Lecture #16: Enzyme Kinetics: Inhibition and Regulation

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1. The mechanism of enzyme inhibition is diagnosed by measuring v_o as a function of [S] in the presence and absence of [I]. (A range of [I] is preferred).
2. Competitive Inhibition:

• A competitive inhibitor (I) displaces substrate (S) by binding to the SAME site. In the presence of I, S appears to have lower affinity for the enzyme. Adding more S can reverse enzyme inhibition. (i.e., A competitive inhibitor will increase apparent K_M by a factor a , but will not decrease V_max).
• Hyperbolic curves will be shifted to the right in presence of increasing [I], but will reach the same maximum if [S] is high enough (Fig. 12-6).
• In double-reciprocal plots, slope will increase as a function of [I] and K_I, but y-intercept (1/V_max) will stay the same (Fig. 12-7).

1. Uncompetitive Inhibition:

• An uncompetitive inhibitor (I) binds to the enzyme-substrate complex and distorts the active site. No amount of [S] can reverse the inhibition. (i.e., Both K_M and V_max are decreased, but the ratio V_max/K_M remains the same).
• Hyperbolic curves will be shifted to the right in presence of increasing [I], but will NOT reach the same maximum even at very high [S].
• In double-reciprocal plots, slope will be constant but y- and x- intercept will change (Fig. 12-8).

1. Physiologically, enzymes are regulated by changing the amount of enzyme (synthesis of new enzyme or degradation of old enzyme) or enzyme activity via allosteric effectors and covalent modification (eg., phosphorylation and dephosphorylation of OH groups on Ser, Thr or Tyr).