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16: Enzyme Kinetics, Inhibitor Kinetics

Reading & Problems: LNC p. 207-212; p. 239 prob. 9a,b, 10; Segel p. 319 prob. 6,7, p. 321, prob. 15,16,19 (except "Z")

I. Determining [P] at a given time "t".

 

  • Solve only for [S]>>Km, and for [S]<<Km.


For [S]>>Km, V = Vmax (zero order for [S]), and [P]t = Vmax•t,
[S]t =[S]0 - [P]t

For [S]<<Km, V = Vmax/Km•[S] (first order for [S]) and

S at t.jpg

II. Determinants of enzyme activity.

 

A. Amount of enzyme present - determined by the relationship between:

 

  • Rate of synthesis, and
  • Rate of degredation

 

B. Inhibitors

 

  • Irreversible inhibitors - irreversibly bind to and inhibit the enzyme. Examples include DIFP and TPCK as inhibitors of chymotrypsin. They are covalently bound and do not dissociate from the enzyme.
  • Reversible inhibitors - bind and release, the amount of inhibitor bound is determined by the concentration of inhibitor and its binding constant (Ki). Reversible inhibitors are important regulators of enzyme activity in cells.

 

III. Inhibitor kinetics

 

A. Competitive inhibitor - can only bind when substrate not bound, prevents substrate binding.

 

B. Noncompetitive inhibitor - can bind whether or not substrate bound, prevents reaction when bound.

 

C. Uncompetitive inhibitor - binds only to ES complex, prevents release of S and production of P.


Some take home information:

Information on reversible inhibitors:

 

Inhibitor Binds to Constant changed
Competitive E Km higher
Noncompetitive ES, E Vmax lower
Uncompetitive ES Vmax lower, Km lower

 

If constant is Higher -> multiple by "the inhibitor factor" = 1+([I]/KI)
If constant is Lower -> divide by "the inhibitor factor" = 1+([I]/KI)

Formulas:
Inhibitor Equations.jpg

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