# Temperature and pH Inhibition

Extremes of pH and high temperature, all of which can denature the enzyme, would inhibit the enzyme in a irreversible fashion, unless it could refold properly.

### Inhibition by Temperature Change

From 0 to about 40-50 °C, enzyme activity usually increases, as do the rates of most reactions in the absence of catalysts (see Arrhenius equation); 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 $$K_m$$
• affect $$E$$ in ways to alter the actual catalysis of bound $$S$$, which would affect $$k_{cat}$$
• 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 chains, with an apparent $$pK_a$$ 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

Figure: Mathematic equations modeling pH effects on enzyme catalyzed reactions

Figure: Graphs of pH effects on enzyme catalyzed reactions