1. Sample Problem 20.1 The Enzyme Active Site
What is the function of the active site in an enzyme?
Solution
The R groups of amino acid residues within the active site of an enzyme bind the substrate by forming hydrogen bonds, salt bridges, and hydrophobic interactions and catalyze the reaction.
Study Check 20.1
Which is a larger molecule, glucose or hexokinase?
Answer
Hexokinase. Typically, enzymes are large protein molecules containing many amino acid residues, and substrates are small molecules such as the monosaccharide glucose.

2. Sample Problem 20.2 The Induced-Fit Model
How does the induced-fit model explain the binding of the substrate at the active site?
Solution
In the induced-fit model, the shapes of the substrate and the active site adjust so that the substrate is in the optimum position needed for the enzyme to carry out reaction catalysis.
Study Check 20.2
How does the lock-and-key model explain the binding of the substrate at an active site?
Answer
In the lock-and-key model, the substrate fits perfectly into the active site of the enzyme similar to a key fitting into a lock. We know today that most enzymes do not have a rigid active site and that the active site flexes to accommodate the chemical reaction being catalyzed.

3. Sample Problem 20.3 Classifying Enzymes
Using Table 20.2, identify the chemical reaction each of the following enzymes catalyzes and classify the enzyme:
a. aminotransferase
b. lactate dehydrogenase

4. Sample Problem 20.3 Classifying Enzymes
Continued
Solution
a. An aminotransferase catalyzes the transfer of an amino group between two substrates, which is classified as a transferase.
b. Lactate dehydrogenase catalyzes the removal of two H atoms from lactate, which is classified as an oxidoreductase.
Study Check 20.3
What is the name and classification of the enzyme that catalyzes the hydrolysis of lipids?
Answer
An enzyme that hydrolyzes a lipid is a lipase, which is classified as a hydrolase.

5. Sample Problem 20.4 Factors Affecting Enzymatic Activity
Describe the effect each of the following changes would have on the rate of the reaction that is catalyzed by urease:
a. increasing the urea concentration b. lowering the temperature to 10 °C
Solution
a. An increase in urea concentration will increase the rate of reaction until all the enzyme molecules bind to urea. Then no further increase in rate occurs.
b. Because 10 °C is lower than the optimum temperature of 37 °C, there is a decrease in the rate of the reaction.
Study Check 20.4
If urease has an optimum pH of 7.0, what is the effect of lowering the pH of a urease sample to 3.0?
Answer
At a pH lower than the optimum pH, the hydrogen bonds and salt bridges of urease will be disrupted, resulting in the denaturation of the enzyme, loss of tertiary and quaternary structure, and a decrease in its biological activity.

6. Sample Problem 20.5 Enzyme Regulation
How is the rate of a reaction sequence regulated in feedback control?
Solution
When the end product of a reaction sequence is produced at sufficient levels for the cell, some product molecules bind to the allosteric enzyme (E1) in the sequence, which shuts down all the reactions that follow and stops the synthesis of intermediate products.
Study Check 20.5
Why is pepsin, a digestive enzyme, produced as a zymogen?
Answer
Pepsin hydrolyzes proteins in the foods we ingest. It is synthesized as a zymogen, pepsinogen, to prevent its digestion of the proteins that make up the organs in the body.

7. Sample Problem 20.6 Enzyme Inhibition
State the type of inhibition in the following:
a. The inhibitor has a structure similar to that of the substrate.
b. The inhibitor binds to the surface of the enzyme, changing its shape in such a way that it cannot bind to substrate.
Solution
a. Step 1 Compare the structure of the inhibitor to that of the substrate.
This inhibitor has a structure similar to that of the substrate.
Step 2 Describe the characteristics of the inhibitor. This inhibitor would compete with the substrate for the active site.
Step 3 Assign the type of inhibition based on the characteristics. This type of inhibition is competitive inhibition, which is reversed by increasing the concentration of the substrate.
b. Step 1 Compare the structure of the inhibitor to that of the substrate.
This inhibitor does not have a structure similar to the substrate.
Step 2 Describe the characteristics of the inhibitor. This inhibitor binds to the surface of the enzyme where it changes the shape of the enzyme and the active site.
Step 3 Assign the type of inhibition based on the characteristics. This type of inhibition is noncompetitive inhibition.

8. Sample Problem 20.6 Enzyme Inhibition
Continued
Study Check 20.6
What type of inhibition occurs when aspirin, also called acetylsalicylic acid, forms a covalent bond with serine in the active site of the enzyme cyclooxygenase?
Answer
irreversible inhibition

9. Sample Problem 20.7 Cofactors
Indicate whether each of the following enzymes is active with or without a cofactor:
a. an enzyme that needs Mg2+ for catalytic activity
b. a polypeptide chain that is biologically active
c. an enzyme that binds to vitamin B6 to become active
Solution
a. The enzyme would be active with the metal ion Mg2+ cofactor.
b. An active enzyme that is only a polypeptide chain does not require a cofactor.
c. An enzyme that requires vitamin B6 is active with a cofactor.
Study Check 20.7
Which cofactor for the enzymes in Sample Problem 20.7 would be called a coenzyme?
Answer
vitamin B6

10. Sample Problem 20.8 Vitamins
Why is it more important to get regular, daily amounts of vitamins B1 and B2, but not vitamins A or D?
Solution
Water-soluble vitamins like vitamin B1 (thiamine) and vitamin B2 (riboflavin) are not stored in the body, whereas fat-soluble vitamins such as vitamin A (retinol) and vitamin D (cholecalciferol) are stored in the liver and body fat. Any excess of thiamine or riboflavin is eliminated in the urine and must be replenished each day from the diet.
Study Check 20.8
Why are fresh fruits rather than cooked fruits recommended as a source of vitamin C?
Answer
Water-soluble vitamins such as vitamin C are easily destroyed by heat.