General Microbiology - Schlegel H. 1987

Phototrophic bacteria and photosynthesis
Primary processes of photosynthesis

Photosynthesis is defined as The conversion of light energy into biochemically accessible energy (ATP) and reducing power [NAD(P)H₂] within the Cells of phototrophic organisms, coupled with the synthesis of cellular components. Photosynthetic phosphorylation and photosynthetic pyridine nucleotide reduction are the processes that lead to The formation of the first stable products of photosynthesis.

This understanding was reached through experimental and theoretical studies, based primarily on comparisons of photosynthesis in PHOTOTROPHIC Bacteria AND green plants. After Winogradsky (1888) established that light is not always the energy source for CO₂ assimilation in certain bacteria, and Engelmann (1883–1888), on The basis of his physiological research, classified purple bacteria as phototrophic

organisms, Buder (1919) demonstrated that this group of bacteria possesses a novel, previously unknown type of METABOLISM. Sulfur and non-sulfur purple bacteria assimilate CO₂ or organic substances in the light. However, this type of photosynthesis differs substantially from that of higher plants: 1) Water cannot serve as a hydrogen donor here, and therefore such photosynthesis is not accompanied by the evolution of O₂; 2) bacteria use H₂S or organic substances rather than water as hydrogen Donors.

Quantitative studies conducted on purple sulfur bacteria (van Niel, 1931) made it possible to derive the following assimilation equation:

Class="center">

Comparing the first of these equations with the equation describing photosynthesis in green plants reveals a striking analogy:

In Chromatium

In green plants

This analogy suggests that hydrogen sulfide plays the Role of water in Bacterial photosynthesis. The General Equation of photosynthesis derived from this

led to the hypothesis that all photosynthesis is based on the same primary process, with Different types of photosynthesis differing only in The Nature of the hydrogen donor (water, hydrogen sulfide, or Organic compounds). Initially, this shared primary process was thought to be the photolysis of water (Н₂O + hv → [Н] + [ОН]), i.e., its decomposition into reducing and oxidizing components. It was believed that bacteria require an exogenous hydrogen donor H₂A to reduce the oxidizing component to water, whereas green plants acquired The ability to split oxygen off from the oxidizing component (4[ОН] → 2Н₂O + O₂). Today, this concept—which viewed the splitting of water as the primary process of photosynthesis—has been abandoned, yet the idea of light-driven reducing equivalent transfer remains central to modern photosynthetic theory.

The first stable products of photosynthesis are ATP and reducing power. These products can be detected both in intact cells and isolated Chloroplasts (from green plants), as well as in Suspensions of photosynthetic membrane vesicles from purple bacteria. CO₂ fixation is not strictly coupled to the light reaction. It can also proceed as a "dark reaction" independent of pigment-containing structures, provided that ATP and NAD(P)H₂ are present. These two processes are also spatially separated: photosynthesis takes place in and on the membranes, whereas CO₂ fixation occurs in the Cytoplasm or the stroma of chloroplasts. As noted above, oxygenic photosynthesis in cyanobacteria and plants differs from anoxygenic photosynthesis in anaerobic phototrophic bacteria in the hydrogen donor utilized. For water to serve as a donor, two successive photoreactions must operate. Conversely, utilizing donors with a more negative redox potential requires only a single photoreaction. Since the primary processes of oxygenic photosynthesis are better understood than those of anoxygenic photosynthesis, we will examine them first.

12.2.1 Oxygenic Photosynthesis

The primary processes of photosynthesis take place in thylakoids—flat, closed membrane vesicles found in the cells of cyanobacteria and in the chloroplasts of Algae and higher plants.

Thylakoid Membranes and Light-Harvesting Pigments (Antenna Pigments). The thylakoid membrane contains pigment molecules (chlorophyll $a$, chlorophyll $b$, and carotenoids), electron carriers, and Enzymes. The vast majority of chlorophyll molecules (approx. 99.5%), along with accessory pigments (carotenoids, phycobiliproteins), are responsible for Light absorption and energy distribution; they form the antenna system. Only a small fraction of chlorophyll $a$ Functions as the photochemical reaction center, where the actual photochemical redox reaction takes place. Antenna pigments (light-harvesting pigments) capture light and transfer energy to the reaction center chlorophyll (Carotenoid → Carotenoid*; Chlorophyll + Carotenoid* → Chlorophyll* + Carotenoid). Carotenoids also serve a protective function: under intense sunlight, they dissipate excess energy into the environment, thereby protecting chlorophyll molecules from photooxidation. The light-harvesting pigment system and the reaction center together constitute what is known as the photosynthetic unit.

Photoreactions. Photoreactions are fundamental to any form of photosynthesis. These photochemical redox reactions take place within reaction centers. A reaction center consists of several components, the most important of which are the primary electron donor (a specialized complex of chlorophyll and protein) and the primary electron acceptor. These two components constitute redox systems. The donor system (P/P⁺) has a positive potential, whereas the acceptor system (X/X^-) has a negative potential. Driven by light energy, a single electron is transferred:

Alternatively, this can be written as

Consequently, the first photoreaction can be represented as follows:

The photoreaction in purple bacteria can be represented in a similar manner:

Thus, As a result of the photoreaction, the donor loses a single electron, creating a "hole" (an electron vacancy). Such "holes" must be filled by electrons, which can arrive via one of two pathways: non-cyclic or Cyclic electron transport. In non-cyclic transport, electrons come from an exogenous external donor: in the case of the second photoreaction, from water molecules; in the case of the first reaction, from the Electron Transport Chain connecting both Photosystems. In cyclic transport, electrons return from the reduced acceptor (X^-) to the oxidized donor. The photochemical redox reaction, in which P is oxidized and X is reduced, is illustrated in the following diagram:

Cyclic electron transport leads to A change in membrane charge, whereas non-cyclic transport additionally results in the reduction of NADP.

Two photoreactions in two pigment systems (photosystems). Oxygenic photosynthesis operates with two pigment systems arranged in series (Fig. 12.14). The pigment system excited by longer-wavelength light (λ < 730 nm) is called Photosystem I, while the one excited by shorter-wavelength light (λ < 700 nm) is called Photosystem II. The photochemically active reaction center of photosystem I contains Chl a_I (P700), which acts as the primary electron donor in the first photoreaction. Light energy absorbed by the light-harvesting pigments of photosystem I is transferred to the reaction center, exciting Chl a_I. This results in The oxidation of Chl a_I, i.e., the loss of a single electron, converting Chl a_I into Chl a_I. In other words, as a result of electron emission, a "hole" or "electron vacancy" is formed in the reaction center. This "hole" is immediately filled by another electron arriving via a specialized electron transport pathway. The acceptor of the ejected electron is presumably an iron-sulfur protein ("X"). It possesses an even more negative redox potential than -420 mV, possibly -530 mV. This acceptor, in turn, passes the electron to ferredoxin, and from the reduced ferredoxin, reducing power can be transferred to NADP or other acceptors. Alongside this, cyclic Electron transport is also possible, in which the electron from "X" is transferred via plastoquinone, Cytochromes, and plastocyanin back to chlorophyll a⁺_I of the reaction center.

Fig. 12.14. Photosynthetic electron transport ("Z-scheme"). The vertical axis represents the redox potential. P700-Chl a_I: electron donor of photosystem I (PS I); P680-Chl a_II: electron donor of photosystem II (PS II); X 320: electron acceptor of PS II; X: electron acceptor of PS I, iron-sulfur protein; Fd: ferredoxin; Cyt: cytochrome. Photochemical reaction centers are enclosed in red boxes. (Explanation in text.)

The reaction center of photosystem II contains Chl a_II (P680), which serves as the primary electron donor In the second photoreaction. Upon receiving energy absorbed by the light-harvesting pigments of photosystem II, this chlorophyll enters an excited state. Excitation of Chl a_II leads to the emission of a single electron, which is a weak reducing agent (E'₀ ≈ 0 V). This electron is accepted by a special plastoquinone molecule (X320), which is thereby reduced to a semiquinone. Water acts as the electron donor for photosystem II. The "hole" formed in Chl a⁺_II as a result of electron loss is filled by one of the electrons released during the formation of O₂ from water (2H₂O → O₂ + 4H⁺ + 4e^-). The splitting of water takes place with the participation of manganese.

The two pigment systems described above are linked by an electron transport chain, an essential component of which is plastoquinone. Much like ubiquinone in the Respiratory Chain, plastoquinone is present in large excess within the photosynthetic electron transport chain and functions as an electron reservoir (depot). This reservoir can hold at least 10 electrons (per 1 molecule of Chl a_II) received from X320. Plastoquinone is oxidized by photosystem I, meaning that electrons from the "reservoir" are consumed to fill the "holes" in Chl a⁺_I. From plastoquinone, electrons are transferred to cytochrome f (a membrane-bound c-type cytochrome), then to plastocyanin (a soluble copper-containing protein), and finally to chlorophyll a⁺_I. Thus, plastoquinone performs the crucial function of storing and subsequently forwarding electrons originating from multiple (at least ten) Electron Transport Chains.

The Main Pathways of electron transport during the primary processes of photosynthesis are shown in Fig. 12.14. This is the well-known Z-scheme, which is the result of studies utilizing flash spectrophotometry, artificial electron donors and acceptors, and specific inhibitors. It provides insight into the redox potentials of pigments and electron carriers and The sequence of their Oxidation and reduction, but it reveals nothing about the localization of these components within the membrane.

Localization of pigments and electron carriers in the membrane. Some insights into this issue have been gained by studying thylakoid functions in the presence of Antibodies, as well as lipophilic or hydrophilic artificial redox systems. Immune antibodies raised against individual purified Components of the photosynthetic electron transport system are unable to penetrate the membrane and therefore react only with those components located on the outer surface of the thylakoids. For example, antibodies against ferredoxin or ferredoxin-NADP reductase effectively inhibit photosystem I function; this indicates that these two components reside on the outer surface of the thylakoids. At the same time, the surface bearing the electron donors is inaccessible to antibodies. Although data regarding the Structural Organization of photosystem II remain rather contradictory, a hypothetical scheme can already be proposed (Fig. 12.15). This scheme of the spatial orientation of directed photosynthetic electron Transport Across the thylakoid membrane should be distinguished from the diagram in Fig. 12.14, where components are arranged vertically according to their redox potentials. The scheme in Fig. 12.15 clearly demonstrates that electrons liberated by water splitting are transported from the thylakoid lumen into the stroma.

Fig. 12.15. Schematic representation of the spatial orientation of The electron transport system within the thylakoid membrane. Components are positioned on and within the membrane in such a way that directed electron transport occurs across the membrane. Mn: manganese complex; PC: plastocyanin; Cyt. f: cytochrome f; Fd: ferredoxin; X: iron-sulfur protein. (See also Fig. 12.14.)

Directed Electron Transport and the generation of a proton gradient.

Concepts regarding the localization of the components of the photosynthetic electron transport chain are consistent with physiological observations and measurements. When a suspension of thylakoids or disrupted chloroplasts is exposed to light, the pH of the suspending medium increases, and upon turning off the light, it drops again. Light triggers the inward movement of protons into the thylakoids (Fig. 12.16). Thus, light energy can be utilized to establish a proton gradient across the thylakoid membrane. It had been known even earlier that increasing the pH of a thylakoid suspension from 4 to 8 (in the dark) leads to ATP synthesis. Experiments of this kind formed the basis of the chemiosmotic hypothesis of energy conversion. More detailed studies showed that The transfer of a single electron through both photosystems is accompanied by the translocation of two protons into the thylakoid lumen. It is now believed that the Two Photosystems, together with the electron transport chain connecting them, provide a directed flow of electrons from water (on the inner side of the thylakoid membrane) to NADP (on the outer side). Thus, photoreactions lead to the reduction of NADP and the generation of a membrane charge. In other words, the light reactions act as a proton pump driven by light energy, creating a positive charge inside the thylakoid; as a result, the membrane accumulates energy in the form of a proton motive force, which is then utilized for ATP synthesis. The proton motive force couples photosynthetic electron transport to phosphorylation in the same manner that it couples Respiratory Electron Transport to phosphorylation (see p. 245).

Fig. 12.16. Proton translocation in cells and Organelles driven by light energy. A. Rhodobacter (Rhodopseudomonas) sphaeroides. B. Chloroplast. (Explanation in text.)

12.2.2 Anoxygenic photosynthesis

Photosynthetic electron transport in anaerobic phototrophic bacteria differs in many respects from the process just described. Anoxygenic photosynthesis involves only a single light reaction, which Supports cyclic electron transport. Electrons leaving the cycle to reduce NAD are not the product of water splitting. Photosynthesis depends on the availability of reduced substrates in the medium and is not accompanied by O₂ evolution. Although the photoreaction itself is analogous to the first photoreaction in green plants, in some bacteria it probably leads only to the generation of a proton motive force and, consequently, to energy storage (ATP) rather than NAD reduction. Thus, non-cyclic electron transport (from an electron donor to a pyridine nucleotide) is absent here. Apparently, NADH₂ is formed as a result of a dark reaction via Reverse Electron Transport, which proceeds with an energy input.

It should be noted, however, that among phototrophic bacteria, distinct groups exhibit much greater differences in pigment composition and photosynthetic mechanisms than green plants do. In our subsequent Structure/133.html">Discussion, we will initially set aside green bacteria.

Photoreaction in purple bacteria. As already mentioned, in purple bacteria, pigments and components of the electron transport system are also located within membranes. The pigment complex of the photochemical reaction center can be successfully isolated from the antenna pigments.

Fig. 12.17. Schematic of photosynthetic electron transport in Rhodospirillales and Chlorobiales (the vertical axis represents the redox potential). Cyt: cytochrome; Fd: ferredoxin; UQ: ubiquinone; P870 or P840: Bhl a [reaction center (RC) electron donor]; X: RC electron acceptor. The photochemical reaction center is enclosed in a red box. (Explanation in text.)

Energy absorbed by antenna pigments (bacteriochlorophyll and carotenoids) is transferred to the reaction centers. Isolated reaction centers consist of a protein complex containing Bhl a, bacteriopheophytin, carotenoids, ubiquinone, and an iron-sulfur protein (FeS protein). The reaction center pigment is designated as P870 based on the wavelength at which light absorption maximally decreases. In the light, P870 is oxidized to P870⁺. The Redox Potential of this electron donor lies between +450 and +490 mV. The primary electron acceptor is presumably a complex of ubiquinone with an FeS protein. The redox potential of this complex must be close to -100 mV. Therefore, it seems unlikely that electrons excited during the light reaction in purple bacteria are capable of reducing NAD. Rather, they return via ubiquinone, cytochromes b and c₂, and possibly FeS Proteins back to P870⁺ (Fig. 12.17). The electrons required for NAD reduction evidently leave the cyclic transport pathway. They are transferred to NAD via reverse transport driven by ATP consumption. This represents a significant difference from the otherwise analogous first photoreaction in oxygenic photosynthesis. To replenish the electron cycle, purple bacteria require external electron donors. Purple sulfur bacteria can utilize hydrogen sulfide, sulfur, or thiosulfate for this purpose; organic compounds (malate, succinate, etc.) and molecular hydrogen serve as electron donors for both groups of purple bacteria.

As numerous experiments have demonstrated, photosynthetic electron transport in purple bacteria also leads to the generation of a proton gradient. Intact cells respond to light exposure by releasing protons into the medium, resulting in medium acidification. In suspensions of vesicles derived from photosynthetic membranes (chromatophores), light induces inward-directed proton translocation. Thus, the membranes of chromatophores and thylakoids possess the same polarity as submitochondrial vesicles. This becomes understandable when one considers that all these membranes are formed by the inward invagination and proliferation of Cell/30.html">The Plasma Membrane or the inner chloroplast membrane. Although the precise localization of individual components within the membrane has not yet been established, it is believed that hydrogen and electron carriers are arranged within the membrane of anaerobic phototrophic bacteria in such a way that charge Separation occurs. In chromatophores, electrons are transported outward, while protons are transported inward. The resulting proton motive force serves as the driving force for photosynthetic phosphorylation.

Photoreaction in green bacteria. The mechanisms of the photoreaction in green bacteria are not yet fully elucidated. There is evidence indicating that the primary electron acceptor participating in the light reaction in green sulfur bacteria has a potential of about -500 mV (compared to only -100 mV in purple bacteria!). With such a strongly negative potential, the direct utilization of electrons from the primary acceptor for the reduction of ferredoxin and pyridine nucleotide becomes possible (Fig. 12.17). Thus, Chlorobiaceae may obtain their reducing power without resorting to energy-demanding reverse electron transport. Such independence from reverse electron transport would be an important distinguishing feature of photosynthesis in green bacteria compared to purple bacteria. Consequently, the photoreaction in Chlorobiaceae would be comparable in efficiency to the first photoreaction of cyanobacteria. From an evolutionary perspective, the photosynthesis of green bacteria could serve as a link between the photosynthesis of purple bacteria and that of cyanobacteria and plants¹.

Conclusion. Photosynthesis involves the conversion of light energy into biochemical energy. The primary action of light is that, within photochemical reaction centers, electrons are transferred from a donor to an acceptor in a thermodynamically unfavorable direction. At least a fraction of these electrons returns to the reaction centers via the electron transport chain. Due to the specific spatial arrangement of the electron transport system components within the membrane, this process is coupled with the directed translocation of protons and the generation of a proton motive force. Consequently, the photosynthetic apparatus functions primarily as a light-driven proton pump. The proton motive force drives energy conversion via phosphorylation. ATP synthesis relies on mechanisms fundamentally identical to those operating in the membranes of aerobic bacteria or Mitochondria. Regarding the conversion of light energy into biochemically useful energy (ATP), there is no fundamental difference between phototrophic bacteria and green plants. In purple bacteria, The Role of photosynthesis appears to be limited to this conversion process. In cyanobacteria and green plants, one can observe a subsequent stage in the Evolution of photosynthesis. Through the sequential integration of two photoreactions, the energy level of electrons During the first reaction is elevated sufficiently to enable the reduction of ferredoxin and NADP. The second photoreaction utilizes water as the electron source. As a result of this combination, energy storage is accompanied by the reduction of NADP and the evolution of O₂.

¹ In Chloroflexaceae, The Mechanism of the primary photosynthetic processes is apparently identical to that of purple bacteria. - Ed. note.

Photosynthesis is undoubtedly the most widespread chemical reaction taking place on our planet. We owe both the continuous synthesis of novel organic matter and the existence of fossil fuels such as coal, oil, and natural gas to this process. Therefore, the immense efforts dedicated to unraveling the mysteries of photosynthesis are fully justified. However, many challenges remain unresolved. Consequently, alongside experimentally proven facts, the model of photosynthesis described above incorporates A number of hypotheses that require further rigorous investigation to be validated.