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Lecture notes

Bacteriology 2

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Bacteriology notes continuation..

Respiration

 

         Compared to fermentation (means of oxidizing organic compounds), respiration is more complicated.

         Respirations result in complete oxidation of the substrate by an outside electron acceptor. In addition to a pathway of glycolysis, four essential metabolic components are needed:

 

1. Tricarboxylic acid (TCA) cycle (the citric acid cycle or the Kreb's cycle):

-an organic compound is utilized as a substrate, TCA cycle is required for complete oxidation of substrate.

-end product that always results from complete oxidation of an organic compound is CO2.

 

2. A membrane and an associated electron transport system (ETS). The ETS is a sequence of electron carriers in the plasma membrane transports electrons taken from substrate through chain of carriers to a final electron acceptor.

- electrons enter ETS at a very low redox potential (E'o) and exit at relatively high redox potential.

- drop in potential releases energy that can be harvested by cells in process of ATP synthesis electron transport phosphorylation .

- operation of the ETS establishes a proton motive force (pmf) due to formation of a proton gradient across membrane.

 

3. An outside electron acceptor ("outside", meaning it is not internal to the pathway, as is pyruvate in a fermentation).

            - For aerobic respiration electron acceptor is O2,

            - molecular oxygen is reduced to H20 in last step of electron transport system.

- in bacterial processes of anaerobic respiration, final electron acceptors may be SO4 or S or NO3 or NO2 or certain other inorganic compounds, or even an organic compound, such as fumarate.

 

4. A transmembranous ATPase enzyme (ATP synthetase).

            - enzyme utilizes proton motive force established on membrane (by the operation of the ETS) to synthesize ATP in process of electron transport phosphorylation.

            - It is believed that transmembranous Fo subunit is a proton transport system that transports 2 H+ to the F1 subunit (the actual ATPase) on inside of membrane.

            - 2 protons required and consumed during synthesis of ATP from ADP plus Pi, in the membrane of E. coli.

            - reaction catalyzed by ATPase enzyme: is ADP + 2 Pi + 2 H + <----------> ATP.

 

  • It is important to appreciate the reversibility of this reaction in order to account for how a fermentative bacterium, without an ETS, could establish a necessary pmf on the membrane for transport or flagellar rotation. If such an organism has a transmembranous ATPase, it could produce ATP by SLP, and subsequently the ATPase could hydrolyze the ATP, thereby releasing protons to the outside of the membrane.
  • Diagram aerobic respiration integrates these metabolic processes into a scheme that represents overall process of respiratory metabolism.
  • Substrate such as glucose completely oxidized to to CO2 by combined pathways of glycolysis and TCA cycle.
  • Electrons removed from glucose by NAD are fed into ETS in membrane. E
  • lectrons traverse the ETS, a pmf becomes established across the membrane.
  • Electrons eventually reduce an outside electron acceptor, O2, and reduce it to H20.
  • The pmf on membrane is used by ATPase enzyme to synthesize ATP by a process referred to as "oxidative phosphorylation”

 

The overall reaction for the aerobic respiration of glucose is:

         Glucose + 6 O2 ----------> 6 CO2 + 6 H20 + 688 kcal (total)

         which can be written

         Glucose ----------> 6 CO2 + 10 NADH2 + 2 FADH2 + 4 ATP

         In E. coli, 2 ATP are produced for each pair of electrons that are introduced into the ETS by NADH2. One ATP is produced from a pair of electrons introduced by FADH2. Hence, the equation can be rewritten

         Glucose + 6 O2 ----------> 6 CO2 + 6 H20 + 20 ATP (ETP) + 2 ATP (ETP) + 4 ATP (SLP) + 688 kcal (total)

         Since a total of 26 ATP is formed during the release of 688 kcal of energy, the efficiency of this respiration is 26x8/688 or about 30 percent. In Pseudomonas (or mitochondria), due to the exact nature of the ETS, 3 ATP are produced for each pair of electrons that are introduced into the ETS by NADH2 and 2 ATP are produced from a pair of electrons introduced by FADH2. Hence, the overall reaction in Pseudomonas, using the same dissimilatory pathways as E. coli, is

         Glucose + 6 O2 ----------> 6 CO2 + 6 H20 + 38 ATP + 688 kcal (total)

         corresponding efficiency is about 45 percent.

         Respiration in some procaryotes is entirely possible using electron acceptors other than oxygen (O2). This type of respiration in the absence of oxygen is referred to as anaerobic respiration. Although anaerobic respiration is more complicated than the foregoing statement, in its simplest form it represents the substitution or use of some compound other than O2 as a final electron acceptor in the electron transport chain. Electron acceptors used by procaryotes for respiration or methanogenesis (an analogous type of energy generation in archaea) are described in the table below.

 

 

Phototrophic Metabolism

 

         Phototrophy - use of light as a source of energy for growth, more specifically conversion of light energy into chemical energy in form of ATP.

         procaryotes - can convert light energy into chemical energy include photosynthetic cyanobacteria, purple and green bacteria and halobacteria.

         cyanobacteria conduct plant photosynthesis, called oxygenic photosynthesis

         purple and green bacteria conduct bacterial photosynthesis or anoxygenic photosynthesis;

         halobacteria use a type of nonphotosynthetic photophosphorylation mediated by bacteriorhodopsin to transform light energy into ATP.

 

Photosynthesis

 

         conversion of light energy into chemical energy used in formation of cellular material from CO2.

         a type of metabolism separable into a catabolic and anabolic component.

         Catabolic component of photosynthesis is light reaction(light energy is transformed into electrical energy, then chemical energy).

         Anabolic component involves fixation of CO2 and its use as a carbon source for growth, usually called dark reaction.

         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 displacement of electron at reaction center.

         Displaced electron is 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.

         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.

 

 

         The differences between plant and bacterial photosynthesis are summarized in the table.

         Bacterial photosynthesis is an anoxygenic process.

         The external electron acceptor for bacterial photosynthesis is never H2O, and therefore, they 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.

 

         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 the region of the spectrum from 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 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.

 

         All phototrophic bacteria are capable of performing cyclic photophosphorylation as described above and in Figure 15 below.

         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 16

 

         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, X, (often an 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 14 above. Bacterial photosynthesis uses only Photosystem I for the conversion of light energy into chemical energy.

 

         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 X 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 (shown by X----->ferredoxin----->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 (Q).

         The electrons are transferred from Q through a chain of electron carriers consisting of cytochromes and quinones until they reach plastocyanin in PSI. The movement of the electrons from Q to plastocyanin is a drop in redox potential that 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. Coupling the light and dark reactions of photosynthesis is illustrated in Figure 17 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 (see Figure 15 above). 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 16 above. Electrons removed from Photosystem I reduce ferredoxin directly. Ferredoxin, in turn, passes the electrons to NADP.

 

         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, which 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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