lecture 16 - 15-25 Allosteric interactions: sequential...

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Assumptions.…. . (for positive cooperativity): 1. Multi-subunit proteins 2. Each subunit has two conformational states “T” = low affinity for ligand “R” = high affinity 3. T R transition of subunit induced upon binding of ligand (= substrate or activator) 4. Binding of ligand switches conformation of subunit to which it’s bound, but does not induce the T R transition in neighboring subunits. . 5. However , T R in one subunit does increase affinity of neighboring subunits for ligand, which they then bind more readily, and then undergo T R themselves (i.e. T subunit with an R neighbor has higher affinity for ligand than T subunit with a T neighbor) Allosteric interactions: “sequential” model MODELS ORIGINATED FROM STUDIES OF O 2 -BINDING PROTEIN HEMOGLOBIN 15-25
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Binding of ligand to a tetrameric allosteric protein Example: hemoglobin, a tetramer w. 2 α and 2 β subunits, O 2 = ligand. according to simple sequential model T state = R state = deoxyhemoglobin is in T4 state binding of O 2 to one subunit changes it from T R, but leaves other subunits in T-form O2-binding affinity of unoccupied sites in RT3 is higher than in T4 R2 T2 and R3T, which each have higher O2-binding affinities than RT3, are formed when 2nd and 3rd O2 bind R4 produced when 4th O2 binds K 1-4 = dissociation constants for O 2 -binding: get increasingly 15-26 KOSHLAND
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Concerted model for allosteric interactions - all subunits change conformation together , but - binding of ligand increases the probability that all subunits are in the R-form R T R favored R favored R & T ~ equally favored T favored T favored K T,R = dissociation constants for O2-binding to T and R states: assume constant for each state 15-27 MONOD, WYMAN, & CHANGEUX
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This note was uploaded on 01/29/2012 for the course MCB 354 taught by Professor Spees during the Spring '09 term at University of Illinois, Urbana Champaign.

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lecture 16 - 15-25 Allosteric interactions: sequential...

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