2 the inhibitor does not have to bind at the active

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2. The inhibitor does not have to bind at the active site. 3. The inhibitor does not have to resemble the substrate ( e.g. allosteric inhibitor). V max is reduced : the amount of ESI formed depends on [I]. The solution to the steady-state equation results in the (1 + [I]/K I ) factor multiplying the [S] term in the denominator of the Michaelis-Menten equation. The slopes of the Lineweaver-Burk plot are unchanged (K M /V MAX ), but the Y-intercept increases by a factor of (1 + [I]/K I ). The X-intercept shifts to the left by a factor (1 + [I]/K I ). Irreversible Inhibition In contrast to the above types of reversible inhibition, where the effects on the enzyme reaction depend on the concentration of an inhibitor (and its K I ), there are many examples of compounds that react chemically with residues in the enzyme active site. In these cases, enzyme activity is destroyed. For example, the "nerve gas" Sarin reacts specifically with an active site Ser residue on the enzyme, acetylcholinesterase. If acetlycholine cannot be hydrolyzed by this enzyme, nerve signals cannot be passed across the synapses of the nervous system. On exposure to this compound, death can result in minutes due to respiratory failure. 6.1 "The Michaelis-Menten equation does not describe the behavior of allosteric enzymes." The above title statement, quoted from Campbell, is misleading. What is true is that the simplest form of the Michaelis-Menten equation does not account for the higher than first order substrate concentration dependence found in many allosteric enzymes. But this is the same as saying that the Scatchard equation does not describe O 2 binding by hemoglobin. In each case, the basic concept of saturation (of an enzyme active site or a ligand binding site) is identical. In fact, the
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4 cooperativity observed in allosteric enzymes is analyzed using the Hill equation and Hill plots to obtain values for K M (or K 0.5 )and n H . Allosteric enzymes are usually found at metabolic control points. Frequently, these are the first steps in pathways. Regulation of the entire sequence of enzymatic steps is most efficiently accomplished by turning on or turning off the pathway at the first step. When the control is
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  • Spring '06
  • clavijo
  • Biochemistry, Enzyme, Enzyme inhibitor, Competitive inhibition, inhibition Nonompetitive inhibition, Zymogens Campbell

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