Chapter 20 Enzymes and Vitamins 20.5 Regulation of Enzyme Activity

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.

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

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

Summary of Digestive Enzymes

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

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

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

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

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