metabolism in bacteria - complete

8/10/2019 Metabolism in Bacteria - Complete

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Phototrophs convert the energy of the sun into chemical energy, which feeds

chemoorganotrophs. Chemoorganotrophs recycle the wastes of other organisms and play

important roles in industry; chemolithotrophsoxidize inorganic molecules and in the process

contribute to biogeochemical cycles such as the iron and sulfur cycles.

Chemoorganotrophs oxidize an organic energy source and conserve the energy released in

the form of ATP. The electrons released are accepted by a variety of electron acceptors, and

whether the acceptor is exogenous (that is, externally supplied) or endogenous (internally

supplied) defines the energy conserving process used by the organism. When the electron

acceptor is exogenous, the metabolic process is called respiration and may be divided into

two different types. In aerobic respiration,the final electron acceptor is oxygen, whereas the

terminal acceptor in anaerobic respiration is a different exogenous acceptor such as NO3-,

SO42-, CO2, Fe

3-, and SeO42-. Organic acceptors such as fumarate and humic acids also may be

used. Respiration involves the activity of an electron transport chain. As electrons pass

through the chain to the final electron acceptor, a type of potential energy called the proton

motive force (PMF) is generated and used to synthesize ATPfrom ADP and Pi. In contrast,

fermentation uses an endogenous electronacceptor and does not involve an electron

transport chain or the generation of PMF.The endogenous electron acceptor is usually an

intermediate (e.g., pyruvate) of the catabolic pathway used to degrade and oxidize the

organic energy source. During fermentation, ATP is synthesized only by substrate level

phosphorylation, a process in which a phosphate group is transferred to ADP from a high-

energy molecule (e.g., phosphoenolpyruvate) generated by catabolism of the energy source.

I. AEROBIC RESPIRATION:

The process of aerobic respiration is represented in the following summary form:

C6H12O6+ 6O2+ 38 ADP + 38 Pi = 6CO2+ 6H2O + 38 ATP

A. The breakdown of glucose to pyruvate

Microorganisms employ several metabolic pathways to catabolize glucose and other sugars. The

way in which microorganisms breakdown sugar to pyruvate are classified into 3 pathways: (1)


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the Embden-Meyerhof pathway, (2) the pentose phosphate pathway, and (3) the

Entner-Doudoroff pathway. Collectively these pathways are called glycolytic pathwaysor

glycolysis.


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1. The Embden-Meyerhof Pathway

The Embden-Meyerhof pathway is undoubtedly the most common pathway for glucose

degradation to pyruvate in stage two of aerobic respiration. It is found in all major groups of

microorganisms and functions in the presence or absence of O2. As noted earlier, it is also an

important amphibolic pathway and provides several precursor metabolites. The Embden-

Meyerhof pathway occurs in the cytoplasmof prokaryotes and eukaryotes.

The pathway as a whole may be divided into two parts. In the initial six-carbon

phase, energy is consumed as glucose is phosphorylated twice, and is converted to fructose

1,6-bisphosphate. This preliminary phase consumes two ATP molecules for each glucose


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molecule and adds phosphates to each end of the sugar. In essence, the organism invests

some of its ATP so that more can be made later in the pathway. The three-carbon, energy-

conserving phase begins when the enzyme fructose 1,6-bisphosphate aldolase catalyzes the

cleavage of fructose 1,6-bisphosphate into two halves, each with a phosphate group. One of

the products, dihydroxyacetone phosphate, is immediately converted to

glyceraldehyde 3-phosphate. This yields two molecules of glyceraldhyde 3-phosphate,

which are then converted to pyruvate in a five-step process. Because dihydroxyacetone

phosphate can be easily changed to glyceraldehyde 3-phosphate, both halves of fructose 1,6-

bisphosphate are used in the three-carbon phase. First, glyceraldehyde 3-phosphate is oxidized

with NAD+ as the electron acceptor (to form NADH), and a phosphate (Pi) is simultaneously

incorporated to give a high energy molecule called 1,3-bisphosphoglycerate. The high-energy

phosphate on carbon one is subsequently donated to ADP to produce ATP. This synthesis of

ATP is called substrate-level phosphorylation because ADP phosphorylation is coupled with

the exergonic breakdown of a high-energy bond.

A somewhat similar process generates a second ATP by substrate-level phosphorylation.

The phosphate group on 3- phosphoglycerate shifts to carbon two, and 2-phosphoglycerate is

dehydrated to form a second high-energy molecule, phosphoenolpyruvate. This molecule

donates its phosphate to ADP forming a second ATP and pyruvate, the final product of thepathway.

The Embden-Meyerhof pathway degrades one glucose molecule to two pyruvate

molecules. ATP and NADH are also produced. The yields of ATP and NADH may be calculated

by considering the two phases separately. In the six-carbon phase, two ATPs are used to form

fructose 1,6-bisphosphate. For each glyceraldehyde 3-phosphate transformed into pyruvate,

one NADH and two ATP are formed. Because two glyceraldehyde 3-phosphates arise from a

single glucose (one by way of dihydroxyacetone phosphate), the three carbon phase generates

four ATPs and two NADHs per glucose. Subtraction of the ATP used in the six-carbon phase

from that produced by substrate-level phosphorylation in the three-carbon phase gives a net

yield of two ATPs per glucose. Thus the catabolism of glucose to pyruvate can be represented

by this simple equation.

Glucose + 2 ADP + 2 Pi + 2 NAD+= 2 pyruvate + 2 ATP + 2 NADH + 2 H+

2. The Pentose Phosphate Pathway

A second pathway, the pentose phosphate or hexose monophosphate pathway, may be

used at the same time as either the Embden-Meyerhof or the Entner-Doudoroff pathways. It

can operate either aerobically or anaerobically and is important in both biosynthesis andcatabolism. The pentose phosphate pathway begins with the oxidation of glucose 6-

phosphateto 6-phosphogluconatefollowed by the oxidation of 6-phosphogluconate to the

pentose sugar ribulose 5-phosphate and CO2. NADPH is produced during these oxidations.

Ribulose 5-phosphate is then converted to a mixture of three- through seven-carbon sugar

phosphates. Two enzymes play a central role in these transformations: (1) transketolase

catalyzes the transfer of two-carbon ketol groups, and (2)transaldolase transfers a three-

carbon group from sedoheptulose 7-phosphate to glyceraldehyde 3-phosphate. The overall

result is that three glucose 6-phosphates are converted to two fructose 6-phosphates,

glyceraldehyde 3-phosphate, and three CO2molecules, as shown in this equation.


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3 glucose 6-phosphate + 6NADP++ 3H2O 2 fructose 6-phosphate + glyceraldehyde 3-

phosphate + 3CO2+ 6NADPH + 6H+

These intermediates are used in two ways. The fructose 6-phosphate can be changedback to glucose 6-phosphate while glyceraldehyde 3-phosphate is converted to pyruvate by

enzymes of the Embden-Meyerhof pathway. Alternatively two glyceraldehyde 3- phosphates

may combine to form fructose 1,6-bisphosphate, which is eventually converted back into

glucose 6-phosphate. This results in the complete degradation of glucose 6-phosphate to CO2

and the production of a great deal of NADPH.

Glucose 6-phosphate + 12NADP++ 7H2O 6CO2 + 12NADPH + 12H++ Pi

The pentose phosphate pathway is a good example of an amphibolic pathway as it has

several catabolic and anabolic functions that are summarized as follows:


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1. NADPH from the pentose phosphate pathway serves as a source of electrons for the

reduction of molecules during biosynthesis.

2. The pathway produces two important precursor metabolites: erythrose 4-phosphate, which is

used to synthesize aromatic amino acids and vitamin B6 (pyridoxal) and ribose 5-phosphate,

which is a major component of nucleic acids. Note that when a microorganism is growing on a

pentose carbon source, the pathway can function biosynthetically to supply hexose sugars (e.g.,

glucose needed for peptidoglycan synthesis).

3. Intermediates in the pentose phosphate pathway may be used to produce ATP.

Glyceraldehyde 3-phosphate from the pathway can enter the three-carbon phase of the

Embden-Meyerhof pathway and be converted to pyruvate, as ATP is produced by substrate-

level phosphorylation. Pyruvate may be oxidized in the tricarboxylic acid cycle to provide more

energy.

Although the pentose phosphate pathway may be a source of energy in many

microorganisms, it is more often of greater importance in biosynthesis.

3. The Entner-Doudoroff Pathway

The Entner-Doudoroff pathway is used by soil microbes, such as Pseudomonas,

Rhizobium, Azotobacter, andAgrobacterium, and a few other gram-negative bacteria. Very few

gram-positive bacteria have this pathway, with the intestinal bacterium Enterococcus faecalis


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being a rare exception. The Entner-Doudoroff pathway begins with the same reactions as the

pentose phosphate pathway: the formation of glucose 6-phosphate, which is then converted to

6-phosphogluconate. Instead of being further oxidized, 6-phosphogluconate is dehydrated to

form 2-keto-3-deoxy-6-phosphogluconate or KDPG, the key intermediate in this pathway. KDPG

is then cleaved by KDPG aldolase to pyruvate and glyceraldehyde 3-phosphate. The

glyceraldehyde 3-phosphate is converted to pyruvate in the Embden-Meyerhof pathway.If

the Entner-Doudoroff pathway degrades glucose to pyruvate in this way, it yields one ATP,one

NADPH, and one NADH per glucose metabolized.

B. THE TRICARBOXYLIC ACID CYCLE


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In the glycolytic pathways, the energy captured by the oxidation of glucose to pyruvate

is limited to no more than two ATP generated by substrate-level phosphorylation. During

aerobic respiration, the catabolic process continues by oxidizingpyruvate to three CO2.The

first step of this process employs a multi-enzyme system called the pyruvate dehydrogenase

complex. It oxidizes and cleaves pyruvate to form one CO2 and the two-carbon molecule

acetyl-coenzyme A (acetyl-CoA). Acetyl-CoA is energy-rich because a high-energy thiol links

acetic acid to coenzyme A. Acetyl-CoA then enters the tricarboxylic acid (TCA) cycle, which

is also called the citric acid cycle or the Krebs cycle.

In the first reaction acetyl-CoA is condensed with (i.e. added to) a four-carbon

intermediate, oxaloacetate, to form citrate, a molecule with six carbons. Citrate (a tertiary

alcohol) is rearranged to give isocitrate, a more readily oxidized secondary alcohol. Isocitrate is

subsequently oxidized and decarboxylated twice to yield -ketoglutarate (five carbons), and

then succinyl-CoA (four carbons), a molecule with a high-energy bond. At this point two NADH

molecules have been formed and two carbons lost from the cycle as CO2. The cycle continues

when succinyl-CoA is converted to succinate. This involves breaking the high-energy bond in

succinyl-CoA and using the energy released to form one GTP by substrate-level

phosphorylation. GTP is also a high-energy molecule, and it is functionally equivalent to ATP.

Two oxidation steps follow, yielding one FADH2 and one NADH. The last oxidation stepregenerates oxaloacetate, and as long as there is a supply of acetyl-CoA the cycle can repeat

itself. TCA cycle generates two CO2molecules, three NADH molecules, one FADH2, and one GTP

for each acetyl-CoA molecule oxidized.

TCA cycle enzymes are widely distributed among microorganisms. In procaryotes, they

are located in the cytoplasmic matrix. In eucaryotes, they are found in the mitochondrial

matrix. The complete cycle appears to be functional in many aerobic bacteria, free-living

protists, and fungi. Even those microorganisms that lack the complete TCA cycle usually have

most of the cycle enzymes, because the TCA cycle is also a key source of carbon skeletons for

use in biosynthesis.

C. ELECTRON TRANSPORT AND OXIDATIVE PHOSPHORYLATION

In oxidizing glucose, the cell has also generated numerous molecules of NADH and

FADH2. Both of these molecules can be used to conserve energy. In fact, most of the ATP

generated during aerobic respiration comes from the oxidation of these electron carriers in the

electron transport chain.

ETC of E.coli

The NADH generated by the oxidation of organic substrates (during glycolysis and the TCA

cycle) is donated to the electron transport chain, where it is oxidized to NAD+ by the

membrane-bound NADH dehydrogenase. The electrons are then transferred to carriers

with progressively more positive reduction potentials. As electrons move through the carriers,

protons are moved across the plasma membrane to the periplasmic space (i.e., outside the cell)

rather than to an intermembrane space as seen in the mitochondria. Another significant

difference between the E. coli chain and the mitochondrial chain is that the bacterial electron

transport chain contains a different array of cytochromes. Furthermore, E. colihas evolved two

branches of the electron transport chain that operate under different aeration conditions. When

oxygen is readily available, the cytochrome bo branch is used. When oxygen levels are reduced,

the cytochrome bdbranch is used because it has a higher affinity for oxygen. However, it is less


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efficient than the bobranch because the bdbranch moves fewer protons into the periplasmic

space.

Oxidative Phosphorylation

Oxidative phosphorylation is the process by which ATP is synthesized as the result of

electron transport driven by the oxidation of a chemical energy source. The most widely

accepted hypothesis for the mechanism of oxidative phosphorylation is the chemiosmotic

hypothesis, which was formulated by British biochemist Peter Mitchell. Chemiosmosis

proposes that electron transport between cytochrome complexes provides the energy to

pump protons(H+) across the membrane; in the case of bacterial and archaeal cells, this is


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from the cytosol to the environment. The result of proton expulsion during electron transport is

the formation of a concentration gradient of protons (pH; chemical potential energy) and a

charge gradient (; electrical potential energy). Thus, the cytoplasm is more alkaline and

more negative than the periplasmic space. The combined chemical and electrical potential

differences make up the proton motive force (PMF). The PMF is used to perform work when

protons flow back across the membrane, down the concentration and charge gradients, and

into the prokaryotic cytoplasm. This flow is exergonic and is often used to phosphorylate ADP to

ATP with the help of enzymeATP synthase.

ATP Yield during Aerobic Respiration


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II. ANAEROBIC RESPIRATION:

As we have seen, during aerobic respiration sugars and other organic molecules are

oxidized and their electrons transferred to NAD+ and FAD to generate NADH and FADH2,

respectively. These electron carriers then donate the electrons to an electron transport chain

that uses O2as the terminal electron acceptor. However, it is also possible for other terminal

electron acceptors to be used for electron transport. Anaerobic respiration, aprocess

whereby an exogenous terminal electron acceptor other than O2is used for electron transport,

is carried out by many bacteria and archaea. The most common terminal electron acceptorsused during anaerobic respiration are nitrate, sulfate, and CO2, but metals and a few organic

molecules can also be reduced.

Although some bacteria and archaea grow using only anaerobic respiration, many can

perform both aerobic and anaerobic respiration, depending on the availability of oxygen. One

example is Paracoccus denitrificans, a gram-negative, facultative anaerobic soil bacterium

that is extremely versatile metabolically. It can degrade a wide variety of organic compounds

and can even grow chemolithotrophically. Under anoxic conditions, P. denitrificans uses NO3-as

its electron acceptor. Two differentelectron transport chains are used by this bacterium, one

for aerobic respiration and the second for anaerobic respiration. Notice that during

chemoorgantrophic growth, the source of electrons in both chains is NADH.The aerobic chain

has four complexes that correspond to the mitochondrial chain. When P. denitrificans grows

without oxygen, using NO3- as the terminal electron acceptor, the electron transport chain is

more complex. Also, not as many protons are pumped across the membrane during anaerobic

growth, but nonetheless a PMF is established. The anaerobic reduction of nitrate makes it

unavailable to the cell for assimilation or uptake. Therefore this process is called dissimilatory

nitrate reduction. Nitrate reductase replaces cytochrome oxidase to catalyze the reaction:

NO3-+ 2e-+ 2H+NO2

-+ H2O

Nitrite is quite toxic.Bacteria such as P. denitrificans avoid the toxic effects of nitrite byreducing it to nitrogen gas, a process known as denitrification. By donating five electrons to a

nitrate molecule, NO3-is converted into a nontoxic product.

2NO3-+ 10e- + 12H+N2+ 6H2O

Denitrification is a multistep process with four enzymes participating: nitrate reductase,

nitrite reductase, nitric oxide reductase, and nitrous oxide reductase.

NO3-NO2

-NO N2O N2

In addition to P. denitrificans, some members of the genera Pseudomonas and Bacillus

carry out denitrification. All three genera use denitrification as an alternative to aerobic

respiration and may be considered facultative anaerobes.

Not all microbes employ anaerobic respiration facultatively. Some are obligate

anaerobesthat can carry out only anaerobic respiration. The methanogens are an example.

These archaea use CO2 or carbonate as a terminal electron acceptor. They are called

methanogens because the electron acceptor is reduced to methane. Bacteria such as

Desulfovibrio are another example. They donate eight electrons to sulfate, reducing it to sulfide

(S2-or H2S).

SO42-

+ 8e-

+ 8H+

S2-

+ 4H2O


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Also, both methanogens and Desulfovibrio are able to function as chemolithotrophs,

using H2as an energy source.

In anaerobic respiration, the amount of ATP produced is less than in aerobic respiration.

There are several reasons for this. First, only a portion of the citric acid cycle functions in

anaerobic respiration, so fewer reduced coenzymes are available to the electron transport

chain. Also, not all of the cytochrome complexes function during anaerobic respiration, so the

ATP yield will be less. The exact amount of ATP produced therefore will depend on the

organism and where in the respiratory pathway intermediates enter.

III. FERMENTATION

Despite the tremendous ATP yield obtained by oxidative phosphorylation, some

chemoorganotrophic microbes do not respire because either they lack electron transport

chains or they repress the synthesis of electron transport chain components under anoxic

conditions, making anaerobic respiration impossible. Yet NADH produced by the Embden-

Meyerhof pathway reactions during glycolysis must still be oxidized back to NAD+. If NAD+is

not regenerated, the oxidation of glyceraldehyde 3-phosphate will cease and glycolysis willstop. Many microorganisms solve this problem by slowing or stopping pyruvate

dehydrogenase activity and using pyruvate or one of its derivatives as an electron acceptor

for the reoxidation ofNADH in a fermentation process. There are many kinds of

fermentations, and they often are characteristic of particular microbial groups.

Three unifying themes should be kept in mind when microbial fermentations are

examined: (1) NADH is oxidized to NAD+, (2) the electron acceptor is often either pyruvate

or a pyruvate derivative, and (3) oxidative phosphorylation cannot operate, reducing theATP

yield per glucose significantly. In fermentation, the substrate is only partially oxidized,ATP is

formed exclusively by substrate-level phosphorylation, and oxygen is not needed.

Many fungi, protists, and some bacteria ferment sugars to ethanol and CO2 in a process

called alcoholic fermentation. Pyruvate is decarboxylated to acetaldehyde, which is then

reduced to ethanol by alcohol dehydrogenase with NADH as the electron donor. Lactic acid

fermentation, the reduction of pyruvate to lactate, is even more common. It is present in

bacteria (lactic acid bacteria, Bacillus), protists (Chlorella and some water molds), and even

in animal skeletal muscle. Alcoholic and lactic acid fermentations are quite useful. Alcoholic

fermentation by yeasts produces alcoholic beverages; CO2 from this fermentation causes

bread to rise. Lactic acid fermentation can spoil foods, but also is used to make yogurt,

cheese, and pickles. Also, many bacteria, especially members of the family

Enterobacteriaceae, can metabolize pyruvate to formic acid and other products in a processsometimes called the formic acid fermentation.


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Photosynthesis

Photosynthesis is a process by which light energy is converted to chemical energy that is then

stored as carbohydrate or other organic compounds. In the cyanobacteria, the process takes place in

special thylakoid membranes, which contain chlorophyll or chlorophyll-like pigments. Among

eukaryotes, photosynthesis occurs in the chloroplasts of such organisms as diatoms, dinoflagellates, and

green algae. In all cases, these microbes carry out oxygenic photosynthesis; that is, where oxygen gas

(O2) is a by-product of the process.

The Energy-Fixing Reactions:

In the first part of photosynthesis, light energy is converted into or fixed as chemical energy in

the form of ATP and NADPH. Thus, the process is referred to as energy-fixing reactions, or light-

dependent reactions, because light energy is required for the process. Like the reactions of cellular

respiration, the energy-fixing reactions of photosynthesis are dependent on electrons and protons. The

source of these atomic particles is water. In the cyanobacteria and algae, the splitting of water not only

produces the needed atomic particles, it releases oxygen as a by-product:

2H2O 4H++ 4e+ O2

1. Light energy is absorbed by the green pigment chlorophyll a, a magnesium-containing, lipid solublecompound. Chlorophylls and accessory pigments make up light-receiving complexescalledphotosystems. The light excites pigment molecules in photosystem II,resulting in the loss of oneelectron.

Photosystem I absorbs longer wavelength light (680 nm) and funnels the energy to a

special chlorophyll apair called P700. The term P700 signifies that this molecule mosteffectively absorbs light at a wavelength of 700 nm. Photosystem II traps light at shorterwavelengths (680 nm) and transfers its energy to the special chlorophyll pair P680.

2. These electrons are replaced from the splitting of water. Excited electrons are immediately acceptedby the first of a series of electron carriers. The electrons are passed along the membrane carriers and

cytochrome complexes, and eventually the electrons are taken up by other chlorophyll pigments thatform photosystem I. As the electrons move between cytochromes, energy is made available forproton pumping across the thylakoid membraneof the cyanobacterium, followed bychemiosmosis.As described for oxidative phosphorylation, ATP is formed when protons pass backacross the membrane and release their energy. Because light was involved in the formation of ATP,this process is called photophosporylation.

3. The electrons in photosystem I again are excited by light energy and are boosted out of the pigment

molecules to the first of another set of membrane carriers;

4. And finally to a coenzyme called nicotinamide adenine dinucleotide phosphate (NADP+).The

coenzyme functions much like NAD+in that NADP+receives pairs of electrons and protons from

water molecules to form NADPH.

The Carbon-Fixing Reactions:

5. In the second stage of photosynthesis, another cyclic metabolic pathway forms carbohydrates. Theprocess is known as the carbon-fixing reactionsbecause the carbon in carbon dioxide is trapped (or

fixed) into carbohydrates and other organic compounds. It also is called theCalvin cycle, namedafter Melvin Calvinwho worked out the sequence of reactions. An enzyme, ribulose bisphosphate

carboxylase, bonds carbon dioxide to a 5-carbon organic substance called ribulose 1, 5-bisphosphate

(RuBP).


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6. The resulting 6-carbon molecule then splits to form two molecules of 3-phosphoglycerate (3PG).Inthe next step, the products of the energyfixing reactions, ATP and NADPH, drive the conversion of

3PG to glyceraldehyde-3-phosphate (G3P).

7. Two molecules of G3Pthen condense with each other to form a molecule of glucose.

Thus, the overall formula for photosynthesis may be expressed as:

6 CO2+ 6 H2O + ATPlight C6H12O6+ 6 O2+ ADP + Pi

Notice that this reaction is the reverse of the equation for aerobic respiration. The fundamentaldifference is that aerobic respiration is a catabolic, energy-yielding process, while photosynthesis is an

anabolic, energy-trapping process.

8. To finish off the cycle, most G3P molecules undergo a complex series of enzyme-catalyzed reactionsthat require ATP to reform RuBP. However, some G3P exits the cycle and combines in pairs to formglucose. The sugar then can be used for cell respiration, stored as glycogen, or used for other cellular

purposes.

In addition to the cyanobacteria, a few other groups of bacteria trap energy by photosynthesis. Two

such groups are the green bacteria and purple bacteria, so named because of the colors imparted by theirpigments. These bacterial organisms have chlorophyll-like pigments known as bacteriochlorophylls todistinguish them from other chlorophylls. In the energy-fixing reactions, the organisms do not use water

as a source of hydrogen ions and electrons. Consequently, no oxygen is liberated and the process istherefore called anoxygenic photosynthesis. Instead of water, a series of inorganic or organic substances,

such as hydrogen sulfide gas (H2S) and fatty acids are used as a source of electrons and hydrogen ions.

2H2S + CO2light Carbohydrates + H2O + 2S

Thus, the green and purple bacteria commonly live under anaerobic conditions in environments

such as sulfur springs and stagnant ponds.


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Rhodopsin-Based Phototrophy

Oxygenic and anoxygenic photosynthesis are chlorophyll-based types of phototrophythat is,chlorophyll or bacteriochlorophyll is the major pigment used to absorb light and initiate the conversion

of light energy to chemical energy. This type of phototrophy is observed only in eucaryotes and bacteria;it has not been observed in any archaea, to date. However, some archaea are able to use light as a source

of energy. Instead of using chlorophyll, these microbes use a membrane protein called


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bacteriorhodopsin (more correctly called archaeorhodopsin). One such archaeon is the halophileHalobacterium salinarum.

H. salinarum normally depends on aerobic respiration for the release of energy from an organicenergy source. However, under conditions of low oxygen and high light intensity, it synthesizes

bacteriorhodopsin, a deep-purple pigment that closely resembles the rhodopsin from the rods and cones ofvertebrate eyes. Bacteriorhodopsin functions as a light-driven proton pump. When retinal absorbs light, a

proton is released and the bacteriorhodopsin undergoes a sequence of conformation changes that

translocate the proton into the periplasmic space. The light-driven proton pumping generates a pHgradient that can be used to power the synthesis of ATP by chemiosmosis. However, that this type of

phototrophy does not involve electron transport.

The overall electron flowresulting in ATPgeneration in photosynthesiscan be of2 main typescyclic &Non cyclic.

CYCLICPHOTOPHOSPHORYLATIONINBACTERIA:-In the cyclic type, the high energy electronsejected by the reaction centerpigment flowthrough

a seriesofelectron acceptorsfrom a higherenergy level to a gradually lower energy level &

return to the reaction center, forming there by a close circuit. The lossof energy ofelectronsin

thiscyclicpath isutilized forphosphorylation ofADPto ATP. The only product ofcyclic path is

ATP.NoNADH2 isproduced.

CYCLIC AND NON CYCLIC PHOTO PHOSPHORYLATION INCYANOBACTERIA:-In non-cyclic photo phosphorylation electrons ejected by the reaction centerpigmentcomplex & accepted by ferredoxin are used forreduction ofNAD

+/NADP

+.


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Chemoautotrophy

Chemoautotrophs can grow in a mineral medium, during carbon from CO2 & energy

from the oxidation of inorganic compounds. Some of these bacteria are capable of

growing both Chemoorganotrophically & chemoautotrophically i.e., they are

facultative autotrophs, e.g. Alcaligenes eutrophus.Other chemoautotrophic bacteria are

obligate in nature e.g. Thiobacillus,Nitrosomonas.

Reaction which yield energy in chemoautotrophs are the oxidation ofH2, NH4+,

NO3-, S & reduced sulphur compounds and Fe

++. All these oxidations, except H2

oxidation, couple electron transport to the cytochrome system & NAD+reduction occurs

by energy dependent reverse electron flow.

The assimilation of CO2 in these organisms occurs through the reaction of

the calvin cycle. When grown chemoautotrophically, cells contain high levels of

the 2 enzymesofthispathway namely carboxy dismutase, phosphoribulokinase.

Depending on the oxidisable in organic substrate, the chemoautotrophic bacteria

can be distinguished into following groups:Nitrifying bacteria, sulfuroxidizing bacteria,

H2oxidizing bacteria, Iron oxidizing bacteria & carbon monoxide bacteria.

Nitrifyingbacteria:

Nitrification isa natural processcarried out by the nitrifying bacteria occurring in soil &

aquatic bodies. It involves oxidation of ammonia liberated by decomposition of

nitrogenousorganic matterlike proteins, nucleic acids, urea etc. The oxidation takesplacein 2 steps ammonia to nitrousacid & nitrous acid to nitric acid. The acids react with

metal ions to produce the corresponding salts, nitrite & nitrate. Nitrate acts asmain N-

source ofplants.

The 2 step nitrification carried out by two different groupsofbacteria.

I. First step involving oxidation of NH3 to nitrous acid is called

nitrosification. Eg. Nitrosomonas. The members of this genus are highly

aerobic & strictly autotrophic.

The energy-yielding oxidation reaction ofthese bacteria can be represented as:

NH3+ O2NO2+ 3H

++ 2e

NH3+ O2+ 2H++ 2e

NH2OH + H2O

NH2OH + H2O NO2-+ 5H

++ 4e

II. The second step ofnitrification involvesoxidation ofnitrousacid to nitric acid

& organismsare known asnitrite oxidizing bacteria., Eg.Nitrobacter


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NO2-+ H2O NO3

-+ 2H

++ 2e

The organisms of both groups are capable of generating ATP by oxidative

phosphorylation in course ofelectron transport through the cytochrome system of

the respiratory chain & final electron acceptorisO2.

ATP generated in this way is utilized for CO2 fixation by Calvin Benson cycle.

The part of the ATP generated by oxidative phosphorylation is spent for driving

electronsfrom nitrite toNADthrough a reverse electron transport.

SULFUROXIDIZINGBACTERIA:

Oxidation of elemental sulfur (S0) & various reduced sulfur compounds, like

sulfide (S2-), thio sulfate (S2O3

2-) etc. takes place in soil & aquatic bodiesmediated by

both Eubacteria & Archae bacteria.

The best known among sulfur oxidizing eubacteria are members of genus

Thiobacillus. Some species like T. thiooxidans, T. thioparus & T. denitrificans areobligately chemoautotrophic while T.novellusorT.intermedinsare facultative.

Some eubacteria, designated asfilamentoussulfuroxidizing bacteriabelonging

to the generaBeggiatoa,Thiothrixare able to oxidise sulfide (H2S)to elemental sulfur(S0)

The anoxygenic sulfurpurple & green bacteria like Chromatium,Chlorobiumetc.are able to oxidize sulfide to sulfur.

Thiobacillus oxidize elemental sulfur or sulfur compounds to sulfuric acid. Thereactionsare represented as:

OtherchemoautotrophicbacteriaincludeIron -oxidizingbacteria-Ferrobacillusferrooxidans,Hydrogenoxidizingbacteria -HydrogenoxidizingbacteriaareAlcaligeneseutrophus,Allare facultativechemoautotrophs.Anumberofextremophilicarchaebactriacangrowashydrogenbacteriaeg.Pyrodictium(Anaerobic,obligatelychemoautotrophic).

IMPORTANCEOFCHEMOAUTOTROPHS:

Chemoautotrophs play an important role in oxidation of reduced N and S

compounds like NH3 and H2S to NO3 and SO4 respectively.Nitrates, sulphates are

the utilized formsofnutrientsforhigherplants.

metabolism in bacteria - complete

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