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Lecture17 - /7/11 Background reading Berg et al Pages 241...

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Lecture 17 11/7/11 Background reading: Berg, et al: Pages 241 - 242 Pages 289 – 296 Garrett and Grisham: Pages 423 – 419; 426 – 430 and 482 – 484 Segel: Pages 305 - 310 Outline: Noncompetitive inhibition Uncompetitive inhibition Irreversible inhibition Allosteric enzymes Homotropic response Heterotropic response Aspartate transcarbamylase
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Noncompetitive Inhibition In this case the inhibitor binds to a site on the enzyme that is different from the active site. The noncompetitive inhibitor binds to both the E and ES. E + S ES E + P k p S and I bind reversibly, randomly and independently at different sites on the enzyme. The ESI complex is catalytically inactive. The I might prevent the positioning of the catalytic center. The noncompetitive inhibitor acts by decreasing the concentration of functional enzyme. K S + I EI + S K i + I EIS K i / K S / Cannot overcome the inhibitor by adjusting the [S]. [E T ] = [E] + [ES] + [EI] + [EIS] K S = K S / K EIS = K S K i = [E] [I] [S] [EIS] K i = K i /
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v V max = [S] K S 1 + [S] K S [I] K i + + [S] [I] K S K i v V max [S] K S 1 + K i + [S] [I] ( ) ( 1 + K i [I] ) = ( If multiply numerator and denominator by K S: If divide denominator with parenthetical character: [S] v V max K i [I] ( ) 1 + = K S + S Thus see that the V max changes but the K S does not change.
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An example of noncompetitive inhibition may be that of poisoning with heavy metal ions, such as mercury or lead, which can form reversible complexes with sulfhydryl groups. If SH group is necessary for catalysis to occur, catalysis will not proceed.
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