Lecture 9 EnzReg - Chapter 10 Enzyme Regulation Allosteric...

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Chapter 10 – Enzyme Regulation Allosteric Regulation – acting at a regulatory site separate from substrate T (low affinity for substrate - less active) R (high affinity for substrate - more active) Isoenzymes -multiple forms of enzymes Reversible covalent modification Proteolytic activation Controlling amount of enzyme present
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Feedback Inhibition by CTP ATCase catalyzes the first committed step in the biosynthesis of pyrimidine nucleotides, such as cytidine triphosphate (CTP) The product of this biosynthetic pathway, CTP, inhibits ATCase through binding at a site separate of the active site. This alternative CTP binding site is referred to as an allosteric site. Aspartate Transcarbamoylase (ATCase)
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Quaternary structure of ATCase: 6 catalytic (c) subunits and 6 regulatory (r) regulatory subunits. 3 c subunits form a trimer (2 trimers each in the holoenzyme) and 2 r subunits for a dimer (3 dimers each in the holoenzyme) ATCase is an r 6 c 6 Heteromeric Enzyme 17 kD 34 kD
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A plot of aspartate substrate concentration vs. rate illustrates that ATCase does not obey a simple Michaelis Menten model for its kinetics. The sigmoidal kinetic plot suggests cooperativity in which the binding of one aspartate to ATCase enhances the enzyme’s binding affinity for additional aspartate molecules. The sigmoidal kinetic behavior of ATCase is reminiscent of the cooperative binding of O 2 by hemoglobin. ATCase Shows Cooperativity
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PALA is a bisubstrate analog of a reaction intermediate and an inhibitor of ATCase Similar to reaction intermediate and a strong competitive inhibitor
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PALA or Aspartate stabilizes the R state and CTP stabilizes the T state of ATCase Aspartate or
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The Substrate (and PALA) have interactions with more than one subunit
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Kinetics with and without the Regulatory subunits
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Lecture 9 EnzReg - Chapter 10 Enzyme Regulation Allosteric...

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