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BIOC12Fall2012 Lecture Week 12 Notes

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University of Toronto Scarborough
Biological Sciences
Rongmin Zhao

1 BIOC12Fall2012 Lecture Week 12: Enzyme regulation (Chapter 15) Control of Enzymatic Activity o There are 2 levels of control of enzyme activity o Control of enzyme availability: control the expression level of the enzyme or the amount of the enzyme protein inside the cells o This is regulated by the synthesis rate and the degradation rate of the enzyme o Control of enzyme activity: the structure of the enzyme is directly altered to change the catalityic activity of the enzyme (allosteric control and covalent modification) o pH can only regulate enzyme activity outside of the cell since in the body pH is maintained constant Allosteric control o alloseteric effect: the binding of a ligand at one site affects thebinding of another ligand at another site o recall the mechanism of hemoglobin and non classic competitive inhibitors  but hemoglobin is not really an enzyme o the binding of small molecules may change the catalytic activity of the enzyme o then the small molecules are called allosteric effectors (inhibitors, activators) The General features of allosteric regulation o action at another site o enzymes situated at key steps in metabolic pathways are modulated by allosteric effectors o these effectors are usually produced elsewhere in the pathway o effectors may be feed forward activators or feedback inhibitors o kinetics are sigmoid (S-shaped) o o Sigmoid v versus [S] plot. The dotted line represents the hyperbolic plot characteristic of normal Michaelis=Menten kinetics Allosteric regulation can be explained by conformational changes in proteins o MWC model o Allosteric proteins can exist in 2 states: relaxed (R – when the enzyme is bound by substrate) and T (taut) o In this model, all the subunits of an oligomer must be in the same state o T state predominates in the absence of substrate S o S binds much tighter to R than to T 2 o o allosteric effects: A and I binding to R and T, respectively o o The sequential model for allosteric regulation is based on ligand-induced conformation changes 3 o an alternative model – proposed by Koshland, Nemethy ,and Filmer (KNF) relies on the idea that ligand binding triggers a conformation change in protein o if the prote
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