Two ways to Regulate a Metabolic Pathway

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Two ways to Regulate a Metabolic Pathway
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Control of Metabolism in Prokaryotes and Eukarotes

• Two major ways to control metabolism
• Allosteric control of Enzyme activity negative
  feedback and feedback inhibition
• A quick short-term response
• E1 E2 E3
  E4
• A ? B ? C ? D ?
  Product
• Recall the role played by PFK (PhosphoFructoKinase
  ) in the Allosteric control of glycolysis
• PFK Activated by _________________________
• PFK Inhibited by _________________________

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2. Regulation of Gene Expression (Prokaryotes)

• Anabolic Pathways (e.g. tryptophan synthesis)
• Product accumulation inhibits the transcription
  (mRNA synthesis) of genes coding for the enzymes
  needed to make the product.
• Enzymes are not made unless they are needed
• Catabolic Pathways (e.g. lactose breakdown)
• Presence of substrate activates the
  transcription (mRNA synthesis) of genes coding
  for the enzymes needed to breakdown the
  substrate.
• Enzymes are not made unless they are needed

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2. Regulation of Gene Expression (Eukaryotes)

• Much more complex in Eukaryotes than in
  Prokaryotes!
• Some mechanisms include..
• Accessibility of genes ? condensed (coiled) DNA
  prevents transcription ? RNA polymerase cant
  access the promoter
• e.g. Barr bodies One X chromosome is
  inactivated in females by producing a
  tightly-wound structure called a Barr body
• Transcriptions factors ? activate or inhibit
  transcription
• e.g. p53 protein is a transcription factor
• Chemical modification of DNA (e.g. DNA
  methylation)
• Permanently inactivates genes

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The trp operon a repressible operon
Anabolic Pathway for Tryptophan Synthesis from
Precursor Molecule A E1
E2 E3 E4
E5 A ? B ? C ?
D ? E ? Tryptophan
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Control of Gene Expression in Prokaryotes

• Vocabulary
• Operon Regulated cluster of structural genes
  that have a common function (e.g. lac Operon, trp
  Operon)
• Structural genes code for mRNA
• Advantage of an Operon?
• Promoter Site on where RNA polymerase binds to
  DNA
• Operator Controls access of RNA polymerase to
  structural genes
• Acts as an on/off switch for transcription
• Located between promoter and operon

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The trp operon regulated synthesis of
repressible enzymes (Layer 1)
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The trp operon regulated synthesis of
repressible enzymes (Layer 2)
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Control of Gene Expression in Prokaryotes

• Vocabulary (continued)
• Repressor
• Protein that binds reversibly to operator
• Binding to operator blocks the transcription of
  the operon
• Co-repressor (involved with repressible
  (anabolic) operonse.g. trp operon)
• Molecule that binds reversibly repressor protein
• Co-repressor binding activates the repressor
• Co-repressor-repressor complex binds to operator
  ? operon not transcribed

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The lac operon regulated synthesis of inducible
enzymes
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The lac operon regulated synthesis of inducible
enzymes
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cAMP Receptor Protein (CRP) Lac Operon
Transcribed only if Lactose is present in the
absence of Glucose
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Lac Operon is not transcribed if glucose is
present
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cAMP (Cyclic AMP) Activates Lac Operon When
Lactose is present in the absence of Glucose

• Cellular concentrations of cAMP increase as
  cellular concentration of glucose decrease.
• cAMP binds to an inactive CRP (Cyclic AMP
  Receptor Protein)
• cAMP-CRP complex ? binds to promoter ? Lac Operon
  Transcribed

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ALE 11. Question 7 on Page 11
Mutation Effect of mutation on lac operon when Effect of mutation on lac operon when
Mutation Allolactose present Allolactose absent
Mutation of regulatory gene Repressor will not bind to allolactose
Mutation of operator Repressor will not bind to operator
Mutation of regulatory geneRepressor will not bind to operator
Mutation of promoter RNA polymerase will not bind to promoter
It was through the effects of mutations that
enabled Jacob and Monod to decipher how the lac
operon works. Predict how the following mutations
would affect lac operon function in the presence
and absence of allolactose. Note use this
question to test your knowledge of the lac
operon. Study the how the lac operon works, then
attempt this question, using only your cerebral
cortex as a reference
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Figure 18.10 A hypothesis to explain how prions
propagate
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Figure 18.11 Replication of the bacterial
chromosome
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Figure 18.x7 E. coli
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Figure 18.x8 E. coli dividing
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Figure 18.x9 Bacterium releasing DNA with
plasmids
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Figure 18.x10 Plasmids
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Figure 18.12 Detecting genetic recombination in
bacteria
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Figure 18.13 Transduction (Layer 1)
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Figure 18.13 Transduction (Layer 2)
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Figure 18.13 Transduction (Layer 3)
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Figure 18.13 Transduction (Layer 4)
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Figure 18.14 Bacterial mating
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Figure 18.15 Conjugation and recombination in E.
coli (Layer 1)
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Figure 18.15 Conjugation and recombination in E.
coli (Layer 2)
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Figure 18.15 Conjugation and recombination in E.
coli (Layer 3)
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Figure 18.15 Conjugation and recombination in E.
coli (Layer 4)
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Figure 18.16 Insertion sequences, the simplest
transposons
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Figure 18.17 Insertion of a transposon and
creation of direct repeats
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Figure 18.18 Anatomy of a composite transposon
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Unnumbered Figure (page 353) Bacterial and viral
growth curves