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 Figure: Mathematic equations modeling pH effects on enzyme catalyzed reactions Figure: Graphs of pH effects on enzyme catalyzed reactions