C7. Inhibition by Temperature and pH Changes

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From 0 to about 40-50° C, enzyme activity usually increases, as do the rates of most reactions in the absence of catalysts. (Remember the general rule of thumb that reaction velocities double for each increase of 10°C.). At higher temperatures, the activity decreases dramatically as the enzyme denatures.

Figure: Temperature and Enzyme Activity

INHIBITION BY pH CHANGE

pH has a marked effect on the velocity of enzyme-catalyzed reactions.

Figure: pH and Enzyme Activity

Think of all the things that pH changes might affect. It might

affect E in ways to alter the binding of S to E, which would affect Km
affect E in ways to alter the actual catalysis of bound S, which would affect kcat
affect E by globally changing the conformation of the protein
affect S by altering the protonation state of the substrate

The easiest assumption is that certain side chains necessary for catalysis must be in the correct protonation state. Thus, some side chain, with an apparent pKa of around 6, must be deprotonated for optimal activity of trypsin which shows an increase in activity with the increase centered at pH 6. Which amino acid side chain would be a likely candidate?

See the following figure which shows how pH effects on enzyme kinetics can be modeled at the chemical and mathematical level.

Figure: Chemical equations showing the mechanism of pH effects on enzyme catalyzed reactions