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HOW ENZYMES FUNCTION
Enzymes are usually proteins that act as
biological catalysts, allowing life sustaining
reactions to occur in living cells.
© 2012 Pearson Education, Inc.
5.13 Enzymes speed up the cell’s chemical
reactions by lowering energy barriers
 Although biological molecules possess much
potential energy, it is not released spontaneously.
– An energy barrier must be overcome before a chemical
reaction can begin.
– This energy is called the activation energy (EA).
© 2012 Pearson Education, Inc.
5.13 Enzymes speed up the cell’s chemical
reactions by lowering energy barriers
 We can think of EA
– as the amount of energy needed for a reactant molecule
to move “uphill” to a higher energy but an unstable state
– so that the “downhill” part of the reaction can begin.
 One way to speed up a reaction is to add heat,
– which agitates atoms so that bonds break more easily
and reactions can proceed but
– could kill a cell.
© 2012 Pearson Education, Inc.
Figure 5.13A
Activation
energy barrier
Reactant
Products
Without enzyme With enzyme
Reactant
Products
Enzyme
Activation
energy
barrier
reduced by
enzyme
Energy
Energy
Figure 5.13Q
Reactants
Products
Energy
Progress of the reaction
a
b
c
5.13 Enzymes speed up the cell’s chemical
reactions by lowering energy barriers
 Enzymes
– function as biological catalysts by lowering the EA
needed for a reaction to begin,
– increase the rate of a reaction without being consumed
by the reaction, and
– are usually proteins, although some RNA molecules can
function as enzymes.
© 2012 Pearson Education, Inc.
Animation: How Enzymes Work
5.14 A specific enzyme catalyzes each cellular
reaction
 An enzyme
– is very selective in the reaction it catalyzes and
– has a shape that determines the enzyme’s specificity.
 The specific reactant that an enzyme acts on is
called the enzyme’s substrate.
 A substrate fits into a region of the enzyme called
the active site.
 Enzymes are specific because their active site fits
only specific substrate molecules.
© 2012 Pearson Education, Inc.
5.14 A specific enzyme catalyzes each cellular
reaction
 The following figure illustrates the catalytic cycle of
an enzyme.
© 2012 Pearson Education, Inc.
Figure 5.14_s1
1
Enzyme
(sucrase)
Active site
Enzyme available
with empty active
site
Figure 5.14_s2
2
1
Enzyme
(sucrase)
Active site
Enzyme available
with empty active
site
Substrate
(sucrose)
Substrate binds
to enzyme with
induced fit
Figure 5.14_s3
3
2
1
Enzyme
(sucrase)
Active site
Enzyme available
with empty active
site
Substrate
(sucrose)
Substrate binds
to enzyme with
induced fit
Substrate is
converted to
products
H2O
Figure 5.14_s4
4
3
2
1
Products are
released
Fructose
Glucose
Enzyme
(sucrase)
Active site
Enzyme available
with empty active
site
Substrate
(sucrose)
Substrate binds
to enzyme with
induced fit
Substrate is
converted to
products
H2O
5.14 A specific enzyme catalyzes each cellular
reaction
 For every enzyme, there are optimal conditions
under which it is most effective.
 Temperature affects molecular motion.
– An enzyme’s optimal temperature produces the highest
rate of contact between the reactants and the enzyme’s
active site.
– Most human enzymes work best at 35–40ºC.
 The optimal pH for most enzymes is near
neutrality.
© 2012 Pearson Education, Inc.
5.14 A specific enzyme catalyzes each cellular
reaction
 Many enzymes require nonprotein helpers called
cofactors, which
– bind to the active site and
– function in catalysis.
 Some cofactors are inorganic, such as zinc, iron, or
copper.
 If a cofactor is an organic molecule, such as most
vitamins, it is called a coenzyme.
© 2012 Pearson Education, Inc.
5.15 Enzyme inhibitors can regulate enzyme
activity in a cell
 A chemical that interferes with an enzyme’s activity
is called an inhibitor.
 Competitive inhibitors
– block substrates from entering the active site and
– reduce an enzyme’s productivity.
© 2012 Pearson Education, Inc.
5.15 Enzyme inhibitors can regulate enzyme
activity in a cell
 Noncompetitive inhibitors
– bind to the enzyme somewhere other than the active
site,
– change the shape of the active site, and
– prevent the substrate from binding.
© 2012 Pearson Education, Inc.
Figure 5.15A
Substrate
Enzyme
Allosteric site
Active site
Normal binding of substrate
Competitive
inhibitor
Noncompetitive
inhibitor
Enzyme inhibition
5.15 Enzyme inhibitors can regulate enzyme
activity in a cell
 Enzyme inhibitors are important in regulating cell
metabolism.
 In some reactions, the product may act as an
inhibitor of one of the enzymes in the pathway that
produced it. This is called feedback inhibition.
© 2012 Pearson Education, Inc.
Figure 5.15B
Feedback inhibition
Starting
molecule
Product
Enzyme 1 Enzyme 2 Enzyme 3
Reaction 1 Reaction 2 Reaction 3
A B C D
5.16 CONNECTION: Many drugs, pesticides,
and poisons are enzyme inhibitors
 Many beneficial drugs act as enzyme inhibitors,
including
– Ibuprofen, inhibiting the production of prostaglandins,
– some blood pressure medicines,
– some antidepressants,
– many antibiotics, and
– protease inhibitors used to fight HIV.
 Enzyme inhibitors have also been developed as
pesticides and deadly poisons for chemical
warfare.
© 2012 Pearson Education, Inc.
NUCLEIC ACIDS
© 2012 Pearson Education, Inc.
3.15 Nucleic acids are polymers of nucleotides
 DNA (deoxyribonucleic acid) and RNA
(ribonucleic acid) are composed of monomers
called nucleotides.
 Nucleotides have three parts:
– a five-carbon sugar called ribose in RNA and
deoxyribose in DNA,
– a phosphate group, and
– a nitrogenous base.
© 2012 Pearson Education, Inc.
Figure 3.15A
Phosphate
group
Sugar
Nitrogenous
base
(adenine)

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Ch 3enzymes 2

  • 1. HOW ENZYMES FUNCTION Enzymes are usually proteins that act as biological catalysts, allowing life sustaining reactions to occur in living cells. © 2012 Pearson Education, Inc.
  • 2. 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers  Although biological molecules possess much potential energy, it is not released spontaneously. – An energy barrier must be overcome before a chemical reaction can begin. – This energy is called the activation energy (EA). © 2012 Pearson Education, Inc.
  • 3. 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers  We can think of EA – as the amount of energy needed for a reactant molecule to move “uphill” to a higher energy but an unstable state – so that the “downhill” part of the reaction can begin.  One way to speed up a reaction is to add heat, – which agitates atoms so that bonds break more easily and reactions can proceed but – could kill a cell. © 2012 Pearson Education, Inc.
  • 4. Figure 5.13A Activation energy barrier Reactant Products Without enzyme With enzyme Reactant Products Enzyme Activation energy barrier reduced by enzyme Energy Energy
  • 6. 5.13 Enzymes speed up the cell’s chemical reactions by lowering energy barriers  Enzymes – function as biological catalysts by lowering the EA needed for a reaction to begin, – increase the rate of a reaction without being consumed by the reaction, and – are usually proteins, although some RNA molecules can function as enzymes. © 2012 Pearson Education, Inc. Animation: How Enzymes Work
  • 7. 5.14 A specific enzyme catalyzes each cellular reaction  An enzyme – is very selective in the reaction it catalyzes and – has a shape that determines the enzyme’s specificity.  The specific reactant that an enzyme acts on is called the enzyme’s substrate.  A substrate fits into a region of the enzyme called the active site.  Enzymes are specific because their active site fits only specific substrate molecules. © 2012 Pearson Education, Inc.
  • 8. 5.14 A specific enzyme catalyzes each cellular reaction  The following figure illustrates the catalytic cycle of an enzyme. © 2012 Pearson Education, Inc.
  • 9. Figure 5.14_s1 1 Enzyme (sucrase) Active site Enzyme available with empty active site
  • 10. Figure 5.14_s2 2 1 Enzyme (sucrase) Active site Enzyme available with empty active site Substrate (sucrose) Substrate binds to enzyme with induced fit
  • 11. Figure 5.14_s3 3 2 1 Enzyme (sucrase) Active site Enzyme available with empty active site Substrate (sucrose) Substrate binds to enzyme with induced fit Substrate is converted to products H2O
  • 12. Figure 5.14_s4 4 3 2 1 Products are released Fructose Glucose Enzyme (sucrase) Active site Enzyme available with empty active site Substrate (sucrose) Substrate binds to enzyme with induced fit Substrate is converted to products H2O
  • 13. 5.14 A specific enzyme catalyzes each cellular reaction  For every enzyme, there are optimal conditions under which it is most effective.  Temperature affects molecular motion. – An enzyme’s optimal temperature produces the highest rate of contact between the reactants and the enzyme’s active site. – Most human enzymes work best at 35–40ºC.  The optimal pH for most enzymes is near neutrality. © 2012 Pearson Education, Inc.
  • 14. 5.14 A specific enzyme catalyzes each cellular reaction  Many enzymes require nonprotein helpers called cofactors, which – bind to the active site and – function in catalysis.  Some cofactors are inorganic, such as zinc, iron, or copper.  If a cofactor is an organic molecule, such as most vitamins, it is called a coenzyme. © 2012 Pearson Education, Inc.
  • 15. 5.15 Enzyme inhibitors can regulate enzyme activity in a cell  A chemical that interferes with an enzyme’s activity is called an inhibitor.  Competitive inhibitors – block substrates from entering the active site and – reduce an enzyme’s productivity. © 2012 Pearson Education, Inc.
  • 16. 5.15 Enzyme inhibitors can regulate enzyme activity in a cell  Noncompetitive inhibitors – bind to the enzyme somewhere other than the active site, – change the shape of the active site, and – prevent the substrate from binding. © 2012 Pearson Education, Inc.
  • 17. Figure 5.15A Substrate Enzyme Allosteric site Active site Normal binding of substrate Competitive inhibitor Noncompetitive inhibitor Enzyme inhibition
  • 18. 5.15 Enzyme inhibitors can regulate enzyme activity in a cell  Enzyme inhibitors are important in regulating cell metabolism.  In some reactions, the product may act as an inhibitor of one of the enzymes in the pathway that produced it. This is called feedback inhibition. © 2012 Pearson Education, Inc.
  • 19. Figure 5.15B Feedback inhibition Starting molecule Product Enzyme 1 Enzyme 2 Enzyme 3 Reaction 1 Reaction 2 Reaction 3 A B C D
  • 20. 5.16 CONNECTION: Many drugs, pesticides, and poisons are enzyme inhibitors  Many beneficial drugs act as enzyme inhibitors, including – Ibuprofen, inhibiting the production of prostaglandins, – some blood pressure medicines, – some antidepressants, – many antibiotics, and – protease inhibitors used to fight HIV.  Enzyme inhibitors have also been developed as pesticides and deadly poisons for chemical warfare. © 2012 Pearson Education, Inc.
  • 21. NUCLEIC ACIDS © 2012 Pearson Education, Inc.
  • 22. 3.15 Nucleic acids are polymers of nucleotides  DNA (deoxyribonucleic acid) and RNA (ribonucleic acid) are composed of monomers called nucleotides.  Nucleotides have three parts: – a five-carbon sugar called ribose in RNA and deoxyribose in DNA, – a phosphate group, and – a nitrogenous base. © 2012 Pearson Education, Inc.

Editor's Notes

  1. Student Misconceptions and Concerns For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output is much greater than the input. Teaching Tips The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids.
  2. Student Misconceptions and Concerns For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output is much greater than the input. Teaching Tips The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids.
  3. Figure 5.13A The effect of an enzyme in lowering EA
  4. Figure 5.13Q Activation energy with and without an enzyme
  5. Student Misconceptions and Concerns For students not previously familiar with activation energy, analogies can make all the difference. Activation energy can be thought of as a small input that is needed to trigger a large output. This is like (a) an irritated person who needs only a bit more frustration to explode in anger, (b) small waves that lift debris over a dam, or (c) lighting a match around lighter fluid. In each situation, the output is much greater than the input. Teaching Tips The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids.
  6. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments.
  7. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments.
  8. Figure 5.14_s1 The catalytic cycle of an enzyme (step 1)
  9. Figure 5.14_s2 The catalytic cycle of an enzyme (step 2)
  10. Figure 5.14_s3 The catalytic cycle of an enzyme (step 3)
  11. Figure 5.14_s4 The catalytic cycle of an enzyme (step 4)
  12. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments.
  13. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. The text notes that the relationship between an enzyme and its substrate is like a handshake, with each hand generally conforming to the shape of the other. This induced fit is also like the change in shape of a glove when a hand is inserted. The glove’s general shape matches the hand, but the final “fit” requires some additional adjustments.
  14. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback inhibition relies upon the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced.)
  15. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback inhibition relies upon the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced.)
  16. Figure 5.15A How inhibitors interfere with substrate binding
  17. Student Misconceptions and Concerns The specific interactions of enzymes and substrates can be illustrated with simple physical models. Many students new to these concepts will benefit from several forms of explanation, including diagrams such as those in the textbook, physical models, and the opportunity to manipulate or create their own examples. Just like pitching a tent, new concepts are best constructed with many lines of support. Teaching Tips 1. The information in DNA is used to direct the production of RNA, which in turn directs the production of proteins. However, in Chapter 3, four different types of biological molecules were noted as significant components of life. Students who think this through might wonder, and you could point out that DNA does not directly control the production of carbohydrates and lipids. So how does DNA exert its influence over the synthesis of these two chemical groups? The answer is largely by way of enzymes, proteins with the ability to promote the production of carbohydrates and lipids. 2. Enzyme inhibitors that block the active site are like (a) a person sitting in your assigned theater seat or (b) a car parked in your parking space. Analogies for inhibitors that change the shape of the active site are more difficult to imagine. Consider challenging your students to think of such analogies. (Perhaps someone adjusting the driver seat of the car differently from your preferences and then leaving it that way when you try to use the car.) 3. Feedback inhibition relies upon the negative feedback of the accumulation of a product. Ask students in class to suggest other products of reactions that inhibit the process that made them when the product reaches high enough levels. (Gas station pumps routinely shut off when a high level of gasoline is detected. Furnaces typically turn off when enough heat has been produced.)
  18. Figure 5.15B Feedback inhibition of a biosynthetic pathway
  19. Teaching Tips Challenge your class to identify advantages of specific enzyme inhibitors for pest control. These advantages include (a) the ability to target chemical reactions of only certain types of pest organisms and (b) the ability to target chemical reactions that are found in insects but not in humans.
  20. Teaching Tips When discussing the sequence of nucleotides in DNA and RNA, consider challenging your students with the following questions based upon prior analogies. If the 20 possible amino acids in a polypeptide represent “words” in a long polypeptide sentence, how many possible words are in the language of a DNA molecule? (Answer: Four nucleotides, GCAT, are possible). Are these the same “words” used in RNA? (Answer: No. Uracil substitutes for thymine.)
  21. Figure 3.15A A nucleotide, consisting of a phosphate group, a sugar, and a nitrogenous base