1. Chapter 20 Enzymes and Vitamins
Describe how enzymes function as catalysts; name and classify them.

2. Enzymes are Biological Catalysts
Enzymes are proteins that
catalyze nearly all the chemical reactions taking place in the cells of the body
increase the rate of reaction by lowering the energy of activation

3. Names of Enzymes The name of an enzyme usually ends in ase
identifies the reacting substance; for example,
sucrase catalyzes the reaction of sucrose
describes the function of the enzyme; for example, oxidases catalyze oxidation
can be a common name, particularly for the digestive enzymes (such as pepsin and trypsin)

4. Classification of Enzymes
Enzymes are classified by the types of reaction they
catalyze.
Class Type of Reactions catalyzed
Oxidoreductases Oxidation–reduction
Transferases Transfer groups of atoms
Hydrolases Hydrolysis
Lyases Add atoms/remove atoms to or
from a double bond
Isomerases Rearrange atoms
Ligases Use ATP to combine small molecules

5. Oxidoreductases and Transferases

6. Hydrolases and Lyases

7. Isomerases and Ligases

8. Learning Check Match the type of reaction with an enzyme
1) aminase 2) dehydrogenase
3) isomerase 4) synthetase
A. converts a cis-fatty acid to a trans-fatty acid
B. removes 2H atoms to form a double bond
C. combines two molecules to make a new compound
D. adds NH3

9. Solution Match the type of reaction with an enzyme
1) aminase 2) dehydrogenase
3) isomerase 4) synthetase
3 A. converts a cis-fatty acid to a trans-fatty acid
2 B. removes 2H atoms to form a double bond
4 C. combines two molecules to make a new compound
1 D. adds NH3

10. Chapter 20 Enzymes and Vitamins
20.2 Enzyme Action
Describe the role of an enzyme in an enzyme–catalyzed reaction.

11. Active Site The active site
How enzyme works p 706
The active site
is a region within an enzyme that fits the shape of the reacting molecule, called a substrate
contains amino acid R groups that bind the substrate
releases products when the reaction is complete

12. Enzyme-Catalyzed Reaction
The proper fit of a substrate (S) in an active site on an enzyme (E) forms an enzyme–substrate (ES) complex.
E + S ES
Within the ES complex, the reaction occurs to convert substrate to product (P).
ES E + P
The products, which are no longer attracted to the active site, are released.
Overall, substrate is converted to product.
E + S ES E + P

13. Enzyme-Catalyzed Reaction (continued)
In an enzyme-catalyzed
reaction,
a substrate attaches to the active site
an enzyme–substrate (ES) complex forms
reaction occurs and products are released
an enzyme (E) is used over and over
E + S ES E + P

14. Enzyme Specificity Enzymes may recognize and catalyze
a single substrate
a group of similar substrates
a particular type of bond

15. Lock-and-Key Model In the lock-and-key model of enzyme action,
the active site has a rigid shape
only substrates with the matching shape can fit
the substrate is the key that fits that lock

16. Induced-Fit Model In the induced-fit model of enzyme action,
enzyme structure is flexible, not rigid
shapes of enzyme and substrate adjust for best fit at the active site to improve catalysis of reaction
substrate specificity increases

17. Example of an Enzyme-Catalyzed Reaction

18. Learning Check A. The active site is 1) the entire enzyme
2) a section of the enzyme
3) the substrate
B. In the induced-fit model, the shape of the enzyme when substrate binds
1) stays the same
2) adapts to the shape of the substrate

19. Solution A. The active site is 2) a section of the enzyme
B. In the induced-fit model, the shape of the enzyme when substrate binds
2) adapts to the shape of the substrate

20. Isoenzymes as Diagnostic Tools
H.N. P 709
Isoenzymes as Diagnostic Tools
Isoenzymes
catalyze the same reaction in different tissues in the body
such as lactate dehydrogenase (LDH), which converts lactate to pyruvate, consist of five isoenzymes
can be used to identify the organ or tissue involved in damage or disease
such as LDH have one form more prevalent in heart muscle and another form in skeletal muscle and liver tissue

21. Isoenzymes (continued)

22. Diagnostic Enzymes Diagnostic enzymes
determine the amount of damage in tissues
that are elevated may indicate damage or disease in a particular organ

23. Diagnostic Enzymes (continued)
Levels of the enzyme creatine kinase (CK), lactic dehydrogenase (LDH), and aspartate transaminase (AST)
are elevated following a heart attack
are used to determine the severity of the attack

24. Chapter 20 Enzymes and Vitamins
20.3 Factors Affecting Enzyme Activity
Describe the effect of temperature, pH, concentration of enzyme, and concentration of substrate on enzyme activity.

25. Temperature and Enzyme Action
Enzymes
are most active at an optimum temperature (usually 37 °C in humans)
show little activity at low temperatures
lose activity at high temperatures as denaturation occurs

26. pH and Enzyme Action Enzymes are most active at optimum pH
contain R groups of amino acids with proper charges at optimum pH
lose activity in low or high pH as tertiary structure is disrupted

27. Optimum pH Values Enzymes in the body have an optimum pH of about 7.4
certain organs have enzymes that operate at lower and higher optimum pH values

28. Enzyme Concentration An increase in enzyme concentration
increases the rate of reaction (at constant substrate concentration)
binds more substrate with enzyme

29. Substrate Concentration
An increase in substrate
concentration
increases the rate of reaction (at constant enzyme concentration)
eventually saturates an enzyme with substrate to give maximum activity

30. Learning Check
Sucrase has an optimum temperature of 37 °C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction.
1) no change ) increase 3) decrease
A. increasing the concentration of sucrase
B. changing the pH to 4
C. running the reaction at 70 °C

31. Solution
Sucrase has an optimum temperature of 37 °C and an optimum pH of 6.2. Determine the effect of the following on its rate of reaction
1) no change ) increase 3) decrease
2 A. increasing the concentration of sucrase
3 B. changing the pH to 4
3 C. running the reaction at 70 °C

32. Chapter 20 Enzymes and Vitamins
20.4 Enzyme Inhibition
Describe competitive and noncompetitive inhibition, and reversible and irreversible inhibition.

33. Enzyme Inhibitors Inhibitors
are molecules that cause a loss of catalytic activity
prevent substrates from fitting into the active sites
E + S ES E + P
E + I EI no P

34. Competitive Inhibitor
A competitive inhibitor
has a structure that is similar to that of the substrate
competes with the substrate for the active site
has its effect reversed by increasing substrate concentration

35. Noncompetitive Inhibitor
A noncompetitive inhibitor
has a structure that is much different than the substrate
distorts the shape of the enzyme, which alters the shape of the active site
prevents the binding of the substrate
cannot have its effect reversed by adding more substrate

36. Malonate and Succinate Dehydrogenase
is a competitive inhibitor of succinate dehydrogenase
has a structure that is similar to succinate
inhibition is reversed by adding succinate

37. Irreversible Inhibition
In irreversible inhibition, an inhibitor
bonds with R groups at the active site
destroys enzyme activity
Enzyme Inhibition p 714

38. Irreversible Enzyme Inhibitors

39. Learning Check
Identify each description as an inhibitor that is 1) competitive or 2) noncompetitive.
A. increasing substrate reverses inhibition
B. binds to enzyme surface but not to the active site
C. structure is similar to substrate
D. inhibition is not reversed by adding more substrate

40. Solution
Identify each description as an inhibitor that is 1) competitive or 2) noncompetitive.
1 A. increasing substrate reverses inhibition
2 B. binds to enzyme surface but not to the active site
1 C. structure is similar to substrate
2 D. inhibition is not reversed by adding more substrate

41. Chapter 20 Enzymes and Vitamins
20.5 Regulation of Enzyme Activity
Describe the role of zymogens, feedback control, and allosteric enzymes in regulating enzyme activity.

42. 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.

43. 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

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

45. Summary of Digestive Enzymes

46. 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

47. 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

48. 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
Enzyme Inhibition p 714

49. 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

50. 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

51. Chapter 20 Enzymes and Vitamins
20.6 Enzyme Cofactors and Vitamins
Describe the types of cofactors found in enzymes.

52. Enzyme Cofactors
A simple enzyme is an active enzyme that consists only of protein.
Many enzymes are active only when they combine with cofactors such as metal ions or small molecules.
A coenzyme is a cofactor that is a small organic molecule such as a vitamin.

53. Enzyme Cofactors (continued)

54. Function of Coenzymes
A coenzyme prepares the active site for catalytic activity.

55. Metal Ions as Cofactors
Many active enzymes require a metal ion. For example, Zn2+, a cofactor for carboxypeptidase, stabilizes the carbonyl oxygen during the hydrolysis of a peptide bond.

56. Some Enzymes and Their Cofactors

57. Learning Check Identify each enzyme as 1) a simple enzyme
2) an enzyme that required a cofactor
A. requires Mg2+ for hydrolysis of phosphate esters
B. requires vitamin B3 to transfer an acetyl group
C. is active with four polypeptide subunits

58. Solution Identify each enzyme as 1) a simple enzyme
2) an enzyme that required a cofactor
2 A. requires Mg2+ for hydrolysis of phosphate esters
2 B. requires vitamin B3 to transfer an acetyl group
1 C. is active with four polypeptide subunits

59. Water-Soluble Vitamins
Water-soluble vitamins are
soluble in aqueous solutions
cofactors for many enzymes
not stored in the body

60. Descriptions of Water-Soluble Enzymes

61. Fat-Soluble Vitamins Fat-soluble vitamins are vitamins A, D, E, and K
soluble in lipids but not in aqueous solutions
stored in the body
important in vision, bone formation, antioxidants, and blood clotting

62. Learning Check
Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS).
A. folic acid
B. retinol (vitamin A)
C. vitamin C
D. vitamin E
E. niacin

63. Solution
Identify each of the following as a water-soluble vitamin (WS) or fat-soluble vitamin (FS)
WS A. folic acid
FS B. retinol (vitamin A)
WS C. vitamin C
FS D. vitamin E
WS E. niacin

64. Learning Check Identify the vitamin associated with each
1) riboflavin (B2) 2) vitamin A
3) vitamin K 4) vitamin D
5) ascorbic acid
A. collagen formation
B. part of the coenzymes FAD and FMN
C. absorption of phosphorus and calcium in bone
D. vision
E. blood clotting

65. Solution Identify the vitamin associated with each
1) riboflavin (B2) 2) vitamin A
3) vitamin K 4) vitamin D
5) ascorbic acid
5 A. collagen formation
1 B. part of the coenzymes FAD and FMN
4 C. absorption of phosphorus and calcium in bone
2 D. vision
3 E. blood clotting