diversity of metabolic process among bacteria

S. Y. B.Sc. BT Unit: 2 Notes

Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.

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DIVERSITY OF METABOLIC PROCESS AMONG BACTERIA

Metabolism refers to all the biochemical reactions that occur in a cell or

organism. The study of bacterial metabolism focuses on the chemical diversity of

substrate oxidations and dissimilation reactions (reactions by which substrate

molecules are broken down), which normally function in bacteria to generate

energy. Also within the scope of bacterial metabolism is the study of the uptake

and utilization of the inorganic or organic compounds required for growth and

maintenance of a cellular steady state (assimilation reactions). These respective

exergonic (energy-yielding) and endergonic (energy-requiring) reactions are

catalyzed within the living bacterial cell by integrated enzyme systems, the end

result being self-replication of the cell. The capability of microbial cells to live,

function, and replicate in an appropriate chemical milieu (such as a bacterial

culture medium) and the chemical changes that result during this transformation

constitute the scope of bacterial metabolism.

Metabolic diversity is the hallmark of the bacteria, just as morphological

complexity is the hallmark of the eukaryotes. Many bacteria and Achaea can

extract energy from reduced carbon compounds, such as sugars, through

fermentation pathways or by transferring high-energy electrons to electron

transport chains with oxygen as the final electron acceptor. But among the

bacteria many different reduced inorganic or organic compounds serve as

electron donors, and a wide variety of oxidized inorganic molecules serve as

electron acceptors. Dozens of distinct organic compounds are fermented,

including proteins, purines, alcohols, and an assortment of carbohydrates.

Few of the diversified bacterial metabolism are indicated here…

Unique fermentations proceeding through the Embden-Meyerhof pathway

Other fermentation pathways such as the phosphoketolase (heterolactic) and Entner-Doudoroff pathways

Lithotrophy: use of inorganic substances as sources of energy

Photoheterotrophy: use of organic compounds as a carbon source during bacterial photosynthesis

Anoxygenic photosynthesis: photophosphorylation in the absence of O2

Methanogenesis: an ancient type of archaean metabolism that uses H2 as an energy source and produces methane

Light-driven nonphotosynthetic photophosphorylation: unique archaean metabolism that converts light energy into chemical energy

In addition, among autotrophic procaryotes, there are three ways to fix CO2, two of which are unknown among eucaryotes, the CODH (acetyl CoA pathway) and the reverse TCA cycle.



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1. Metabolism of organic matter (heterotrophic

metabolism): Heterotrophic bacteria, which include all pathogens, obtain energy from

oxidation of organic compounds. Carbohydrates (particularly glucose), lipids, and

protein are the most commonly oxidized compounds. Biologic oxidation of these

organic compounds by bacteria results in synthesis of ATP as the chemical

energy source. This process also permits generation of simpler organic

compounds (precursor molecules) needed by the

bacteria cell for biosynthetic or assimilatory reactions.

All heterotrophic bacteria require preformed organic compounds. These

carbon- and nitrogen-containing compounds are growth substrates, which are

used aerobically or anaerobically to generate reducing equivalents (e.g., reduced

nicotinamide adenine dinucleotide; NADH + H+); these reducing equivalents in

turn are chemical energy sources for all biologic oxidative and fermentative

systems.

Heterotrophs are the most commonly studied bacteria; they grow readily in

media containing carbohydrates, proteins, or other complex nutrients such as

blood. Also, growth media may be enriched by the addition of other naturally

occurring compounds such as milk (to study lactic acid bacteria) or hydrocarbons

(to study hydrocarbon-oxidizing organ isms).

Two most common metabolism of organic matter (heterotrophic metabolism)

are..

Metabolism of Organic compound

Respiration

Aerobic respiration

Anaerobic respiration

Fermentation

Acidogenic fermentation

Anoxic fermentation



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A. Respiration: “Biochemical process in which organic compound serve as electron donor while

external compound serve as terminal electron acceptor”.

Aerobic respiration:

Glucose is the most common substrate used for studying heterotrophic

metabolism. Most aerobic organisms oxidize glucose completely by the following

reaction equation: This equation expresses the cellular oxidation process called

respiration.

Respiration occurs within the cells of plants and animals, normally generating 38

ATP molecules (as energy) from the oxidation of 1 molecule of glucose. This

yields approximately 380,000 calories (cal) per mode of glucose (ATP ~ 10,000

cal/mole).

Thermodynamically, the complete oxidation of one mole of glucose should yield

approximately 688,000 cal; the energy that is not conserved biologically as

chemical energy (or ATP formation) is liberated as heat (308,000 cal). Thus, the

cellular respiratory process is at best about 55% efficient.

Metabolically, bacteria are unlike cyanobacteria (blue-green algae) and

eukaryotes in that glucose oxidation may occur by more than one pathway. In

bacteria, glycolysis represents one of several pathways by which bacteria can

catabolically attack glucose. The glycolytic pathway is most commonly

associated with anaerobic or fermentative metabolism in bacteria and yeasts. In

bacteria, other minor heterofermentative pathways, such as the phosphoketolase

pathway, also exist.



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Anaerobic respiration:

Instead of O2 other inorganic compound serve as terminal electron acceptor. Some

bacteria exhibit a unique mode of respiration called anaerobic respiration. These

heterotrophic bacteria that will not grow anaerobically unless a specific chemical

component, which serves as a terminal electron acceptor, is added to the medium.

Among these electron acceptors are NO3–, SO42–, the organic compound fumarate,

and CO2. Bacteria requiring one of these compounds for anaerobic growth are said

to be anaerobic respirers.



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B. Fermentation: “Special kind of redox reaction in which both electron donors and acceptors are

organic in nature”.

Internally generated organic compounds such as pyruvic acid can serve as

electron acceptor when external electron acceptor is absent.

Reduced compounds produced during such reactions are secreted in

extracellular.

Fermentation yields less amount of ATP molecules than respiration, as in

fermentation reaction organic compounds can’t be fully oxidized to CO2 &

H2O.

In Fermentation, the organic compounds are simply rearranged into a form

containing less energy than the organic substrate.

Fermentation generate few ATPs per molecule of substrate than respiration,

hence more substrate molecules should be metabolize during it.

ATP synthesis during fermentation is substrate level phosphorylation and

remains largely restricted to amount formed during glycolysis.

Chemiosmotic synthesis and oxidative phosphorylation of ATP don’t occur in

fermentation.

Fermentation pathways Fermentation Vs Respiration



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For most microbial fermentations, glucose dissimilation occurs through the

glycolytic pathway. The simple organic compound most commonly generated is

pyruvate, or a compound derived enzymatically from pyruvate, such as

acetaldehyde, α-acetolactate, acetyl ~ SCoA, or lactyl ~ SCoA Acetaldehyde can

then be reduced by NADH + H+ to ethanol, which is excreted by the cell.

For thermodynamic reasons, bacteria that rely on fermentative process for

growth cannot generate as much energy as respiring cells. In respiration, 38 ATP

molecules (or approximately 380,000cal/mole) can be generated as biologically

useful energy from the complete oxidation of 1 molecule of glucose (assuming 1

NAD(P)H = 3 ATP and 1 ATP → ADP + Pi = 10,000 cal/mole). Although only 2 ATP

molecules are generated by this glycolytic pathway, this is apparently enough energy

to permit anaerobic growth of lactic acid bacteria and the ethanolic fermenting yeast,

Saccharomyces cerevisiae.

Fermentation Vs Respiration:

Aerobic respiration Vs Anaerobic respiration:



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Metabolism of Inorganic matter: Nitrate reduction

Sulfate reduction

Lithotrophy: a) Hydrogen bacteria b) Sulfur bacteria

Mainly followed during anaerobic respiration by Nitrate reducing bacteria,

Sulfate reducing bacteria and many other lithotrophic bacteria.

These metabolic pathways are the most integral parts of Biogeochemical

cycles.

1. Nitrate reduction/ Denitrification:

Nitrate reduction takes place through both assimilatory and dissimilatory cellular

functions.

Assimilatory denitrification: nitrate is reduced to ammonia, which then serves

as a nitrogen source for cell synthesis. Thus, nitrogen is removed from the liquid

stream by incorporating it into cytoplasmic material.

Dissimilatory denitrification: nitrate serves as the electron acceptor in energy

metabolism and is converted to various gaseous end products but principally

molecular nitrogen, N2, which is then stripped from the liquid stream.

A relatively small fraction of the nitrogen is removed through assimilation.

Dissimilatory denitrification is, therefore, the primary means by which nitrogen

removal is achieved.

A carbon source is also essential as electron donor for denitrification to take

place..

Denitrification releases nitrogen which escapes as an inert gas to the

atmosphere while oxygen released stays dissolved in the liquid and thus reduces

the oxygen input needed into the system. Each molecule of nitrogen needs 4

molecules of oxygen during nitrification but releases back 2.5 molecules in

denitrification. Thus, theoretically, 62.5% of the oxygen used is released back in

denitrification.



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a) Assimilatory denitrification:

Nitrate is reduced to ammonia, which then serves as a nitrogen source for

cell synthesis. Thus, nitrogen is removed from the liquid stream by

incorporating it into cytoplasmic material.

Since oxidation state of nitrogen in nitrate is +5 and in ammonia it is -3. Total 8 e-



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must be transferred to nitrate in order to reduce it to ammonia.

b) Dissimilatory denitrification:

In dissimilatory denitrification, nitrate serves as the electron acceptor in

energy metabolism and is converted to various gaseous end products but

principally molecular nitrogen, N2, which is then stripped from the liquid

stream.

Dissimilatory denitrification is the primary means by which nitrogen removal is

achieved.



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Nitrogenous oxides, principally NO3-, NO2- are used as terminal electron

acceptor in the absence of O2 and reduced molecular nitrogen N2 during

microbial metabolism.

Enzymes for denitrification procedure are oxygen sensitive and works under

anaerobic condition.

All denitrifying organisms are facultative anaerobes such as Pseudomonas

and Alcaligens. Achromobacter, Vibrio, Flavobacterium

Assimilatory denitrification Vs Dissimilatory denitrification:

2. Sulfate reduction:

SO42-( most oxidized form of S) can be used as terminal electron acceptor by

a specialized group of microbes which are known as Sulfur reducing

bacteria….

SO42- first reduced to sulfite(SO3

- ) and then to sulfide(H2S) or S2- and then

incorporated into Cysteine.



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The oxidation level of S in SO42- is (+6) and in sulfide it is (-2), so total 8

electrons are required to reduce SO42- to S2-.

Gram +ve: Desulfotomaculum

Gram –ve: Desulfovibrio

Archaebacteria: Archaeglobus

a) Assimilatory sulfate reduction:



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There is sound thermodynamic reason behind the formation of APS, which is

AMP derivative of sulfate.

The reduction potential of sulfate can be increased by attaching AMP, which

makes it a better electron acceptor than free sulfate.

Formation of PAPS: The reductant is sulfhydryl protein called thioridoxin, which

accepts e- from NADPH.

3 ATP molecules are used:

o 2 ATP for PAPS

o 1 ATP for AMP formation

b) Desimmilatory sulphate reduction :

Dissimilation of sulfate is very rare as it yields less amount of energy than any

alternative e- donor as nitrate or oxygen.

Since energy influences growth and metabolism of these SRBs, it shows slow

growth.



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3. Lithotrophy:

The study of metabolism of organism using reduced inorganic material is called

lithotrophy.

CO2- Lithoautotrophs(Most of)

H2, NH3, H2S, NO2, Fe+2, CO- Lithotrophs(Some)

Consist of one of the major class of autolithotrophs and very important for it.

Classification of lithotrophs:



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Hydrogen bacteria:

Facultative lithotrophs

Also known as hydrogen oxidizing bacteria capable of utilizing H2 as source

of energy

Majority of these bacteria are aerobic capable of utilizing O2 as terminal e-

acceptor.

However they are not purely dependent on H2 as energy source but are

capable of utilizing other organic sources. That’s why it is facultative

lithotrophs.

CO2+ 2H2[CH2O]n+H2O

Hydrogen is oxidized by membrane bound hydrogenase



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Sulfur oxidizing bacteria:

It includes….

1. Photosynthetic sulfur oxidizers: Green sulfur bacteria & purple sulfur bacteria

2. Non- Photosynthetic sulfur oxidizers: colourless sulfur bacteria such as

Baggiatoa, Thiothrix

Almost all are gram –ve

e- donors for SOBs: H2S, S2-, S2O3

Comprises physiologically diverse group of bacteria:

1. Obligatory autotrophs(CO2- sole C source)

2. Facultative heterotrophs(Mixotrophic) Eg: Baggiatoa

Few sulfur oxidizing bacteria are archaebacteria such as Sulfolobus.

Lives on sulfur rich spring- hot spring in temperature range upto 90º C and

pH=1.

H2S+2CO2 SO42- + 2H+

S+ H2O+O2 SO42-+ 2H+

S2O3+ H2O+2O2 SO42- + 2H+

diversity of metabolic process among bacteria

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