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Diversity of Metabolism in Procaryotes (page 6)

(This chapter has 8 pages)

© Kenneth Todar, PhD

Phototrophic Metabolism

Phototrophy is the use of light as a source of energy for growth, more specifically the conversion of light energy into chemical energy in the form of ATP. Procaryotes that can convert light energy into chemical energy include the photosynthetic cyanobacteria, the purple and green bacteria and the "halobacteria" (actually archaea). The cyanobacteria conduct plant photosynthesis, called oxygenic photosynthesis; the purple and green bacteria conduct bacterial photosynthesis or anoxygenic photosynthesis; the extreme halophilic archaea use a type of nonphotosynthetic photophosphorylation mediated by bacteriorhodopsin to transform light energy into ATP.

Photosynthesis is the conversion of light energy into chemical energy that can be used in the formation of cellular material from CO2. Photosynthesis is a type of metabolism separable into a catabolic and anabolic component. The catabolic component of photosynthesis is the light reaction, wherein light energy is transformed into electrical energy, then chemical energy. The anabolic component involves the fixation of CO2 and its use as a carbon source for growth, usually called the dark reaction. In photosynthetic procaryotes there are two types of photosynthesis and two types of CO2 fixation.

The Light Reactions depend upon the presence of chlorophyll, the primary light-harvesting pigment in the membrane of photosynthetic organisms. Absorption of a quantum of light by a chlorophyll molecule causes the displacement of an electron at the reaction center. The displaced electron is an energy source that is moved through a membrane photosynthetic electron transport system, being successively passed from an iron-sulfur protein (X ) to a quinone to a cytochrome and back to chlorophyll (Figure 16 below). As the electron is transported, a proton motive force is established on the membrane, and ATP is synthesized by an ATPase enzyme. This manner of converting light energy into chemical energy is called cyclic photophosphorylation.

Figure 16. Photosystem I: cyclical electron flow coupled to photophosphorylation.

The functional components of the photochemical system are light harvesting pigments, a membrane electron transport system, and an ATPase enzyme. The photosynthetic electron transport system of is fundamentally similar to a respiratory ETS, except that there is a low redox electron acceptor (e.g. ferredoxin) at the top (low redox end) of the electron transport chain, that is first reduced by the electron displaced from chlorophyll.

There are several types of pigments distributed among various phototrophic organisms. Chlorophyll is the primary light-harvesting pigment in all photosynthetic organisms. Chlorophyll is a tetrapyrrole which contains magnesium at the center of the porphyrin ring. It contains a long hydrophobic side chain that associates with the photosynthetic membrane. Cyanobacteria have chlorophyll a, the same as plants and algae. The chlorophylls of the purple and green bacteria, called bacteriochlorophylls are chemically different than chlorophyll a in their substituent side chains. This is reflected in their light absorption spectra. Chlorophyll a absorbs light in two regions of the spectrum, one around 450nm and the other between 650 -750nm; bacterial chlorophylls absorb from 800-1000nm in the far red region of the spectrum.

The chlorophylls are partially responsible for light harvesting at the photochemical reaction center. The energy of a photon of light is absorbed by a special chlorophyll molecule at the reaction center, which becomes instantaneously oxidized by a nearby electron acceptor of low redox potential. The energy present in a photon of light is conserved as a separation of electrical charge which can be used to generate a proton gradient for ATP synthesis.

Carotenoids are always associated with the photosynthetic apparatus. They function as secondary light-harvesting pigments, absorbing light in the blue-green spectral region between 400-550 nm. Carotenoids transfer energy to chlorophyll, at near 100 percent efficiency, from wave lengths of light that are missed by chlorophyll. In addition, carotenoids have an indispensable function to protect the photosynthetic apparatus from photooxidative damage. Carotenoids have long hydrocarbon side chains in a conjugated double bond system. Carotenoids "quench" the powerful oxygen radical, singlet oxygen, which is invariably produced in reactions between chlorophyll and O2 (molecular oxygen). Some nonphotosynthetic bacterial pathogens, i.e., Staphylococcus aureus, produce carotenoids that protect the cells from lethal oxidations by singlet oxygen in phagocytes.

Phycobiliproteins are the major light harvesting pigments of the cyanobacteria. They also occur in some groups of algae. They may be red or blue, absorbing light in the middle of the spectrum between 550 and 650nm. Phycobiliproteins consist of proteins that contain covalently-bound linear tetrapyrroles (phycobilins). They are contained in granules called phycobilisomes that are closely associated with the photosynthetic apparatus. Being closely linked to chlorophyll they can efficiently transfer light energy to chlorophyll at the reaction center.

Figure 17. The distribution of photosynthetic pigments among photosynthetic microorganisms.

All phototrophic bacteria are capable of performing cyclic photophosphorylation as described above and in Figure 16 and below in Figure 18. This universal mechanism of cyclic photophosphorylation is referred to as Photosystem I. Bacterial photosynthesis uses only Photosystem I (PSI), but the more evolved cyanobacteria, as well as algae and plants, have an additional light-harvesting system called Photosystem II (PSII). Photosystem II is used to reduce Photosystem I when electrons are withdrawn from PSI for CO2 fixation. PSII transfers electrons from H2O and produces O2, as shown in Figure 20.

Figure 18. The cyclical flow of electrons during bacterial (anoxygenic) photosynthesis. A cluster of carotenoid and chlorophyll molecules at the Reaction Center harvests a quantum of light. A bacterial chlorophyll molecule becomes instantaneously oxidized by the loss of an electron. The light energy is used to boost the electron to a low redox intermediate, ferredoxin, (or some other iron sulfur protein) which can enter electrons into the photosynthetic electron transport system in the membrane. As the electrons traverse the ETS a proton motive force is established that is used to make ATP in the process of photophosphorylation. The last cytochrome in the ETS returns the electron to chlorophyll. Since light energy causes the electrons to turn in a cycle while ATP is synthesized, the process is called cyclic photophosphorylation. Compare bacterial photosynthesis with the scheme that operates in Photosystem I in Figure 16 above. Bacterial photosynthesis uses only Photosystem I for the conversion of light energy into chemical energy.

Figure 19. The normally cyclical flow of electrons during bacterial photosynthesis must be opened up in order to obtain electrons for CO2 fixation. In the case of the purple sulfur bacteria, they use H2S as a source of electrons. The oxidation of H2S is coupled to PSI. Light energy boosts an electron, derived from H2S, to the level of ferredoxin, which reduces NADP to provide electrons for autotrophic CO2 fixation.

Figure 20. Electron flow in plant (oxygenic) photosynthesis. Photosystem I and the mechanisms of cyclic photophosphorylation operate in plants, algae and cyanobacteria, as they do in bacterial photosynthesis. In plant photosynthesis, chlorophyll a is the major chlorophyll species at the reaction center and the exact nature of the primary electron acceptors (X or ferredoxin) and the components of the ETS are different than bacterial photosynthesis. But the fundamental mechanism of cyclic photophosphorylation is the same. However, when electrons must be withdrawn from photosystem I (ferredoxin--e--->NADP in upper left), those electrons are replenished by the operation of Photosystem II. In the Reaction Center of PSII, a reaction between light, chlorophyll and H2O removes electrons from H2O (leading to the formation of O2) and transfers them to a component of the photosynthetic ETS (primary electron acceptor). The electrons are then transferred through a chain of electron carriers consisting of cytochromes and quinones until they reach chlorophyll in PSI. The resulting drop in redox potential allows for the synthesis of ATP in a process called noncyclic photophosphorylation. The operation of photosystem II is what fundamentally differentiates plant photosynthesis from bacterial photosynthesis. Photosystem II accounts for the source of reductant for CO2 fixation (provided by H2O), the production of O2, and ATP synthesis by noncyclic photophosphorylation

Most of the phototrophic procaryotes are obligate or facultative autotrophs, which means that they are able to fix CO2 as a sole source of carbon for growth. Just as the oxidation of organic material yields energy, electrons and CO2, in order to build up CO2 to the level of cell material (CH2O), energy (ATP) and electrons (reducing power) are required. The overall reaction for the fixation of CO2 in the Calvin cycle is CO2 + 3ATP + 2NADPH2 ----------> CH2O + 2ADP + 2Pi + 2NADP. The light reactions operate to produce ATP to provide energy for the dark reactions of CO2 fixation. The dark reactions also need reductant (electrons). Usually the provision of electrons is in some way connected to the light reactions. A model for coupling the light and dark reactions of photosynthesis is illustrated in Figure 21 below.

The general scheme for finding electrons for CO2 fixation is to open up Photosystem I and remove the electrons, eventually getting them to NADP which can donate them to the dark reaction. In bacterial photosynthesis the process may be quite complex. The electrons are removed from Photosystem I at the level of a cytochrome, then moved through an energy-consuming reverse electron transport system to an iron-sulfur protein, ferredoxin, which reduces NADP to NADPH2. The electrons that replenish Photosystem I come from the oxidation of an external photosynthetic electron donor, which may be H2S, other sulfur compounds, H2, or certain organic compounds.

In plant photosynthesis, the photosynthetic electron donor is H2O, which is lysed by photosystem II, resulting in the production of O2. Electrons removed from H2O travel through Photosystem II to Photosystem I as described in Figure 20 above. Electrons removed from Photosystem I reduce ferredoxin directly. Ferredoxin, in turn, passes the electrons to NADP.

Figure 21. Model for coupling the light and dark reactions of photosynthesis.

The differences between plant and bacterial photosynthesis are summarized in Table 6 below. Bacterial photosynthesis is an anoxygenic process. The external electron donor for bacterial photosynthesis is never H2O, and therefore, purple and green bacteria never produce O2 during photosynthesis. Furthermore, bacterial photosynthesis is usually inhibited by O2 and takes place in microaerophilic and anaerobic environments. Bacterial chlorophylls use light at longer wave lengths not utilized in plant photosynthesis, and therefore they do not have to compete with oxygenic phototrophs for light. Bacteria use only cyclic photophosphorylation (Photosystem I) for ATP synthesis and lack a second photosystem.

Table 6. Differences between plant and bacterial photosynthesis

plant photosynthesis bacterial photosynthesis
organisms plants, algae, cyanobacteria purple and green bacteria
type of chlorophyll chlorophyll a 

absorbs 650-750nm


absorbs 800-1000nm

Photosystem I

(cyclic photophosphorylation)

present present
Photosystem I

(noncyclic photophosphorylation)

present absent
Produces O2 yes no
Photosynthetic electron donor H2O H2S, other sulfur compounds or 

certain organic compounds

While photosynthesis is highly-evolved in the procaryotes, it apparently originated in the Bacteria and did not spread or evolve in Archaea. But the Archaea, in keeping with their unique ways, are not without representatives which can conduct a type of light-driven photophosphorylation. The extreme halophiles, archaea that live in natural environments such as the Dead Sea and the Great Salt Lake at very high salt concentration (as high as 25 percent NaCl) adapt to the high-salt environment by the development of "purple membrane", actually patches of light-harvesting pigment in the plasma membrane. The pigment is a type of rhodopsin called bacteriorhodopsin which reacts with light in a way that forms a proton gradient on the membrane allowing the synthesis of ATP. This is the only example in nature of non photosynthetic photophosphorylation. These organisms are heterotrophs that normally respire by aerobic means. The high concentration of NaCl in their environment limits the availability of O2 for respiration so they are able to supplement their ATP-producing capacity by converting light energy into ATP using bacteriorhodopsin.

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