Método directo de diseño, .1 Limitaciones

+

+ + + ⎯⎯⎯⎯⎯→

° + + +

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 two different reaction centers.

3. Electron fl ow is primarily noncyclic, pro-ducing both ATP and NADPH. (Noncyclic

Fig. 6.5 A model for electron fl ow in the green sulfur bacteria. Both cyclic and noncyclic fl ows of electrons are possible. Reaction center bacteriochlorophyll a (P₈₄₀) becomes energized and reduces A₀, which is bacte-riochlorophyll 663. The electron then travels to A₁, a quinonelike molecule, and then through two or three iron–sulfur centers (FeS). The fi rst reduced product outside the reaction center is the iron–sulfur protein ferre-doxin (Fd). The ferreferre-doxin reduces NAD(P)⁺ in the noncyclic pathway. Cyclic fl ow occurs when the electron reduces menaquinone (MQ) instead of NAD(P)⁺ and returns to the reaction center via a bc₁ complex and cytochrome c₅₅₅. A ∆p is created in the bc₁ complex. In the noncyclic pathway the electron donor is a reduced inorganic sulfur compound, here shown as hydrogen sulfi de. Elemental sulfur or thiosulfate can also be used.

The inorganic sulfur is oxidized by cytochrome c₅₅₅, which feeds electrons into the reaction center.

it with light at shorter wavelengths (e.g., 600 nm). This is explained by pointing out that there are two reaction centers, one energized by light at a wavelength of around 700 nm (reac-tion center I, or RC I) and one energized by lower wavelengths of light (reaction center II, RC II). These are also called photosystems I and II (PS I and PS II). Photosystems I and II oper-ate in series, and therefore both must be ener-gized to maintain the electron fl ow from water to NADP⁺. The lower wavelengths of light can energize both reaction centers, but 700 nm light can energize only reaction center I. It is for this reason that 700 nm light is effective only when given in combination with supplemental doses of shorter wavelengths.

6.4.2 Photosynthetic electron transport A schematic drawing of the overall pattern of electron fl ow in cyanobacteria and chloroplasts is shown in Fig. 6.7. These systems have two reaction centers, PS I and PS II, connected by a short elec-tron transport chain that includes the analogue to the bc₁ complex (i.e., the b₆f complex) that carries out a Q cycle similar to that carried out by the bc₁ complex discussed in Chapter 5. (For a description of the b₆f complex, see ref. 23 and note 24.) Let us begin with PS II, in which the pathway of electron fl ow is the same as in the type II reaction center in the purple photosynthetic bacteria (see Fig. 6.1A).

Chlorophyll a, with a major absorption peak at 680 nm (i.e., P₆₈₀—probably a dimer), becomes energized and reduces an acceptor molecule, which is pheophytin (pheo). Having lost an electron, the P /P₆₈₀⁺ ₆₈₀ couple has a redox potential (esti-mated at about +1.1 V) high enough to replace the lost electron with one from water, thus oxidizing

12H O2 ^{to 1}4O2. The energized electron travels to pheophytin, which has an Em′ of about –0.6 V. The pheophytin reduces plastoquinone (PQ), which is structurally similar to ubiquinone (Fig. 5.3D).

A two-electron gate, similar to the two-elec-tron gate in the reaction center of the purple photosynthetic bacteria, operates during qui-none reduction. Electrons leave the reaction center in PQH₂ and are transferred to a copper-containing protein called plastocyanin (Pc) through a b₆f complex (structurally and func-tionally similar to the bc₁ complex, except that cytochrome f, which has a c-type heme, replaces cytochrome c₁ and there are some differences in the cytochrome b). A ∆p is created by the b6f complex via a Q cycle similar to the respiratory bc₁ complex. The plastocyanin reduces P₇₀₀⁺ in reaction center I, previously oxidized by the light reaction we shall describe next.

Photosystem I is similar to the type I reaction center in the green photosynthetic bacteria (see Fig. 6.5). A photon of light energizes P₇₀₀, which is chlorophyll a with a major absorption peak Fig. 6.6 Quantum yield of photosynthesis. When the rate of O₂ evolution per quantum absorbed is plotted against wavelength, the rate drops off sharply above 680 nm. Source: After Stryer, L. 1988. Biochemistry. W.

H. Freeman, New York.

at 700 nm. The P^*₇₀₀ reduces a chlorophyll a mol-ecule, called A₀. For this initial redox reaction, the energized electron travels from an E_m′ ^of about +0.5 V (P /P₇₀₀⁺ ₇₀₀^{) to an} Em′ of about –1.0 V(A /A ). Note that this is a far more negative _{0 0}^– potential than that generated in reaction center II. From A₀ the electrons travel to A₁, which is a phylloquinone. Phylloquinones have a struc-ture similar to menaquinone (Fig. 5.3), but with only one double bond in the isoprenoid chain.

The electron is transferred from the quinone through several iron–sulfur centers (FeS), which reduce the iron–sulfur protein ferredoxin (Fd) that is outside the reaction center. Ferredoxin in turn reduces NADP⁺. Thus, the two light reac-tions in series energize electron fl ow from H₂O to NADP⁺, which is over a net potential differ-ence of about 1.1 V. This is called noncyclic electron fl ow because the electron never returns to the reaction center. However, cyclic electron fl ow is possible.

There is a branch point at the ferredoxin step, and it is possible for the electron to cycle back to reaction center I via the b₆f complex, augment-ing the ∆p, rather than reducing NADP⁺. This may be a way to increase the amounts of ATP made relative to NADPH. The Calvin cycle,

which is the pathway for reducing CO₂ to car-bohydrate in oxygenic phototrophs, requires three ATPs for every two NADPHs to reduce one CO₂ to the level of carbohydrate.

6.4.3 Summary of photosynthesis by green plants, algae, and cyanobacteria

light and Chl

2 i

1 2 2

H O NADP ADP P

O NADPH H ATP

+

+

+ + + ⎯⎯⎯⎯⎯→

+ + +

6.5 Effi ciency of Photosynthesis

In this section, as an exercise, we calculate the effi ciencies of photosynthesis based upon input light energy and products of photosynthesis.

The calculated effi ciencies are only approxima-tions based upon assumpapproxima-tions regarding ATP yields, actual redox potentials, standard free energies, and so on.

6.5.1 ATP synthesis

Basically, photosynthesis is work done by ener-gized electrons. The work is the phosphoryla-tion of ADP and the reducphosphoryla-tion of NAD(P)⁺. Each electron is energized by a photon (quantum) of light, and each mole of electrons by an einstein (6.023 × 10²³ quanta) of light. It is instructive Fig. 6.7 Photosynthesis in cyanobacteria and chloroplasts. Light stimulates electron fl ow from water through reaction center II (RC II) to reaction center I (RC I) to NADP⁺. Cyclic fl ow is possible by using RC I from ferredoxin through the b₆f complex. Abbreviations: P₆₈₀, chlorophyll a with a major absorption peak at 680 nm; Pheo, chlorophyll pheophytin; PQ, plastoquinone; b₆f, cytochrome b₆f complex; Pc, plastocyanin; P₇₀₀, chlorophyll a with a major absorption peak at 700 nm; A₀, chlorophyll a; A₁, phylloquinone; FeS, one of sev-eral iron–sulfur centers; Fd, ferredoxin.

to ask how much of this energy is conserved in ATP and NADPH.

Let us consider the synthesis of ATP. Assume that each energized electron results in the trans-location of two protons (e.g., during the Q cycle in the bc₁ complex) but that three protons must reenter through the ATP synthase to make one ATP. As mentioned before, a value of 3H⁺/ATP is reasonable in light of experimental data. Thus, each energized electron (or each photon) results in the synthesis of two-thirds of an ATP. How much energy is required to synthesize two-thirds of an ATP? The ∆G_p for the phosphorylation of one mole of ADP to make ATP is about 45 kJ.

Therefore, two-third of a mole of ATP should require approximately +45( ) kJ ²₃ = 30 kJ. An einstein of 870 nm light, which corresponds to the absorption maximum of bacteriochloro-phyll a (found in purple phototrophs), has 138 kJ of energy. Therefore, the effi ciency is (30/138) (100), or about 22%. One can also calculate the effi ciency by using electron volts instead of joules. The energy in a photon of light at 870 nm is 1.43 eV. The synthesis of two-thirds of a mole of ATP requires 30,000 J. Dividing this number by the Faraday constant gives the energy in elec-tron volts, (i.e., 0.31 eV).

6.5.2 ATP and NADPH synthesis

What about photosystems that reduce NADP⁺ as well as make ATP (i.e., photosystems I and II)? The Em′ ^{for O}2/H₂O is +0.82 V, and for NADP⁺/NADPH it is –0.32 V. Therefore, the energized electron must have 0.82 – (–0.32), or 1.14 eV, to move from water to NADP⁺. (The answer would not be very different if E_h values were used instead of Em′ .) If two-thirds of an ATP is made, then a total of 0.31 + 1.14 = 1.45 eV would be required to make two-thirds of an ATP and half an NADPH. A photon of light at a wavelength of 680 nm (the major long-wave-length absorption peak of chlorophyll a in reac-tion center II) has 1.82 eV. Two photons, or the equivalent of about 3.6 eV, are used. Therefore, approximately 40% of the light energy is con-served as ATP and NADPH.

6.5.3 Carbohydrate synthesis and oxygen production

One can also estimate the approximate effi -ciency by considering the number of light quanta required to produce oxygen:

6CO₂ + 12H₂O C₆H₁₂O₆+ 6H₂O + 6O₂

∆G′₀ = 2,870 kJ

Therefore, for every mole of O₂ produced, the standard free energy requirement at pH 7 is 2,870/6, or 478 kJ. The number of einsteins of light required to produce one mole of O₂ is 8.

An einstein of 680 nm light carries 176 kJ of energy. Therefore, 176 × 8, or 1,408 kJ of light energy, is used to produce one mole of O₂. The effi ciency is thus (478/1,408)(100), or 34%.

6.6 Photosynthetic Pigments

The photosynthetic pigments are divided into two categories: reaction center pigments (pri-marily chlorophylls) and light-harvesting pig-ments (carotenoids, phycobilins, chlorophylls).

The light-harvesting pigments are sometimes called accessory pigments or antennae pigments.

By far, most of the photosynthetic pigments are light-harvesting pigments. The light-harvesting pigments are critical to photosynthesis because they absorb light of different wavelengths and funnel the energy to the reaction center. Figure 6.8 shows whole-cell absorption spectra of a purple photoysynthetic bacterium and a cyanobacte-rium, including the absorption wavelengths of the pigments; the various pigments and their absorption peaks are listed in Table 6.2. The nature of the light-harvesting pigments will vary with the type of organism, but some important generalizations about them can be made:

1. They absorb light at wavelengths different