Humboldt State University ® Department of Chemistry

Richard A. Paselk

Chem 431

Biochemistry

Fall 2008

Lecture Notes: 8 October

© R. Paselk 2008
 
PREVIOUS  

NEXT

Enzyme Kinetics and Inhibition, cont.

Last time we left off our discussion with Competitive inhibition. next we have:

Noncompetitive inhibition

In Noncompetitive inhibition the inhibitor can bind to either E or ES. S & I do not bind to the same sites! (text Figure 6-15c)

Model: noncompetitive inhibition equilibria model; and MM equation for noncompetitive inhibition.

Note that will have two inhibitor binding constants, they may be the same, as in the equation above, or could be different, leading to more complex behavior.

Plots for classic, simple situation (overhead MvH 11.5):

L-B plot for noncompetitive inhibition

Mixed inhibition

Like non-competitive above,

Model: noncompetitive inhibition equilibria model; and MM equation for noncompetitive inhibition

but with different values of Ki. This is in fact the more common situation, with a L-B plot with an intersection on the plot above the 1/vo axis. (text Box 6-2 figure 3)

Uncompetitive inhibition

In uncompetitive inhibition the inhibitor binds ONLY to the ES complex (text Figure 6-15b).

Model: uncompetitive inhibition equilibria model; and MM equation for uncompetitive inhibition.

For double reciprocal plots get parallel lines! (text Box 6-2 figure 2) This is not generally found for single substrate enzymes, but is found in multi-substrate systems.


L-B plot for uncompetitive inhibition

Multi-substrate Enzyme Kinetics

Look at three common and easily understood types. We will use Cleland Nomenclature and "Kinetic mechanism diagrams."

Two-substrate Enzyme Product Inhibition Patterns

(Based on: E. B. Cunningham, Biochemistry: Mechanisms of Metabolism. McGraw-Hill Book Company, New York (1978), and W. Cleland, "Substrate Inhibition: in Contemporary Enzyme Kinetics and Mechanism. (Daniel L. Purich, ed.) Academic Press, New york (1983))

Kinetic Mechanism Variable Substrate Product Type of Inhibition

ordered sequential bi bi Kinetic mechanism diagram

Ordered Sequential Bi Bi

Ordered Sequential Bi Bi A Q Competitive
  B Q Noncompetitive
  A P Noncompetitive
  B P Noncompetitive

random sequential bi bi Kinetic mechanism diagram

Random Sequential Bi Bi

Random Sequential Bi Bi A Q Noncompetitive
  B Q Noncompetitive
  A P Noncompetitive
  B P Noncompetitive

ping pong bi bi Kinetic mechanism diagram

Ping pong Bi Bi

Ping pong Bi Bi A Q Competitive
  B Q Noncompetitive
  A P Noncompetitive
  B P Competitive

Note that in each case we can predict/explain the pattern of inhibition on the basis of the substrate and inhibitor binding to the same "enzyme form." Thus for the Ordered Sequential mechanism only the first substrate and last product bind to the same form, in this case the free enzyme. Similarly for the Ping pong mechanism the first substrate and last product should be competitive as the both bind the free enzyme. In this case we also see a competitive inhibition between the second substrate and the first product, since they both bind to the E-X complex. The Random Sequential mechanism is a bit more subtle. Here we see across the board noncompetitive since in each case the substrates (and products) can each bind to more than one substrate form, so competitive inhibition will not be possible! (Think of the product as competing with one order of binding but not the other.)

Temperature Effects on Enzymes (and Proteins)

Temperature profile reflects two underlying phenomena:

Together these effects lead to the plot below where the rising leg is due to activation energy effects (increasing rate) and the falling leg is due to protein denaturation.

 

plot of enzyme velocity vs. temperature


Pathway Diagrams

C431 Home

Lecture Notes

Last modified 8 October 2008