Photophosphorylation: Anoxygenic*#
- Page ID
- 21320
Photophosphorylation
Photophosphorylation an overview
Photophosphorylation is
As electrons pass from one electron carrier to another via red/ox reactions, enzymes can couple these exergonic electron transfers to the endergonic transport (or pumping) of protons across a membrane to create an electrochemical gradient. This electrochemical gradient generates a proton motive force (PMF). Enzymes can couple the exergonic drive of these protons to reach equilibrium to the endergonic production of ATP, via ATP synthase. As we will see in more detail, the electrons involved in this electron transport chain can have one of two fates: (1) they may return to their initial source in a process called cyclic photophosphorylation; or (2) they can transfer onto a close relative of NAD+ called NADP+. If electrons return to the original pigment in a cyclic process, the whole process can start over. If, however, the electron transfers onto NADP+ to form NADPH (**shortcut note—we didn't explicitly mention any protons but assume that they
What happens when a compound absorbs a photon of light?
When a compound absorbs a photon of light, the compound is said to leave its ground state and become "excited".
What are the fates of the "excited" electron? There are four
- The
e - can relax to a lower quantum state, transferring energy as heat. - The
e - can relax to a lower quantum state and transfer energy into a photon of light—a process known as fluorescence. The energy can be transferred by resonance to a neighboring molecule as thee - returns to a lower quantum state.- The energy can change the reduction potential such that the molecule can become an
e - donor. Linking this excitede - donor to a propere - acceptor can lead to an exergonic electron transfer.The excited state can be involved in red/ox reactions.
As the excited electron decays back to its lower energy state, it can transfer its energy in a variety of ways. While many so-called antenna or auxiliary pigments absorb light energy and transfer it to something known as a reaction center (by mechanisms depicted in option III in Figure 2), it is what happens at the reaction center that we are most concerned with (option IV in the figure above). Here a chlorophyll or bacteriochlorophyll molecule absorbs a photon's energy, and an electron is excited. This energy transfer suffices to allow the reaction center to donate the electron in a red/ox reaction to a second molecule. This starts the electron transport reactions. The result is an oxidized reaction center that must now
Simple photophosphorylation systems: anoxygenic photophosphorylation
Early in the evolution of photophosphorylation, these reactions evolved in anaerobic environments where there was very little molecular oxygen available. Two sets of reactions evolved under these conditions, both directly from anaerobic respiratory chains as described previously. We know these as the light reactions because they require the activation of an electron (an "excited" electron) from the absorption of a photon of light by a reaction center pigment, such as bacteriochlorophyll. We classify the light reactions either as cyclic or as noncyclic photophosphorylation, depending upon the final state of the electron
oxidized form |
|
|
Eo´ (volts) |
---|---|---|---|
PS1* (ox) |
PS1* (red) |
- |
-1.20 |
ferredoxin (ox) version 1 |
ferredoxin (red) version 1 |
1 |
-0.7 |
PSII* (ox) |
PSII* (red) |
- |
-0.67 |
P840* (ox) |
PS840* (red) |
- |
-0.67 |
acetate |
acetaldehyde |
2 |
-0.6 |
CO2 |
Glucose |
24 |
-0.43 |
ferredoxin (ox) version 2 |
ferredoxin (red) version 2 |
1 |
-0.43 |
CO2 |
formate |
2 |
-0.42 |
2H+ |
H2 |
2 |
-0.42 (at [H+] = 10-7; |
NAD+ + 2H+ |
NADH + H+ |
2 |
-0.32 |
NADP+ + 2H+ |
NADPH + H+ |
2 |
-0.32 |
Complex I FMN (enzyme bound) |
FMNH2 |
2 |
-0.3 |
|
|
2 |
-0.29 |
FAD+ (free) + 2H+ |
FADH2 |
2 |
-0.22 |
Pyruvate + 2H+ |
lactate |
2 |
-0.19 |
FAD+ + 2H+ (bound) |
FADH2 (bound) |
2 |
0.003-0.09 |
CoQ (Ubiquinone - UQ + H+) |
UQH. |
1 |
0.031 |
UQ + 2H+ |
UQH2 |
2 |
0.06 |
Plastoquinone; (ox) |
Plastoquinone; (red) |
- |
0.08 |
Ubiquinone; (ox) |
Ubiquinone; (red) |
2 |
0.1 |
Complex III Cytochrome b2; Fe3 |
Cytochrome b2; Fe2 |
1 |
0.12 |
Complex III Cytochrome c1; Fe3 |
Cytochrome c1; Fe2 |
1 |
0.22 |
Cytochrome c; Fe3 |
Cytochrome c; Fe2 |
1 |
0.25 |
Complex IV Cytochrome |
Cytochrome |
1 |
0.29 |
1/2 O2 + H2O |
H2O2 |
2 |
0.3 |
P840GS (ox) |
PS840GS (red) |
- |
0.33 |
Complex IV Cytochrome a3; Fe3 |
Cytochrome a3; Fe2 |
1 |
0.35 |
Ferricyanide |
ferrocyanide |
2 |
0.36 |
Cytochrome f; Fe3 |
Cytochrome f; Fe2 |
1 |
0.37 |
PSIGS (ox) |
PSIGS (red) |
. |
0.37 |
Nitrate |
nitrite |
1 |
0.42 |
Fe3+ |
Fe2+ |
1 |
0.77 |
1/2 O2 + 2H+ |
H2O |
2 |
0.816 |
PSIIGS (ox) |
PSIIGS (red) |
- |
1.10 |
* Excited State, after absorbing a photon of light GS Ground State, PS1: Oxygenic photosystem I P840: Bacterial reaction center containing bacteriochlorophyll ( PSII: Oxygenic photosystem II |
Cyclic photophosphorylation
In cyclic photophosphorylation the
Figure 4. Cyclic electron flow. The reaction center P840 absorbs light energy and becomes excited, denoted with an *. The excited electron
Possible NB Discussion Point
The figure of cyclic photophosphorylation above depicts the flow of electrons in a respiratory chain. How does this process help generate ATP? Why might running the process in a cyclical fashion be advantageous for a cell?
Noncyclic photophosphorylation
In cyclic photophosphorylation, electrons cycle from bacteriochlorophyll (or chlorophyll) to a series of electron carriers and eventually back to bacteriochlorophyll (or chlorophyll); there is theoretically no net loss of electrons and they stay in the system. In noncyclic photophosphorylation, electrons leave from the photosystem and red/ox chain and eventually end up on NADPH. That means there needs to be a source of electrons, a source that has a smaller reduction potential than bacteriochlorophyll (or chlorophyll) that can donate electrons to
Noncyclic electron flow
Figure 5. Noncyclic electron flow. In this example, the P840 reaction center absorbs light energy and becomes energized; the emitted electron reduces a
We note that for bacterial photophosphorylation pathways, for each electron donated from a reaction center [remember only one electron is actually donated to the reaction center (or chlorophyl molecule)], the resulting output from that electron transport chain is either the formation of NADPH (requires two electrons) or ATP can be made but NOT not both. The path the electrons take in the ETC can have one or two outcomes. This puts limits on the versatility of the bacterial anoxygenic photosynthetic systems. But what would happen if a process evolved that used both systems? More precicely, a cyclic and noncyclic photosynthetic pathway which could form both ATP and NADPH from a single input of electrons? A second limitation is that these bacterial systems require compounds such as reduced sulfur to act as electron donors to reduce the oxidized reaction centers, but they are not necessarily widely found compounds. What would happen if a chlorophyll