Lecture%208-Kumar%20Regulation%20of%20Enzyme%20Activity

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Lecture 8-KumarRegulation of Enzyme Activity

• Regulation by modification
• Proteolytic cleavage
• Covalent modification
• Protein-protein interaction
• Allosteric regulation
• Properties of allosteric enzymes (important)
• Sigmoid kinetics (what does Km mean in this case)
  (important)
• Positive and negative modulators (where do they
  act and how do they modify activity at constant
  substrate concentration) (most important)
• Models of allosteric transitions (important)

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Regulation of Enzyme Activity

• Normal metabolic control may be exerted in a
  variety of ways. Examples are
• Proteolytic Cleavage of inactive Proenzymes to
  active enzymes
• Pepsinogen pepsin small
  peptide
• in gastrointestinal tract for protein digestion
• Coagulation cascadea series of proenzymes are
  converted to active enzymes. The last step is
• Fibrinogen Fibrin

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Coagulation Cascade
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Covalent Modification as Control

• Chemical modification can either increase or
  decrease activity. Some examples are
• Glycogen Synthetase
  
• Phosphatase
  Kinase
• 
  Phosphorylated Glycogen
• 
  synthetase
• Glycogen phosphorylase
• Covalent Phosphorylated Glycogen phosphorylase

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Protein-Protein Interaction

• Example is activation of Protein Kinase A
• R2C2 4 cAMP R2C2(4cAMP)
• Inactive
• 2R(cAMP)2 2C
  (active)
• The catalytic unit (C) is able to phosphorylate
  and modulate the activity of other enzymes

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Allosteric Regulation
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Properties of Allosteric enzymes

1. Catalyze essentially irreversible reactions are
   rate limiting
2. Generally contain more than one polypeptide chain
3. Do not follow Michaelis-Menten Kinetics
4. Are regulated by allosteric activators or
   inhibitors
5. Can be up-regulated by allosteric activators at
   constant S
6. Can be down regulated by allosteric inhibitors at
   constant S
7. Activators and Inhibitors need not have any
   structural resemblance to substrate structure

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Sigmoid kinetics for allosteric enzymes
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Effects of allosteric activators and allosteric
inhibitors on enzyme activity
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Effect of allosteric activators and inhibitors on
rate at cellular concentration of the substrate
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Models of Allosteric ModulationSymmetry model
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Sigmoidal Curve Effect
vo
Noncooperative (Hyperbolic)
Positive effector (ATP) brings sigmoidal
curve back to hyperbolic Negative effector
(CTP) keeps
Sigmoidal curve
Cooperative (Sigmoidal)
Exaggeration of sigmoidal curve yields a
drastic zigzag line that shows the On/Off
point clearly
Consequently, Allosteric enzyme can sense the
concentration of the environment and adjust
its activity
Off
On
Substrate
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Learning objectives for lecture 8

• Learn various methodologies that enzymes employ
  to control metabolism
• Know the properties of an allosteric enzyme.
• Understand the significance of sigmoid kinetics.
  Can one determine Km and Vmax for these enzymes
  from the sigmoid plot
• Understand how the activity of an allosteric
  enzyme is regulated by allosteric activators and
  inhibitors
• Understand the mechanism of allosterism and
  negative feedback inhibition