Biochemistry Online: An Approach Based on Chemical Logic

CHAPTER 6 - TRANSPORT AND KINETICS

C: ENZYME INHIBITION

BIOCHEMISTRY - DR. JAKUBOWSKI

06/12/2014

Learning Goals/Objectives for Chapter 6C: After class and this reading, students will be able to

differentiate among competitive, uncompetitive, and mixed inhibition of enzymes by reversible, noncovalent inhibitors by writing coupled chemical equilibria equations and drawing cartoons showing molecular interactions among, E, S, and I;
using LeChatelier's principle and coupled chemical equilibria equations, draw double reciprocal (Lineweaver-Burk plots) and semilog plots for enzyme catalyzed reactions in the presence of different fixed concentrations of inhibitors and activators of enzyme
define KIS and KII for competitive, uncompetitive, and mixed inhibition from coupled chemical equilibria and double reciprocal plots;
differentiate between apparent and actual dissociations constants constants of an inhibitor and enzyme from double reciprocal plots and equations initial rate mathematical equations;
define agonist, partial agonist, antagonist, and mixed (noncompetitive antagonists) from analogy to enzymes and their inhibitors;
describe different ways that pH changes could affect the activity of an enzyme and suggest how each could affect Km and kcat.

C7. Inhibition by Temperature and pH Changes

From 0 to about 40-50o 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 10oC.). 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

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