lecture07 - Lecture 7 09 September 2009(P J Hollenbeck BIOL...

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-1- Lecture 7 09 September 2009 (P. J. Hollenbeck) BIOL 231 Reading: pp. 101-03; 151-153 Problem assignment: 21-23; exam I‘04, #4b-d; exam I ‘06, #5b ENZYME INHIBITION AND ALLOSTERY <Substances that inhibit enzymes (inhibitors, “I”) can be classified as reversible or irreversible. Irreversible inhibitors bind the enzyme permanently - usually covalently - and disrupt the ability of the active site to bind S, catalyze the reaction, or release products. Reversible inhibitors bind the enzyme non-covalently and can dissociate. Today we will consider the kinetics of reversible inhibition.> I. Inhibition of Michaelis enzymes (A) Competitive inhibitors (see plots below) (1) Recall from lecture 6, competitive inhibitors are so named because they compete with S for access to the active site. Their molecular structure resembles that of S. (2) The degree of inhibition depends on two things: the affinity of inhibitor (I) for the E’s active site; and the [S] (at fixed [I]), or ratio of [S] to [I]. MAX (3) In the presence of a competitive I, an E can approach its normal V , but only at much higher [S] (and [S]/[I]). As [S] approaches 4 , it swamps out the access of the I to the active site, so the active site will always be occupied by S. Thus, the E can approach its V . In the presence of a competitive I, the E also requires a higher [S] to reach ½V than it would without I. Thus, in the presence of competitive I, V M remains the SAME but the K INCREASES. (B) Non-competitive inhibitors (1) Non-competitive inhibitors, as their name implies, do not compete with S for the active site. Instead, they bind to the E at a different site, and their binding has no effect on the binding of [S] to the active site, only on catalysis. Thus, their inhibition cannot be reversed by increasing the [S]. (2) For a particular non-competitive I, the degree of inhibition depends only on [I], NOT on [S]. Thus, some fraction of the E molecules will be inhibited at any [S].
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lecture07 - Lecture 7 09 September 2009(P J Hollenbeck BIOL...

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