lecture7 - Allosteric Enzymes Allosteric Allosteric: Greek...

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Allosteric Enzymes Allosteric Enzymes Allosteric: Allosteric: Greek allo + steric , other shape Allosteric enzyme: Allosteric enzyme: multi-subunit protein whose activity is affected by binding of other substances change the enzyme’s activity by altering its quaternary structure Allosteric effector: Allosteric effector: a substance that modifies the activity of an allosteric enzyme; may be an allosteric inhibitor allosteric activator Aspartate transcarbamoylase (ATCase) Aspartate transcarbamoylase (ATCase) Cytidine triphosphate (CTP) is an allosteric Cytidine triphosphate (CTP) is an allosteric inhibitor inhibitor through through feedback inhibition feedback inhibition of ATCase of ATCase
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ATCase ATCase H 2 N-C - OPO 3 2 - COO - CH 2 CH- COO - H 3 N + + H 2 N-C- NH- CH-COO - O COO - CH 2 HPO 4 2 - ATCase Carbamoyl phosphate Aspartate N-Carbamoylaspartate - O- P-O- P-O- P-O-CH 2 O OH OH H H H H N N NH 2 O O O - O - O - O Series of steps Cytidine triphosphate (CTP) CTP inhibits ATCase! O O
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ATCase ATCase Rate of ATCase catalysis vs. substrate conc. there is an initial lag, then exponential increase in velocity
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ATCase ATCase ATCase catalysis in presence of CTP and ATP if lots of ATP but little CTP, not enough nucleotides for DNA and RNA synthesis-> MAKE MORE CTP! ACTIVATE ATCase!
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Allosteric Enzymes Allosteric Enzymes The key to allosteric behavior is the existence of multiple forms for the 4° structure of the enzyme An An allosteric effector allosteric effector modifies the 4° structure of an allosteric enzyme homotropic effects: homotropic effects: allosteric interactions that occur when several identical molecules are bound to the protein; e.g., the binding of aspartate to ATCase heterotropic effects: heterotropic effects: allosteric interactions that occur when different substances are bound to the protein; e.g., inhibition of ATCase by CTP and activation by ATP There are 2 models that describe allosteric effects
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The Concerted Model The Concerted Model Wyman, Monod, and Changeux - 1965 The enzyme has two conformations R (relaxed): R (relaxed): binds substrate tightly; active form T (tight or taut): T (tight or taut): binds substrate less tightly; inactive form in the absence of substrate, there are much more
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lecture7 - Allosteric Enzymes Allosteric Allosteric: Greek...

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