Regulation of Metabolism

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Regulation of Metabolism
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Rationale for Regulation
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Biological Efficiency

• Flexibility adaptaton to dietary changes
• Need for biosynthetic products
• Control of pre-existing enzymes
• Modulation biosynthesis only as fast as needs
  for macromolecular syntesis

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Competing Reactions Regulation
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Kinetic Controls
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Control Mechanisms

• Control of Enzyme Amount
• Induction and Repression
• Catabolite Repression
• Attenuaton
• Control of Enzyme Activity
• Modulation of k or Vmax (rare)
• Control of Kms
• Control of Substrate Availability

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Review of Genetic Regulation

• General Principles

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Types of Enzymes

• Constitutive Enzymes e.g. glycolytic enzymes and
  gluconeogenic enzymes
• Inducible Enzymes e.g. b-galactosidase
• Repressible Enzymes e.g. ten enzymes of
  histidine biosynthesis

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Sites of Regulation
Prokaryotes usually at transcription
initiation. Eukaryotes can be anywhere!
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Types of Regulation

• Specific one pathways substrate or product
• General needs for C or N sources or growth rates
  (e.g. energy charge)

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Signals Mediating Regulation

• Availability of
• Substrates or Products
• (Ligands)

Regulatory Proteins
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Gene Organization and Control
Property Prokaryotes Eukaryotes
Regulation Coordinate Coordinate
Organization Operons Dispersed
Magnitude Large Small
Complexity Simple??? Complex
Transcription Translation Coupled Uncoupled
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Gene Expression in Bacteria(Operon Model)
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Upstream Regulatory Sequences

• Promoter (general term)
• UAS (Upstream Activation Sequence)
• Enhancers
• URS (Upstream Repression Sequence)
• Operator

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Binding of RNA Polymerase to Promoter

• Affected by regulators
• Affected by strength of promoter provides
  appropriate variation in enzyme levels

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Gene Expression in Eukaryotes

• Dispersed Genes

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Mechanisms of Gene Regulation
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Negative RegulatorsBind to operators or
upstream repression sequences (URS)
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Positive RegulatorsBind to promoters, enhancers
or upstream activation sequences (UAS)
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Attenuation in Bacteria(Coupled Transcription
and Translation)
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Mechanism of Attenuation
NOTE Negative Regulatory System
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Discovery of Attenuation

• Charles Yanofsky

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Control of Enzyme Activity
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Irreversible Covalent Modification

• Zymogen Activation
• Proteolysis
• Lysosomes
• Proteosomes (ubiquitin)

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Reversible Covalent Modification
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Non-covalent Modification

• Effectors or Ligands

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Negative Effectors
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Positive Effectors
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Allosteric Proteins
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Energy Charge(Daniel Atkinson)
Steady-State E.C. 0.93 ATP, ADP and AMP
Regulatory Ligands
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Regulation of Degradative Pathways
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Degradative Pathways
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Enzyme Amount

• Induction
• (Inducer Substrate)
• Catabolite Repression

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Negative Regulators
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Positive Regulators
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Enzyme Activity

• Regulation Unnecessary
• No Substrate No Flux

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Lactose Utilization
NOTE function is to provide carbon and energy
when substrate is available and when products are
needed.
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Regulation of Enzyme Amount
Conditions (C Source) Enzyme Levels (b-galactosidase) Terminology
Glucose 0.0 Uninduced (Basal)
Lactose 1,000 Induction
Lactose Glucose 0.0 Catabolite Repression
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Regulation

• Specific Regulation mediated by availability of
  substrate called effector (or inducer) e.g.
  lactose (allolactose) through its interaction
  with a regulatory protein.
• General Regulation e.g. catabolite repression
  analogous to repression in that endproduct
  effector (catabolite co-repressor) prevents gene
  expression, often by interacting with a
  regulatory protein, but may use second messenger
  system e.g. cAMP.

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Physiological Manifestations of Catabolite
Repression
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Structure of Lac Operon
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Regulation of the Lac Operon
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Requirements for Gene Expression

• Availability of Substrate Lactose (or
  allolactose)
• and
• Need for Product low glucose) gt cAMP

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Mechanism of Catabolite Repression
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Inducible Operon(Positive Regulator)
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Hut Operon of Klebsiella aerogenes
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Pathway
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Regulation
hisR Enzyme Levels hisRC Enzyme Levels
C Source N Source His His His His
Glucose NH3 0 0 0 0
Glucose Limit NH3 0 105 100 100
Limit Glc NH3 0 120 100 100
Limit Glc Limit NH3 0 120 100 100
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Mechanism of Regulation
Carbon Catabolite Repression Nitrogen Metabolite
Repression
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Regulation of Biosynthetic Pathways
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Biosynthetic Pathways
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Enzyme Amount

• Repression
• Endproduct Corepressor

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Negative Regulator
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Positive Regulators
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Enzyme Activity

• Feedback Inhibition
• Endproduct Ligand or Effector

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Simple Feedback Inhibition
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Complex Feedback Inhibition
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Mechanisms of Complex Feedback Inhibition

• Cumulative sum of individual inhibitions
• Concerted both end products required for
  inhibition
• Isoenzyme two enzymes, each inhibitable by
  different end product
• Sequential inhibition by accumulating
  intermediate

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Amino Acid Biosynthetic Operon

• Positive Regulator

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Pathway
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Regulation
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Eukaryotes versus Prokaryotes
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Properties

• Increased Size reduced membrane surface to
  volume ratio
• Increased Complexity limited solvent capacity
• Uncoupled Transcription and Translation slower
  gene expression

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Evolutionary Response

• Organelles
• Constitutive Enzymes

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Problems

• Intracellular Metabolite Transport
• Competing Pathways

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Regulatory Solutions

• Separate Metabolic Pathways
• Different intermediates
• Different enzymes (control of enzyme activities)
• Physical Separation of Metabolic Pathways
• Location
• Multienzyme Complexes
• (Control of Substrate Availability)