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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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:
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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-
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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:
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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.
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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:
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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
S. Y. B.Sc. BT Unit: 2 Notes
Compiled by: Gunjan Mehta, Asst Professor, Virani Science College.
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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+