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Biology LibreTexts

17: Inhibitor Kinetics (cont.), Sigmoidal kinetics, Enzyme Activation

Reading & Problems: LNC p. 210-213

I. Other effects on enzyme activity.

A. Temperature

increase in temperature speeds reaction, but high temperature will reduce enzyme stability. Most enzymes have a temperature optimum.

B. pH 

can change state of catalytic groups or substrates accelerating or decelerating reaction. Usually a pH optimum that is characteristic of each enzyme.

C. Reversible covalent modification

 phosphorylation most common. Occurs on specific Ser, Thr or Tyr residues. Can activate or inactivate depending on the particular enzyme. Addition of phosphate catalyzed by specific protein kinase (using ATP as donor), removal by specific protein phosphatase which releases free phosphate.

D. Proteolytic activation

 enzyme synthesized as a pre-enzyme called "zymogen" that is cleaved by a protease to produce the active enzyme.


  1. Trypsin and chymotrypsin are produced this way and the zymogen precursors of these are called "trypsinogen" and "chymotrypsinogen".
  2. The formation of blood clots is activated by a series of proteolytic activation events that finally result in conversion of fibrinogen to fibrin and the formation of large fibrin polymers as the main components of the blood clots.

Fibrinogen activation.jpg
G&GIV Fig. 15.4 Every protein in the pathway, excepting the last protein, fibrinogen/fibrin, is a protease that cleaves and activates the next protein in the progressive activation pathway.

II Multi-substrate reactions - mechanism can be complex, but still amenable to kinetic analysis.


III. Solving enzyme kinetic problems - a few examples

Some take home information:

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)

Inhibitor Equations.jpg