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Microbiology Miniworkshop
3rd lecture: Chapter 8
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Microbial Metabolism
The Metabolism of Microbes
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metabolism – all chemical reactions and physical workings of a
cell
Two types of chemical reactions:
• anabolism – biosynthesis; process that forms larger
macromolecules from smaller molecules; requires energy input
• catabolism – degradative; breaks the bonds of larger
molecules forming smaller molecules; releases energy
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Microbial Metabolism
The Metabolism
of Microbes
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Microbial Metabolism
Enzymes -> Catalysators
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Enzymes are biological catalysts that increase the rate of a
chemical reaction by lowering the energy of activation. (no not
interfere with the equilibrium of reaction)
The enzyme is not permanently altered in the reaction.
Enzyme promotes a reaction by serving as a physical site for
specific substrate molecules to position.
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Microbial Metabolism
Enzymes -> Catalysators
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Microbial Metabolism
Enzyme Structure
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Simple enzymes – consist of protein alone
Conjugated enzymes or holoenzymes – contain protein and
nonprotein molecules
• apoenzyme –protein portion
• cofactors – nonprotein portion
 metallic cofactors – iron, copper, magnesium
 coenzymes -organic molecules (Acetyl-CoA) – vitamins
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Microbial Metabolism
Co-factors
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Microbial Metabolism
Enzymes: Specificity and the Active Site
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Exhibits primary, secondary, tertiary, and some, quaternary
structure
Site for substrate binding is active site, or catalytic site
A temporary enzyme-substrate union occurs when substrate
moves into active site – induced fit
Appropriate reaction occurs; product is formed and released
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Microbial Metabolism
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Microbial Metabolism
Enzymes: Enzyme – Substrate Reactions
ES complex ->
transition state
Induced fit ->
enzyme adapts
slightly -> more
precise binding
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Microbial Metabolism
Enzymes: Location and Regularity of Enzyme Action
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Exoenzymes – transported extracellularly, where they
break down large food molecules or harmful chemicals
• cellulase, amylase, penicillinase
endoenzymes – retained intracellularly and function
there
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Microbial Metabolism
Enzymes: Location and Regularity of Enzyme Action
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Constitutive enzymes – always
present, always produced in
equal amounts or at equal
rates, regardless of amount of
substrate
• enzymes involved in glucose
metabolism
Regulated enzymes – not
constantly present; production
is turned on (induced) or
turned off (repressed) in
response to changes in
concentration of the
substrate
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Microbial Metabolism
Enzymes:
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Microbial Metabolism
Enzymes: Synthesis and Hydrolysis Reactions
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Synthesis or condensation reactions –
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Hydrolysis reactions– catabolic reactions that break down substrates
anabolic reactions to form
covalent bonds between smaller substrate molecules, require ATP, release
one molecule of water for each bond formed
into small molecules; requires the input of water to break bonds
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Microbial Metabolism
Enzymes: Transfer Reactions
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Oxidation-reduction reactions – transfer of electrons
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compound that loses electrons – oxidized
compound that gains electrons – reduced
Aminotransferases – convert one type of amino acid to another
by transferring an amino group
Phosphotransferases – transfer phosphate groups, involved in
energy transfer
Methyltransferases – move methyl groups from one molecule to
another
Decarboxylases – remove carbon dioxide from organic acids
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Microbial Metabolism
Enzymes: Sensitivity to their Environment
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Temperature
pH
Osmotic pressure
Organism’s habitat
-> Changes induce instability
Denaturation: non-covalent bonds are broken
-> loss of structure and function
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Microbial Metabolism
Metabolic Pathways
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Sequence of metabolic
reactions that proceed
in a systematic, highly
regulated manner –
metabolic pathways
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Microbial Metabolism
Control of Enzyme Activity
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Competitive inhibition – substance that resembles normal
substrate competes with substrate for active site
Noncompetitive inhibition – enzymes are regulated by the
binding of molecules other than the substrate on the
active site
• feedback inhibition – concentration of product at the
end of a pathway blocks the action of a key enzyme
• feedback repression – inhibits at the genetic level by
controlling synthesis of key enzymes
-> enzyme induction – enzymes are made only when
suitable substrates are present
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Microbial Metabolism
Control of Enzyme Activity
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Microbial Metabolism
Control of Enzyme Activity
-> enzyme induction/repression – enzymes are made only
when suitable substrates are present
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Microbial Metabolism
The Pursuit and Utilization of Energy
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Energy – the capacity to do work or to cause change
The ultimate source of energy is the sun (with the
exception of certain chemoautotrophs).
-> in the cell : Energy = chemical Energy (ATP)
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Microbial Metabolism
The Pursuit and Utilization of Energy: Cell Energetics
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Cells manage energy in the form of chemical reactions that make
or break bonds and transfer electrons.
Endergonic reactions – consume energy
Exergonic reactions – release energy
Energy present in chemical bonds of nutrients are trapped by
specialized enzyme systems as the bonds of the nutrients are
broken.
Energy released is temporarily stored in high energy phosphate
molecules (ATP). The energy of these molecules is used in
endergonic cell reactions.
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Microbial Metabolism
The Pursuit and Utilization of Energy: Cell’s Energy Machine
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Microbial Metabolism
The Pursuit and Utilization of Energy:
-> Biological Oxidation and Reduction
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Redox reactions - always occur in pairs
There is an electron donor (reduced) and electron acceptor
(oxidized) which constitute a redox pair.
Process salvages electrons and their energy
Released energy can be captured to phosphorylate ADP (->
ATP) or another compound.
Electron and Proton Carriers:
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Repeatedly accept and release electrons and hydrogen to
facilitate the transfer of redox energy
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Most carriers are coenzymes:
NAD, FAD, NADP, coenzyme A and compounds of the
respiratory chain
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Microbial Metabolism
The Pursuit and Utilization of Energy:
-> Biological Oxidation and Reduction
Reduction of
NAD+
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Microbial Metabolism
The Pursuit and Utilization of Energy:
-> Adenosine Triphosphate: ATP
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Three part molecule consisting of:
• adenine – a nitrogenous base
• ribose – a 5-carbon sugar
• 3 phosphate groups
ATP utilization and replenishment is
a constant cycle in active cells.
Removal of the terminal phosphate
releases energy.
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Microbial Metabolism
The Pursuit and Utilization of Energy:
-> Formation of ATP
ATP can be formed by three different mechanisms:
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Substrate-level phosphorylation – transfer of phosphate group
from a phosphorylated compound (substrate) directly to ADP
Oxidative phosphorylation – series of redox reactions occurring
during respiratory pathway
Photophosphorylation – ATP is formed utilizing the energy of
sunlight
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Microbial Metabolism
Pathways of Bioenergetics
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Bioenergetics – study of the mechanisms of cellular energy
release and use
Includes catabolic and anabolic reactions
Primary catabolism of fuels (glucose) proceeds through a series
of three coupled pathways:
•
glycolysis
•
tricarboxylic acid cycle, Kreb’s cycle
•
respiratory chain, electron transport
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Microbial Metabolism
Pathways of Bioenergetics
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Microbial Metabolism
Metabolic Strategies
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Nutrient processing is varied, yet in many cases is based on
three catabolic pathways that convert glucose to CO2 and
gives off energy (ATP).
Aerobic respiration – glycolysis, the TCA cycle, respiratory
chain; O2 final electron acceptor
Anaerobic respiration - glycolysis, the TCA cycle,
respiratory chain; molecular oxygen is not final electron
acceptor
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Microbial Metabolism
Metabolic Strategies
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
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Series or enzyme-catalyzed reactions in which electrons are
transferred from fuel molecules (glucose) to oxygen as a final
electron acceptor
Glycolysis – glucose (6C) is oxidized and split into 2 molecules of
pyruvic acid (3C)
TCA – processes pyruvic acid and generates 3 CO2 molecules and
NADH + FADH2
Electron transport chain – accepts electrons NADH and FADH2;
generates energy through sequential redox reactions called
oxidative phosphorylation -> generates ATP
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
Electron Transport and Oxidative Phosphorylation
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Final processing of electrons and hydrogen and the major
generator of ATP
Chain of redox carriers that receive electrons from reduced
NADH and FADH2 (from TCA cycle)
ETS shuttles electrons down the chain, energy is released and
subsequently captured and used by ATP synthase complexes to
produce ATP. – oxidative phosphorylation
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
Electron Transport and Oxidative Phosphorylation
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
The Formation of ATP and Chemiosmosis
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Chemiosmosis – as the
electron transport carriers
shuttle electrons, they
actively pump hydrogen ions
(protons) across the
membrane setting up a
gradient of hydrogen ions proton motive force.
Hydrogen ions diffuse back
through the ATP synthase
complex causing it to rotate,
causing a 3-dimensional
change resulting in the
production of ATP.
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
The Formation of ATP and Chemiosmosis
1 mol glucose -> aerobic 38 mol ATP
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Microbial Metabolism
Metabolic Strategies: Aerobic Respiration
The Terminal Step
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Oxygen accepts 2 electrons from the ETS and then picks up 2
hydrogen ions from the solution to form a molecule of water.
Oxygen is the final electron acceptor.
2H+ + 2e- + ½O2 → H2O
-> in anaerobic respiration: different electron acceptors !!!
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Microbial Metabolism
Metabolic Strategies: Anaerobic Respiration
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Functions like aerobic respiration except it utilizes oxygen
containing ions, rather than free oxygen, as the final electron
acceptor (facultative anaerob: E. coli, Pseudomonas, Bacillus,…)
• Nitrate (NO3-) and nitrite (NO2-)
Most obligate anaerobes use the H+ generated during glycolysis
and TCA to reduce some compound other than O2.
Methanogens: reduce CO2 -> CH4
Sulfate Bacteria: reduce SO42- -> H2S
Some bacteria use metals or organic compounds as H+ acceptor
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Microbial Metabolism
Metabolic Strategies: Fermentation
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Incomplete oxidation of
glucose or other carbohydrates
in the absence of oxygen
Uses organic compounds as
terminal electron acceptors
Yields a small amount of ATP
Production of Ethanol by
yeasts acting on glucose
Formation of acid, gas and
other products by the action
of various bacteria on pyruvic
acid
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Microbial Metabolism
Metabolic Strategies: Fermentation
Products of different microorganisms growing under anaerobic conditions
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Microbial Metabolism
Biosynthesis and the Crossing Pathways of Metabolism
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Many pathways of metabolism are bi-directional or
amphibolic.
Catabolic pathways contain molecular intermediates
(metabolites) that can be diverted into anabolic pathways.
• pyruvic acid can be converted into amino acids through
amination
• amino acids can be converted into energy sources
through deamination
• glyceraldehyde-3-phosphate can be converted into
precursors for amino acids, carbohydrates and fats
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Microbial Metabolism
Biosynthesis and the Crossing Pathways of Metabolism
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Microbial Metabolism
Biosynthesis and the Crossing Pathways of Metabolism
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