cen-ter pigments from photodestruction.13 (For a further explanation, see note 14.)
Electron transfer in the reaction center The sequence of redox reactions in the reaction center summarized in Fig. 6.1A (boxed area) is shown in more detail in Fig. 6.3, along with the proposed time scale for the electron transfer events. The terminal electron acceptors in the reaction center (Fig. 6.1) are quinines, which are called type II reaction centers and are found in purple bacteria (Proteobacteria) and green nonsulfur bacteria (Chlorofl exus group). (Type I reaction centers use iron–sulfi de clusters as the terminal electron acceptor, and these are present in green sulfur bacteria and heliobac-teria, as discussed later.) As shown in Fig. 6.3, the arrangement of the pigment molecules and the quinones reveals twofold symmetry, with right and left halves that are very similar. On the periplasmic side of the membrane there sits a pair of bacteriochlorophyll molecules (Bchl)2 (Fig. 6.3). These bacteriochlorophyll mole-cules are P870 in Fig. 6.1. When the energy from a quantum of light is absorbed by (Bchl)2, an electron becomes excited [i.e., (Bchl)2 becomes (Bchl)2*] (Fig. 6.3, step 1). The redox potential of (Bchl)2* is low enough to reduce bacteriochlo-rophyll A (BchlA), forming (Bchl)2− and (Bchl)A− (Fig. 6.3, step 2). The electron then moves to bacteriopheophytinA (BpheoA), forming BpheoA− (Fig. 6.3, step 3). [As refl ected by the question mark in step 2 of Fig. 6.3, the exact route of the electron is not known. It has not
Fig. 6.3 The reaction center in purple photosynthetic bacteria. The reaction center, which spans the mem-brane, is represented by the boxed areas. The absorp-tion of light creates a transient membrane potential, positive (P-phase) on the periplasmic side and negative (N-phase) on the cytoplasmic side. Step 1: absorption of a photon of light energizes a bacteriochlorophyll dimer (Bchl)2. Step 2: the energized (Bchl)2* reduces BchlA; the question mark indicates that this has not been unequivocally demonstrated. Step 3: bacterio-pheophytin A (BpheoA) is reduced. Step 4: the electron is transferred to a quinone (UQA). Step 5: the oxidized (Bchl)2 is reduced via cytochrome c2. Step 6: the elec-tron moves from UQA to UQB. Steps 1 to 6 are repeated, leading to the formation of UQB2–. Two protons are acquired from the cytoplasm to produce UQH2, which enters the reduced quinone pool in the membrane and returns the electrons to oxidized cytochrome c2 via the bc1 complex. It may be that one proton is acquired by UQB– and the second proton is acquired when the sec-ond electron arrives. Source: Nicholls, D. G., and S.
J. Ferguson. 1992. Bioenergetics 2. Academic Press, London.
been unequivocally demonstrated that the elec-tron moves through the BchlA monomer. Some researchers suggest that the electron travels from Bchl2* to BchlA and very quickly moves on to BpheoA, whereas other investigators pos-tulate that the electron moves directly from (Bchl*)2 to BpheoA.] Then the electron moves to ubiquinone bound to site A on the cytoplas-mic side of the reaction center (UQA) forming the semiquinone, UQ−A (Fig. 6.3, step 4). At this time an electron is returned to (Bchl)2+ from reduced cytochrome c2, which is periplasmic (step 5). Then, the electron is transferred from
A−
UQ to UQB to form UQB− (Fig. 6.3, step 6).
Electron fl ow is therefore across the membrane from periplasmic c2 to UQB located on the cytoplasmic side. This generates a membrane potential, outside positive. A second light reac-tion occurs, and steps 1 through 6 in Fig. 6.3 are repeated so that UQB has two electrons (UQ )2A− . Two protons are picked up from the cytoplasm to form UQH2, which leaves the reaction cen-ter to join the quinone pool in the membrane.
(Although Fig. 6.3 indicates that two protons enter after both electrons have arrived at UQB, it has been suggested that one proton enters after UQB has received the fi rst electron.) Eventually, the protons carried by UQH2 are released on the periplasmic side during oxidation of UQH2 by the bc1 complex.
Thus, the reaction center and the bc1 complex cooperate to translocate protons to the outside.
As explained earlier, UQB is called a two-elec-tron gate because it cannot leave the reaction center until it has accepted two electrons. The length of time that it takes the electron to travel from (Bchl)2 to UQA is a little more than 200 ps.
[One picosecond (ps) is one trillionth of a sec-ond, or 10–12 s.] The rates of subsequent steps are slower (but still very fast), being measured in microseconds. [One microsecond (1 μs) is a millionth of a second, 10–6 s.]
Determining the pattern and timing of electron transfer in the reaction center requires the use of picosecond laser pulses and very rapid recording of the absorption spectra of the electron carriers.
Interestingly, only one side of the reaction center (the A side) appears to be involved in electron transport. For example, there is photoreduction of only one of the bacteriopheophytin mole-cules. Why only one branch appears to function in electron transport is not known.
Th e contribution to the ∆p by the reaction center
When reaction centers absorb light energy, a ∆Ψ and a proton gradient are created. The reason for the ∆Ψ is that the electrons move electrogenically from cytochrome c2 to UQB across the membrane from outside to inside (Fig. 6.2). (This will produce a ∆Ψ, outside positive, because the inward movement of a negative charge is equivalent to the outward movement of a positive charge.) The creation of the membrane potential is detected by a shift in the absorption spectra of membrane carotenoids. (For additional explanation, see note 15.) The value of this technique is that very rapid changes in membrane potential can be monitored. The ∆Ψ produced by the reaction center is delocalized, and increases in the ∆Ψ are made in the bc1 complex. Additionally, two protons are taken from the cytoplasm to reduce UQB to UQH2. Eventually, the two protons will be released on the outside during the oxidation of UQH2 by the bc1 complex. Thus, the reaction center contributes to both the ∆Ψ and the ∆pH components of the ∆p, as well as creating a ∆E between UQ/UQH2 and c2,ox/c2,red.
Summary of photosynthesis by purple photosynthetic bacteria
light and Bchl
i 2
ADP P+ ⎯⎯⎯⎯⎯→ ATP H O+ 6.2.3 Source of electrons for growth NADH and NADPH are important sources of electrons for growth but serve separate func-tions. NADH donates electrons to the respi-ratory chain, resulting in ATP synthesis, and NADPH donates electrons in biosynthetic reac-tions. Although they serve different functions, interconversion is possible via phosphorylation of NAD+, dephosphorylation of NADP+, and transhydrogenation. When the electron donor is of a higher potential than the NAD+/NADH couple, energy is required to reduce NAD+. For example, this is the case for the purple photo-synthetic bacteria that use certain inorganic sulfur compounds or succinate as a source of electrons. The purple photosynthetic bacte-ria use the ∆p created by light energy to drive electron transport in reverse (Fig. 6.4). During reversed electron fl ow, ubiquinol reduces NAD+ via the NADH:ubiquinone oxidoreductase.
(See Section 4.7.1 for a discussion of reversed electron transport.)
Figure 6.4 illustrates the situation in which light energizes the creation of a ∆p (proton motive force), which in turn energizes the synthesis of ATP as well as reversed electron fl ow to reduce NAD+.However, another scenario has been suggested, which is useful to consider because it illustrates the role that an increase in the ∆p can play in slowing the rate of electron transfer.
It should be recalled that ubiquinone can both accept electrons from bacteriopheophytin dur-ing cyclic electron transport and reduce the bc1 complex, which generates a ∆p (Fig. 6.1). It has been suggested that as the ∆p grows larger, it might exert “back-pressure” on the oxidation of ubiquinol by the bc1 complex, thus slowing down the oxidation of ubiquinol via this route and making it available for NAD+ reduction via reversed electron transport.7 To the extent that this might occur, the electron donor (succinate or inorganic sulfur compounds) would replen-ish electrons to the bacteriochlorophyll via either ubiquinone or cytochrome c.
6.3 Th e Green Sulfur Bacteria (Chlorobiaceae)
6.3.1 Photosynthetic electron transport The green sulfur photosynthetic bacteria and the heliobacteria have reaction centers that are distinguished from the reaction centers of the purple photosynthetic bacteria in two respects, as detailed in the subsections that follow.
Possessing iron–sulfur centers as terminal electron acceptors
Reactions centers whose terminal electron acceptors are iron–sulfur centers are called type I to distinguish them from the reactions centers in the purple photosynthetic bacteria, which use quinones as the terminal electron acceptors and are called type II.
Reducing NAD(P)+ instead of quinone
The reaction centers of green sulfur bacteria are similar to the type I reaction centers of cyanobac-teria and chloroplasts. However, the principles underlying the transformation of electrochemi-cal energy into a ∆E and a membrane potential are the same as for the purple photosynthetic bacteria.16,17 Light energizes a bacteriochloro-phyll a molecule (P840), which reduces a primary acceptor (A0), establishing a redox potential dif-ference greater than 1 V (Fig. 6.5). The primary electron acceptor (A0) in the green sulfur bacte-ria has recently been reported to be an isomer of chlorophyll a called bacteriochlorophyll 663.18 It might be added that the primary acceptor of heliobacteria is also a chlorophyll a derivative, hydroxychlorophyll a, refl ecting the similari-ties known to exist between the reaction centers of the green sulfur bacteria, heliobacteria, and photosystem I of chloroplasts and cyanobacte-ria. (See note 19.) The electron fl ows from A0 to a quinonelike acceptor called A1. The electron is then transferred from A1 through three iron–
sulfur centers to ferredoxin. There can be both cyclic and noncyclic electron fl ow. In cyclic fl ow the electron returns to the reaction center via menaquinone (MQ) and a bc1 complex, creat-ing a ∆p. In noncyclic electron fl ow inorganic sulfur donates electrons that travel through the reaction center to NAD+.
According to the scheme in Fig. 6.5, electrons from inorganic sulfur enter at the level of cyto-chrome c and the bc1 complex is bypassed. This is a widely held view based upon available data obtained by using isolated oxidoreductases.
From an energetic point of view, however, it is wasteful because a coupling site is bypassed, even though the E′ values of some of the sulfur m couples are low enough to reduce menaquinone.
For example, the E′ for sulfur/sulfi de (n = 2) is m –0.27; for sulfi te/sulfi de (n = 6) it is –0.11 V; and for sulfate/sulfi te (n = 2) it is –0.54 V. All these couples are at a potential low enough to reduce Fig. 6.4 Relationship between cyclic electron fl ow
and reversed electron transport in the purple pho-tosynthetic bacteria. The electron is driven by light from a cytochrome c2 (c2) to a ubiquinone (UQ) and then returns via electron carriers to the cytochrome c2 with production of a ∆p. The ∆p is used to drive ATP synthesis as well as reversed electron transport.
fl ow also occurs in the green sulfur bacteria, as described in Section 6.3.1.)
6.4.1 Two light reactions
As we shall see, chloroplasts and cyanobacteria have essentially combined the light reactions of purple photosynthetic bacteria and green sulfur photosynthetic bacteria in series, so that two light reactions energize a single electron that energizes ATP synthesis and reduces NADP+. The initial evidence for two light reactions came from early studies of photosynthesis performed with algae by Emerson and his colleagues.21,22 They observed that the effi ciency of photosyn-thesis, measured as the moles of oxygen evolved per einstein absorbed (i.e., the quantum yield) is high over all the wavelengths absorbed by chlorophyll and the light-harvesting pigments, but it drops off sharply at 685 nm even though chlorophyll continues to absorb light between 680 and 700 nm (Fig. 6.6). This became known as the “red drop” effect because 700 nm light is red. One can restore the effi ciency of pho-tosynthesis of 700 nm light by supplementing menaquinone, which has an E′ of –0.074 V. m
However, the point of entry of the electron is still an unresolved issue.20
6.3.2 Summary of photosynthesis by green sulfur bacteria
+
+
+ + + ⎯⎯⎯⎯⎯→
° + + +
light and Bchl
2 i
H S NAD ADP P S NADH H ATP
(These organisms can also use elemental sulfur, oxidizing it to sulfate.)
6.4 Cyanobacteria and Chloroplasts
Photosynthesis in cyanobacteria and chloro-plasts differs from photosynthesis discussed thus far in three important respects:
1. H2O is the electron donor and oxygen is evolved.
2. There are two light reactions in series, hence