Download Chapter 5- Metabolism of bacteria

MICROBIOLOGY
WITH DISEASES BY TAXONOMY, THIRD EDITION
Chapter 5
Microbial Metabolism
Lecture prepared by Mindy Miller-Kittrell, University of Tennessee, Knoxville
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Microbial Metabolism
Metabolism =
 The sum of all chemical reactions that take place in an
organism
 Ability of an organism to obtain, convert and utilize energy
Metabolic pathway =
 A sequence of chemical reactions in a cell in which the end
product becomes the substrate for the next reaction
 Enzymatically catalyzed
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Basic Chemical Reactions Underlying Metabolism
 Catabolism and anabolism
 Oxidation and reduction reactions
 ATP production and energy storage
 The roles of enzymes in metabolism
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Catabolism and Anabolism
 Catabolism = breakdown reactions
 complex molecules to simple
 hydrolytic (use water)
 energy-releasing (exergonic)
 Anabolism = synthesis reactions
 simple molecules to complex
 dehydration (produce water)
 energy-using (endergonic)
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Catabolism and Anabolism
Catabolism provides the building blocks and energy (in the form of ATP)
for anabolism.
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Oxidation and Reduction Reactions
• Oxidation-Reduction (Redox) reactions
– Transfer electrons from one molecule to another
– Oxidized molecule – Donates electron
– occurs when electrons, and often protons, (hydrogen
atoms) are released from a molecule
– Reduced molecule – Accepts electron
– occurs when electrons, and often protons, (hydrogen
atoms) are accepted by a molecule
– OILRIG (Oxidation Is Loss, Reduction Is Gain)
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Oxidation-Reduction (Redox) Reactions
 The transfer of electrons from one molecule to another. These
reactions occur simultaneously.
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Figure 5.2
Oxidation-Reduction (Redox) Reactions
 Reducing agent:
 Causes the reduction of other molecules by donating electrons to
them (and itself gets oxidized in the process)
 Sodium thioglycollate media
 Thioglycolic acid (TGA) is readily oxidized by air to disulfide
 Oxidizing agent:
 Causes the oxidation of other molecules by accepting their
electrons (and itself gets reduced in the process)
 Oxygen
 Hydrogen peroxide
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Electron Carrier Molecules
 Electron carrier molecules are used to carry electrons from one
location in the cell to another.
 Often they are carried in the form of hydrogen atoms (one
proton and one electron)
 Important electron carriers in cell metabolism
– NAD+ → NADH
– Nicotinamide adenine dinucleotide
– FAD → FADH2
– Flavine adenine dinucleotide
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ATP Production and Energy Storage
 Energy produced by catabolism is stored as ATP
 Is needed for many metabolic reactions
 ATP is generated by the phosphorylation of ADP
 (Addition of Phosphate)
 Occurs ~ 3 million times/second
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ATP Structure
When energy is needed
the high energy bond
that binds phosphate to
ADP (ADP~P) is broken
and energy is released
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ATP Production
Substrate-level phosphorylation –
 high energy phosphate is transferred from a
phosphorylated compound to ADP
Oxidative phosphorylation –
 energy from redox reactions used to attach inorganic
phosphate to ADP
Photophosphorylation –
 light energy used to attach inorganic phosphate to ADP
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The Roles of Enzymes in Metabolism
Enzymes
• Proteins that speed up a reaction by decreasing the
‘activation’ energy required to start a chemical reaction
• Biological catalysts- are NOT used up in reaction
• Necessary for chemical reactions to occur fast enough to
maintain life
• Do not make reactions happen that would not ordinarily take
place
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The effect of enzymes on chemical reactions
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Figure 5.4
The Roles of Enzymes in Metabolism
– Enzymes have active sites that bind to a very specific substrate
• Named according to type of catalyzed reaction or substrate
• Most end in “ –ase”
• Examples:
– Transferase Transfer functional groups
– Ligase
Joining of molecules
– Lipase
Fat digestion
– Sucrase
Hydrolysis of sucrose
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Enzyme Components
• Many enzymes are made of protein alone
• Some enzymes are composed of:
– Protein portions (Apoenzymes) and
– Non-protein portions:
– Cofactors -inorganic ions (Fe, Cu,Zn) or
– Coenzymes –organic molecules
– (NAD+, NADP+, FAD)
• Holoenzyme -Complete active enzyme
• Active site
– Region that interacts with the substrate
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Enzyme Components
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Enzyme Activity
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Enzyme Activity
 Many factors influence the rate of enzymatic reactions




Temperature
pH
Enzyme and substrate concentrations
Presence of inhibitors
 Competitive
 Non-competitive
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Temperature and pH
Enzymes can be
denatured by
temperature and
pH
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Figure 5.8a
Substrate Concentration
Saturation point
•Active sites on
enzymes are filled
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Figure 5.8c
Competitive Inhibitors
• Substances that block an enzyme’s active site
• Do not denature enzymes
– Sulfa drugs compete with normal substrate for bacterial
enzymes that make folic acid
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Noncompetitive Inhibitors
• Substances that alter an enzyme’s active site structure by
binding to enzyme at allosteric site
– Nevirapene (HIV) attaches to reverse transcriptase so it
can’t make DNA
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Feedback Inhibition
 Feedback inhibition
stops the metabolic
pathway when final end
product accumulates
 Conserves energy in
organism
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Metabolic Pathway
Sequence of chemical reactions that slowly releases energy
from molecules.
Usually each step catalyzed by a specific enzyme.
Product of one reaction serves as the substrate for the next
one.
Occurs through oxidation-reduction reactions.
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Carbohydrate Catabolism
• The breakdown of carbohydrates to release energy (in the form
of ATP)
• Carbohydrates are catabolized by:
1. Cellular respiration :
– Utilizes glycolysis, Krebs cycle, and electron transport chain
– Glucose is completely decomposed
– Aerobic – Oxygen is final e- acceptor
– Anaerobic – Inorganic substrate is final e- acceptor (ex.
sulfur, iron, nitrogen)
2. Fermentation :
– Utilizes glycolysis- then converts pyruvic acid into another
compound (organic waste products)
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Aerobic Respiration
 C6H12O6 + 6O2 + 38ADP+ 38P  6CO2 + 6H2O +38ATP
 Divided into “four” metabolic steps:
1. Glycolysis
2. Transition step
3. Krebs cycle
4. Electron Transport Chain
• Alternative pathways:
Pentose phosphate pathway- breakdown of 5C sugar
Entner-Doudoroff pathway- used by Pseudomonas aeruginosa &
Enterococcus faecalis
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Carbohydrate Catabolism
• Glycolysis
– Divided into three stages involving 10 total steps
– Energy-investment stage
– Lysis stage
– Energy-conserving stage
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Glycolysis
• Oxidation of glucose to pyruvic acid
• 2 ATP are invested to begin the biochemical pathway.
• Four molecules of ATP are generated through substrate level
phosphorylation.
• Result –
– net 2 ATP and 2 molecules of NADH. Each molecule of
NADH will be converted to 3 ATP in the Electron Transport
Chain.
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Glycolysis
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Figure 5.13
Carbohydrate Catabolism
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Substrate-level phosphorylation
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Figure 5.14
Transition Step
• Pyruvic acid loses a carbon (released as CO2) to become
acetyl-coA which enters the Krebs Cycle.
• 1 NADH produced for each acetyl-coA formed from pyruvic
acid– 2 pyruvic acid molecules from one glucose molecule.
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Krebs Cycle
•2 ATP’s produced by
substrate-level
phosphorylation
•6 NADH (potential 18
ATP)
•2 FADH2 (potential of 4
ATP)
•Each Acetyl Co-A goes
through the Krebs Cycle
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Carbohydrate Catabolism
• Cellular Respiration
– Electron transport
– Most significant production of ATP occurs from series of
redox reactions known as an electron transport chain (ETC)
– Series of carrier molecules that pass electrons from one to
another to final electron acceptor
– Energy from electrons used to pump protons (H+) across the
membrane, establishing a proton gradient
– Located in cristae of eukaryotes and in cytoplasmic
membrane of prokaryotes
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Electron Transport Chain
NADH and FADH2 carry protons and electrons in the form of
hydrogen to the ETC.
Sequence of carrier molecules where oxidation-reduction
reactions occur.
As electrons are passed along the carrier molecules in the ETC,
some hydrogen ions are pushed across the membrane in which
they are located
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An electron transport chain
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Figure 5.17
Carbohydrate Catabolism
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Carbohydrate Catabolism
• Cellular Respiration
– Electron transport
– Four categories of carrier molecules
– Flavoproteins
– Ubiquinones
– Metal-containing proteins
– Cytochromes
– Aerobic respiration: oxygen serves as final electron acceptor
– Anaerobic respiration: molecule other than oxygen serves as
final electron acceptor
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Electron Transport Chain
http://vcell.ndsu.nodak.edu/animations/etc/movie.htm
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Figure 5.20
Chemiosmosis
 The process of releasing electrons from the carrier
molecules in the ETC to produce ATP
 As hydrogen ions collect across the membrane, they
produce a potential energy source (like a battery) called the
Proton Motive Force.
 These hydrogen ions move from this area of high
concentration back across the membrane through channels
in the membrane that contain ATP synthase.
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Chemiosmosis
 ATP synthase converts the energy produced by the H+
flow into ATP.
 Electrons are passed to the final electron acceptor
(oxygen or another inorganic compound).
 34 ATP produced in ETC by oxidative phosphorylation.
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Chemiosmosis
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ATP From Aerobic Respiration
• Total Net ATP from 1 glucose molecule
4 ATP
=
4 ATP
10 NADH = 30 ATP
2 FADH2 =
Total
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4 ATP
38 ATP for prokaryotes
Aerobic Respiration
Pathway
Eukaryote
Prokaryote
Glycolysis
Cytoplasm
Cytoplasm
Transition step
Cytoplasm
Cytoplasm
Krebs cycle
Mitochondrial
matrix
Cytoplasm
Electron Transport Mitochondrial
inner membrane
Chain
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Plasma
membrane
Anaerobic respiration
•The final electron acceptor in the ETC is not O2.
•Yields less energy than aerobic respiration because
only part of the Krebs cycles operates under anaerobic
conditions.
Electron acceptor
Products
E. coli
NO3–
NO2–, N2 + H2O
Desulfovibrio
SO42-
H2S + H2O
Methanococcus CO2
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CH4 + H2O
Fermentation
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Figure 5.13
Fermentation
 Produces 2 ATP from
glycolysis
 Most of the potential
energy remains in the
bonds of fermentation
products
 Fermentation
products are wastes
to cells that make
them, many are useful
to humans
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Figure 5.22
Fermentation
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Catabolism of Organic Molecules
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Lipid Catabolism
LIPID =
•1 glycerol
•3 fatty acids
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Protein Catabolism
• Protein is too large to pass through plasma membrane
• Extracellular enzymes break down protein into amino acids
that can pass through plasma membrane
Extracellular proteases
Protein
and peptidases
Deamination, decarboxylation,
dehydrogenation
Amino acids
Organic acid
Krebs cycle
• Amino acids are enzymatically converted to organic acids
that can enter the Krebs cycle
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Protein Catabolism
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How Metabolism Is Used in Bacterial Identification
 Test for
presence of
enzymes
 Detect end
product of
metabolic
pathways
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