1. Chapter 20 Enzymes and Vitamins
20.5 Regulation of Enzyme Activity

2. Zymogens Zymogens (proenzymes) are inactive forms of enzymes
are activated when one or more peptides are removed
Example: The zymogen proinsulin is converted to its active form, insulin, by removing a small peptide chain.

3. Digestive Enzymes Digestive enzymes are
produced as zymogens in one organ and transported to another, such as the pancreas, when needed
activated by removing small peptide sections

4. Digestive Enzymes (continued)
Zymogen Active Enzyme
(from pancreas) (in small intestine)
Enteropeptidase
Trypsinogen trypsin + peptide
Trypsin
Chymotrypsinogen chymotrypsin + 2 dipeptides
Procarboxypeptidase carboxypeptidase + peptide

5. Summary of Digestive Enzymes

6. Allosteric Enzymes An allosteric enzyme is
an enzyme in a reaction sequence that binds a regulator substance
a positive regulator when it enhances the binding of substrate and accelerates the rate of reaction
a negative regulator when it prevents the binding of the substrate to the active site and slows down the rate of reaction

7. Feedback Control In feedback control, a product acts as a regulator
an end product binds with the first enzyme (E1) in a sequence when sufficient product is present so shuts down the reaction

8. Feedback Control by Threonine Deaminase
When isoleucine accumulates in a cell,
it binds to threonine deaminase, the first enzyme in the pathway from threonine to isoleucine
a change in the shape of threonine deaminase blocks the binding of threonine
feedback control decreases the synthesis of isoleucine

9. Learning Check Identify each statement as:
zymogen (Z) allosteric enzyme (A)
positive regulator (PR) feedback control (FC)
A. an enzyme in a pathway that controls the rate of the
reaction
B. speeds up a reaction by combining with an
enzyme in the pathway
C. removal of a peptide activates the enzyme
D. some product binds to the first enzyme to limit the
synthesis of product

10. Solution Identify each statement as: zymogen (Z) allosteric enzyme (A)
positive regulator (PR) feedback control (FC)
A A. an enzyme in a pathway that controls the rate of the reaction
PR B. speeds up a reaction by combining with an
enzyme in the pathway
Z C. removal of a peptide activates the enzyme
FC D. some product binds to the first enzyme to limit the synthesis of product