|MadSci Network: Biochemistry|
Britney, The pH optimum for an enzyme corresponds to the proton concentration at which ionizable groups (groups that can be protonated or deprotonated) on the enzyme and/or the substrate molecules are in the most favorable form for reactivity. For example, if the activity of an enzyme decreases at low pH, this indicates that a group (for example, a catalytic amino acid residue) that must be deprotonated for activity has become protonated, resulting in a loss of activity. Similarly, if a catalytic reaction depends on a group that must be protonated for activity, raising the pH so high that this critical residue becomes deprotonated will result in lower activity. Some enzymes have very broad optimum pH ranges, within which the rate is constant. If your enzymatic rate is unchanged between 4 and 10, it means that any groups necessary for catalysis (including the substrate) are not changing their protonation state in this pH range. Good luck with your future experiments! [Moderator's Note : A detailed discussion of pH and enzyme activity, and amino-acid side group protonation and deprotonation can be found in a college-level Biochemistry text, such as Biochemistry by L. Stryer. ]
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