MadSci Network: Biochemistry

Re: Why might the reaction rate of enzyme catalysis be the same at a pH of 7&10

Date: Sat Nov 1 15:45:04 2003
Posted By: Alvan Hengge, Faculty, Chemistry and Biochemistry, Utah State University
Area of science: Biochemistry
ID: 1066764238.Bc

     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

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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