodynamically favorable reaction (a transition from a high-energy state to a lower-energy state) to a thermodynamically unfavorable reaction (such as a separation of charges, or the creation of an osmotic gradient), in such a way that the overall free energy of the system decreases (making it thermodynamically possible), while useful work is done at the same time. Biological macromolecules that catalyze a thermodynamically favorable reaction if and only if a thermodynamically unfavorable reaction occurs simultaneously underlie all known forms of life.

Electron transport chains produce energy in the form of a transmembrane electrochemical potential gradient. This energy is used to do useful work. The gradient can be used to transport molecules across membranes. It can be used to do mechanical work, such as rotating bacterial flagella. It can be used to produce ATP and NADPH, high-energy molecules that are necessary for growth.

Photosynthetic electron transport chains in chloroplasts
The cells of all plants and photosynthetic algae contain chloroplasts, which produce ATP and NADPH using the energy of sunlight. Chloroplasts have a deep resemblance to certain eubacteria (cyanobacteria), which suggests that chloroplasts originated as intracellular symbionts in primitive eukaryotic cells.

The electron transport chain in chloroplasts is contained in two extremely complex transmembrane structures, Photosystem II (PS II) and Photosystem I (PS I). PS II and PS I are linked by a transmembrane proton pump, cytochrome b6f, which is similar to mitochondrial Complex III.

The overall process is the transfer of electrons from water to NADPH via a transmembrane proton pump:

H2O → PS II → plastoquinone → b6f → plastocyanin → PS I → NADPH

The resulting transmembrane proton gradient is used to make ATP via ATP synthase. NADPH is used by the Calvin cycle to make organic molecules from CO2.

Photosystem II
PS II is an extremely complex, highly organized transmembrane structure that contains a water-splitting complex, chlorophylls a and b, a reaction center (P680), pheophytin (a pigment similar to chlorophyll), and two quinones. It uses the energy of sunlight to transfer electrons from water to a mobile electron carrier in the membrane called plastoquinone:

H2O → P680 → P680* → plastoquinone

Plastoquinone, in turn, transfers electrons to b6f, which feeds them into PS I.

The water-splitting complex
The step H2O → P680 is performed by a poorly-understood structure embedded within PS II called the water-splitting complex or the oxygen-evolving complex. It catalyzes a reaction that splits water into electrons, protons and oxygen:

2H2O → 4H+ + 4e- + O2

The electrons are transferred to special chlorophyll molecules (embedded in PS II) that are promoted to a higher-energy state by the energy of photons.

The light-harvesting system
The light harvesting-system is composed of antenna proteins or antenna pigments which are complex structures embedded in the membrane that absorb energy from sunlight and transfer it to special chlorophyll molecules in PS II. There are hundreds of antenna proteins for every PS II. The transfer process (exciton transfer) is extremely efficient, due to the ability of antenna proteins to transfer their excitation energy to neighboring molecules in a quantum, all-or-none fashion. Most of the energy gathered by antenna proteins is ultimately transferred to special reaction center proteins P680 in PS II, which are promoted to a high-energy state.

The reaction center
The excitation P680 → P680*of the reaction center pigment P680 occurs here. These special chlorophyll molecules embedded in PS II absorb the energy of photons, with maximal absorption at 680 nm. Electrons within these molecules are promoted to a higher-energy state. This is one of two core processes in photosynthesis, and it occurs with astonishing efficiency (greater than 90%) because, in addition to direct excitation by light at 680 nm, the energy of light first harvested by antenna proteins at other wavelengths in the light-harvesting system is also transferred to these special chlorophyll molecules.

This is followed by the step P680*→ pheophytin, and then on to plastoquinone which occurs within the reaction center of PS II. High-energy electrons are transferred to plastoquinone. Plastoquinone is then released into the membrane as a mobile electron carrier.

This is the second core process in photosynthesis. The initial stages occur within picoseconds, with an efficiency of 100%. The seemingly impossible efficiency is due to the precise positioning of molecules within the reaction center. This is a solid-state process, not a chemical reaction. It occurs within an essentially crystalline environment created by the macromolecular structure of PS II. The usual rules of chemistry (which involve random collisions and random energy distributions) do not apply in solid-state environments.

Link of water splitting complex and chlorophyll excitation
When the chlorophyll passes the electron to pheophytin, it obtains an electron from P680*. In turn, P680* can oxidize the Z (or YZ) molecule. Once oxidized, the Z molecule can derive electrons from the water splitting complex.

Summary
PS II is a transmembrane structure found in all chloroplasts. It splits water into electrons, protons and molecular oxygen. The electrons are transferred to plastoquinone, which carries them to a proton pump. Molecular oxygen is released into the atmosphere.

The emergence of such an incredibly complex structure, a macromolecule that converts the energy of sunlight into potentially useful work with efficiencies that are impossible in ordinary experience, seems almost magical at first glance. Thus it is of considerable interest that essentially the same structure is found in purple bacteria.

Cytochrome b6f
PS II and PS I are connected by a transmembrane proton pump, cytochrome b6f complex (plastoquinol&mdash;plastocyanin reductase; . Electrons from PS II are carried by plastoquinone to b6f, where they are removed in a stepwise fashion and transferred to a water-soluble electron carrier called plastocyanin. This redox process is coupled to the pumping of four protons across the membrane. The resulting proton gradient (together with the proton gradient produced by the water-splitting complex in PS II) is used to make ATP via ATP synthase.

The similarity in structure and function between cytochrome b6f (in chloroplasts) and cytochrome bc1 (Complex III in mitochondria) is striking. Both are transmembrane structures that remove electrons from a mobile, lipid-soluble electron carrier (plastoquinone in chloroplasts; ubiquinone in mitochondria) and transfer them to a mobile, water-soluble electron carrier (plastocyanin in chloroplasts; cytochrome c in mitochondria). Both are proton pumps that produce a transmembrane proton gradient.

Photosystem I
PS I accepts electrons from plastocyanin and transfers them either to NADPH (noncyclic electron transport) or back to cytochrome b6f (cyclic electron transport):

plastocyanin → P700 → P700* → ferredoxin → NADPH ↑                           ↓                             b6f          ←          plastoquinone b6f

PS I, like PS II, is a complex, highly organized transmembrane structure that contains antenna chlorophylls, a reaction center (P700), phylloquinine, and a number of iron-sulfur proteins that serve as intermediate redox carriers.

The light-harvesting system of PS I uses multiple copies of the same transmembrane proteins used by PS II. The energy of absorbed light (in the form of delocalized, high-energy electrons) is funneled into the reaction center, where it excites special chlorophyll molecules (P700, maximum light absorption at 700 nm) to a higher energy level. The process occurs with astonishingly high efficiency.

Electrons are removed from excited chlorophyll molecules and transferred through a series of intermediate carriers to ferredoxin, a water-soluble electron carrier. As in PS II, this is a solid-state process that operates with essentially 100% efficiency.

There are two different pathways of electron transport in PS I. In noncyclic electron transport, ferredoxin carries the electron to the enzyme ferredoxin NADP+ oxidoreductase that reduces NADP+ to NADPH. Alternately, in cyclic electron transport, electrons from ferredoxin are transferred (via plastoquinone) to a proton pump, cytochrome b6f. They are then returned (via plastocyanin) to P700.

NADPH and ATP are used to synthesize organic molecules from CO2. The ratio of NADPH to ATP production can be adjusted by adjusting the balance between cyclic and noncyclic electron transport.

It is noteworthy that PS I closely resembles photosynthetic structures found in green sulfur bacteria, just as PS II resembles structures found in purple bacteria.

Photosynthetic electron transport chains in bacteria
PS II, PS I and cytochromeb6f are found in chloroplasts. All plants and all photosynthetic algae contain chloroplasts, which produce NADPH and ATP by the mechanisms described above. Essentially the same transmembrane structures are also found in cyanobacteria.

Unlike plants and algae, cyanobacteria are prokaryotes. They do not contain chloroplasts. Rather, they bear a striking resemblance to chloroplasts themselves. This suggests that organisms resembling cyanobacteria were the evolutionary precursors of chloroplasts. One imagines primitive eukaryotic cells taking up cyanobacteria as intracellular symbionts.

Cyanobacteria
Cyanobacteria contain structures similar to PS II and PS I in chloroplasts. Their light-harvesting system is different from that found in plants (they use phycobilins, rather than chlorophylls, as antenna pigments), but their electron transport chain

H2O → PS II → plastoquinone → b6f → cytochrome c6 → PS I → ferredoxin →  NADPH ↑                    ↓                                                  b6f       ←     plastoquinone

is essentially the same as the electron transport chain in chloroplasts. The mobile water-soluble electron carrier is cytochrome c6 in cyanobacteria, plastocyanin in plants.

Cyanobacteria can also synthesize ATP by oxidative phosphorylation, in the manner of other bacteria. The electron transport chain is

NADH dehydrogenase → plastoquinone → b6f → cytochrome c6 → cytochrome aa3 → O2

where the mobile electron carriers are plastoquinone and cytochrome c6, while the proton pumps are NADH dehydrogenase, b6f and cytochrome aa3.

Cyanobacteria are the only bacteria that produce oxygen during photosynthesis. The Earth’s primordial atmosphere was anoxic. Organisms like cyanobacteria produced our present-day oxygen containing atmosphere.

The other two major groups of photosynthetic bacteria, purple bacteria and green sulfur bacteria, contain only a single photosystem and do not produce oxygen.

Purple bacteria
Purple bacteria contain a single photosystem that is structurally related to PS II in cyanobacteria and chloroplasts:

 P870 → P870* → ubiquinone → bc1 → cytochrome ''c2 → P870

This is a cyclic process in which electrons are removed from an excited chlorophyll molecule (bacteriochlorophyll; P870), passed through an electron transport chain to a proton pump (cytochrome bc1 complex, similar but not identical to cytochrome bc1 in chloroplasts), and then returned to the cholorophyll molecule. The result is a proton gradient, which is used to make ATP via ATP synthase. As in cyanobacteria and chloroplasts, this is a solid-state process that depends on the precise orientation of various functional groups within a complex transmembrane macromolecular structure.

In order to make NADPH, purple bacteria use an external electron donor (hydrogen, hydrogen sulfide, sulfur, sulfite, or organic molecules such as succinate and lactate) to feed electrons into a reverse electron transport chain.

Green sulfur bacteria
Green sulfur bacteria contain a photosystem that is analogous to PS I in chloroplasts:

 P840 → P840* → ferredoxin → NADH ↑               ↓                            cyt c553 ← bc1 ← menaquinone

There are two pathways of electron transfer. In cyclic electron transfer, electrons are removed from an excited chlorophyll molecule, passed through an electron transport chain to a proton pump, and then returned to the chlorophyll. The mobile electron carriers are, as usual, a lipid-soluble quinone and a water-soluble cytochrome. The resulting proton gradient is used to make ATP.

In noncyclic electron transfer, electrons are removed from an excited chlorophyll molecule and used to reduce NAD+ to NADH. The electrons removed from P840 must be replaced. This is accomplished by removing electrons from H2S, which is oxidized to sulfur (hence the name “green sulfur bacteria”).

Purple bacteria and green sulfur bacteria occupy relatively minor ecological niches in the present day biosphere. They are of interest because of their importance in precambrian ecologies, and because they were the evolutionary precursors of modern plants.

Links

 * Photosynthesis: The Role of Light contains a description of the molecules present in photosystems I and II, and in the light harvesting complexes.


 * Photosynthetic Antennas and Reaction Centers: Current Understanding and Prospects for Improvement

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