Enzyme Regulation

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• Enzyme Regulation

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Regulation of Enzyme Activity

• Enzyme quantity regulation of gene expression
  (Response time minutes to hours)
• Transcription
• Translation
• Enzyme turnover
• Enzyme activity (rapid response time fraction
  of seconds)
• Allosteric regulation
• Covalent modification
• Association-disassociation
• Proteolytic cleavage of proenzyme

3
Allosteric Regulation

• End products are often inhibitors
• Allosteric modulators bind to site other than the
  active site
• Allosteric enzymes usually have 4o structure
• Vo vs S plots give sigmoidal curve for at least
  one substrate
• Can remove allosteric site without effecting
  enzymatic action

4
Regulation of Enzyme Activity(biochemical
regulation)

• 1st committed step of a biosynthetic pathway or
  enzymes at pathway branch points often regulated
  by feedback inhibition.
• Efficient use of biosynthetic precursors and
  energy

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Phosphofructokinase( PFK)
Fructose-6-P ATP -----gt Fructose-1,6-bisphosphat
e ADP

• PFK catalyzes 1st committed step in glycolysis
  (10 steps total)
• (Glucose 2ADP 2 NAD 2Pi ? 2pyruvate 2ATP
  2NADH)
• Phosphoenolpyruvate is an allosteric inhibitor of
  PFK
• ADP is an allosteric activator of PFK

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Allosteric modulators bind to site other than the
active site and allosteric enzymes have 4o
structure
Fructose-6-P ATP -----gt Fructose-1,6-bisphosphat
e ADP
ADP
Allosteric Activator (ADP) binds distal to active
site
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Vo vs S plots give sigmoidal curve for at least
one substrate
Binding of allosteric inhibitor or activator does
not effect Vmax, but does alter Km Allosteric
enzyme do not follow M-M kinetics
8
Allosteric T to R transition
S
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Covalent modification

• Regulation by covalent modification is slower
  than allosteric regulation
• Reversible
• Require one enzyme for activation and one enzyme
  for inactivation
• Covalent modification freezes enzyme T or R
  conformation

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Phosphorylation /dephosphorylation

• most common covalent modification
• involve protein kinases/phosphatase
• PDK inactivated by phosphorylation
• Amino acids with OH groups are targets for
  phosphorylation
• Phosphates are bulky (-) charged groups which
  effect conformation

11
Enzyme Regulation by Association/Disassociation

• Acetyl-CoA Carboxylase
• acetyl-CoA CO2 ATP ? malonyl-CoA ADP Pi
• 1St committed step in fatty acid biosynthesis
• In presence of citrate activated
• In presence of fatty acyl-CoA inactivated

citrate
polymerized
unpolymerized
Fatty acyl-CoA
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Proteolytic cleavage of proenzyme(zymogen)
13
Proinsulin to Insulin
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Blood Clotting

• Clotting involves series of zymogen activations
• Seven clotting factors are serine proteases
  involved in clotting cascade rxns

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