Control of Enzyme Activity

• Feedback inhibition

  • the excess end-product of a pathway inhibits the activity of the first enzyme
  • reversible
  • allosteric enzyme (e.g. phosphofructokinase)
  • allosteric: binding site for end product; active: binding for substrate
• Isoenzymes


  different enzymes that catalyze the same reaction but subjected to different regulatory controls

  • Ex: DAHP synthases may form different end products depending on isoenzymes present
• Covalent Modification


  A regulatory mechanism wherein the attachment or removal of small molecules to the enzyme changes its conformation, thereby affecting its catalytic activity

  • Modifiers:

  1. AMP
  2. ADP
  3. inorganic phosphate
  4. methyl

  • Example: GS is modulated by AMP (adenylation)
• Glutamine Synthetase Structure

  • key role in ammonia assimilation of bacteria
  • has 12 identical subunits (dodecamer)
  • arranged in face to face hexagonal rings
  • held by hydrophobic and hydrogen-bonding reactions
  • Exposed loops: site for proteolysis and ADP-ribosylation
  • target for adenylation; tyrosyl residue
  • has two metal binding sites for Mn and Mg
  • n1: responsible for protein stabilization and binding of glutamate to catalytic site
  • n2: for phosphoryl transfer
• Regulation of Glutamine Synthetase

  1. Cumulative Feedback Inhibition: produces multiple end products of glutamine metabolism
  2. Interconversion between taut and relaxed protein configurations: in response to binding and dissociation of divalent cations at one of its two metal binding site
• Regulation (Part II)

  3. Novel bicyclic cascade system: stimulates dynamic interconversion of enzymes between covalently modified (adenylated) and unmodified form

  4. Repression and Derepression: forms cyclic phosphorylation and dephosphorylation of an RNA factor

  5, Coupling of site specific metal ion-catalyzed oxidation with proteolytic degradation regulates GS turnover
• Regulation

  • SH reagents: transform relaxed to subunits
  • basic pH of urea (8.0) affects relaxed form of subunits
  • EDTA (pH 7.0) turns taut into relaxed
• Adenylation


  inactivates GS

  • too much glutamine in the cell makes GS"

  1. adenylated
  2. inactive
  3. transcription inhibited
• Glutamine Synthetase

  • catalyzes the ATP-dependent condensation of glutamate and ammonia to glutamine
  • key role in ammonia assimilation
  • glutamine limitation in E. coli leads to adenylation of GS