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Biology 12 - Enzymes and Metabolic Pathways - Section 5-2
1.
2. UNIT A: Cell Biology
Chapter 2: The Molecules of Cells
Chapter 3: Cell Structure and Function
Chapter 4: DNA Structure and Gene
Expression
Chapter 5: Metabolism: Energy and
Enzymes: Section 5.2
Chapter 6: Cellular Respiration
Chapter 7: Photosynthesis
3. UNIT A Chapter 5: Metabolism: Energy and Enzymes
Chapter 5: Metabolism: Energy
In this chapter you will learn about the numerous chemical reactions
in our bodies involved in breaking down food to produce essential
biological molecules and energy.
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and Enzymes
What is the role of
an enzyme?
What factors
influence the rate of
enzyme activity?
4. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
5.2 Enzymes and Metabolic Pathways
Many chemical reactions in the cell are linked in metabolic
pathways.
โขThe product of one reaction is the reactant for the next
reaction in the pathway. These pathways may be linear (with
a final product) or cyclical (reactant is regenerated)
โขSpecific enzymes are proteins that catalyze each step. The
reactants are called enzyme substrates
A is a substrate for the enzyme E1 to produce product B. B is a substrate for E2 to
produce C. This process continues until the final product G.
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5. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Energy of Activation
For chemical reactions,
energy must be added for
reactants to react. This is the
energy of activation, Ea.
โขEven if ฮG is negative, Ea
must be overcome
โขEnzymes speed up the rate
of a reaction by lowering the
Ea barrier
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Figure 5.2 Energy of activation (Ea).
6. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
How Enzymes Function
In enzyme-catalyzed reactions, the active site of the
enzyme interacts with the substrate(s) to form an enzyme-substrate
complex. After the reaction, product is released
and the enzyme can bind another substrate.
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Figure 5.3 Enzymatic action.
7. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Induced-Fit Model
When a substrate binds to an enzyme, the active site undergoes
a slight change in shape, called the induced-fit model, to form
the enzyme-substrate complex.
Figure 5.4 Induced
fit model.
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8. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Enzyme Names
Because enzymes form complexes with specific substrates,
they are often named by adding the suffix โase to the name of
the substrate.
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9. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Factors Affecting Enzymatic Speed
Several factors can regulate the activity of an enzyme. These
include
โขthe amount of substrate(s) present for the reaction
โขenvironmental conditions, such as temperature and pH
โขenzyme activation
โขenzyme inhibition
โขpresence of cofactors
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10. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Substrate Concentration
Generally, enzyme activity increases as substrate
concentration increases because
โขthere are more collisions between the enzyme and substrate
molecules
โขmore substrate molecules are available to fill more active
sites of enzymes
However, a maximum rate exists. Once all active sites on an
enzyme are filled with substrate, the reaction cannot go any
faster.
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11. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Temperature and pH
As temperature increases, enzyme activity also increases
because there are more effective collisions between enzyme
and substrate.
โขAbove a certain temperature the enzyme will no longer be
active because it is denatured and cannot bind substrate.
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SLIDE Figure 5.5 The effect of temperature on rate of reaction.
12. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Temperature and pH
Every enzyme has an ideal pH where its activity is greatest.
โขThe protein is in a configuration that makes it most active.
โขChanges in pH can disrupt normal interactions such as
hydrogen bonding, causing a change in enzyme shape and a
decrease in activity. Extreme pH changes can cause
denaturation.
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Figure 5.6 The effect of temperature on rate of reaction.
13. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Enzyme Activation
Some enzymes do not need to be always active in the cell and
can exist in an inactive form. When the cell signals a need for
the enzyme, the inactive form is changed to an active form.
There are different ways this can occur:
โขinteraction with another protein or molecule
โขremoval of part of the protein
โขaddition or removal of one or more phosphate groups; kinase
enzymes add phosphates to proteins
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14. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Enzyme Inhibition
โข Enzyme inhibition
decreases the activity of
the enzyme by no longer
allowing substrate(s) to
bind to the active site.
โข An important type of
inhibition is feedback
inhibition: reaction
product binds the enzyme,
causing a change in
enzyme conformation and
inactivation.
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Figure 5.7 Feedback inhibition.
15. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
Enzyme Cofactors
Many enzymes need an inorganic ion or organic nonprotein
molecule to function properly.
โขThe inorganic ions are called cofactors and include metals
such as iron and zinc.
โขThe organic nonproteins are called coenzymes and may
contribute atoms to the reaction. Vitamins are small organic
molecules required in our diet that are often components of
coenzymes (for example, the vitamin niacin is part of the
coenzyme NAD).
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16. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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Check Your Progress
1. Summarize why enzymes are needed in biochemical
pathways and how cells may regulate their activity.
2. Explain why denaturing an enzyme causes a change
in its ability to act as a catalyst.
3. Discuss why the three-dimensional shape of an
enzyme is important to its function.
17. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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18. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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19. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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20. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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21. UNIT A Chapter 5: Metabolism: Energy and Enzymes Section 5.2
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Editor's Notes
Presentation title slide
Chapter opener background notes
Are you lactose intolerant? Do you know someone who is? As much as three-quarters of the world's population has some difficulty digesting lactose. Digesting a piece of cheese pizza can be very uncomfortable for those who are lactose intolerant because cheese and other dairy products contain lactose. What causes lactose intolerance?
ย Lactose is a disaccharide sugar that must be broken down chemically into its two smaller building blocks (galactose and glucose) before it can be absorbed into the bloodstream and used for energy. If it is not broken down, lactose remains in the digestive tract and can cause uncomfortable digestive symptoms. The breakdown of lactose requires an enzyme called lactase. People who are lactose intolerant do not produce enough lactase. A lactose intolerant person who wants to consume dairy products without discomfort must consume products that have been specially treated with enzymes or a lactase enzyme supplement with the dairy in their meal. Yogurt and buttermilk can often be tolerated by those with lactose intolerance because they contain bacterial cultures that aid in the digestion of lactose.
ย Why is it that some people can easily digest lactose when so many other people cannot? After young mammals are weaned, milk becomes a smaller component of the diet. In most mammals, lactase activity decreases and lactose intolerance usually begins. However, some humans have developed what is called lactase persistency. These individuals continue to produce lactase. Factors that contribute to lactase persistency include diet, culture, and genetics.
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The breakdown of lactose is just one of myriad chemical reactions that occur in the human body. Almost every reaction that takes place in our bodies requires a specific enzyme. In this chapter, you will learn about the characteristics of enzymes and how enzymes function in the flow of energy and metabolism.
metabolic pathway: a series of linked reactions that begin with a particular reactant and terminate with an end product
enzymes: types of proteins that function as catalysts to speed up chemical reactions
substrates: reactants in an enzymatic reaction
Caption text
Figure 5.2 Energy of activation (Ea). Enzymes speed the rate of reactions because they lower the amount of energy required for the reactants to react.
energy of activation (Ea): the energy that must be added to cause molecules to react with one another
Caption text
Figure 5.3 Enzymatic action. An enzyme has an active site where the substrates and enzyme fit together in such a way that the substrates react. Following the reaction, the products are released, and the enzyme is free to act again. a. The enzymatic reaction can result in the degradation of a substrate into multiple products (catabolism) or, b. the synthesis of a product from multiple substrates (anabolism).
active site: a small part of an enzyme that forms a complex with a substrate(s)
Caption text
Figure 5.4 Induced fit model. These computer-generated images show an enzyme called lysozyme that hydrolyzes its substrate, a polysaccharide that makes up bacterial cell walls. a. Shape of enzyme when no substrate is bound to it. b. After the substrate binds, the shape of the enzyme changes so that hydrolysis can better proceed.
Induced-fit model: when the enzyme undergoes a slight change in shape in order to accommodate the substrate
Caption text
Figure 5.5 The effect of temperature on rate of reaction. a. Usually, the rate of an enzymatic reaction doubles with every 10ยฐC rise in temperature. This enzymatic reaction is maximum at about 40ยฐC. Then it decreases until the reaction stops altogether, because the enzyme has become denatured. b. The body temperature of ectothermic animals, which require an environmental source of heat, often limits rates of reactions. c. The body temperature of endothermic animals, which generate heat through their own metabolism, promotes rates of reaction.
denatured: describes a protein that has had an irreversible change in its shape; occurs when proteins are exposed to extremes in heat and pH
Caption text
Figure 5.6 The effect of temperature on rate of reaction. The preferred pH for pepsin, an enzyme that acts in the stomach, is about 2, while the preferred pH for trypsin, an enzyme that acts in the small intestine, is about 8. At the preferred pH, an enzyme maintains its shape so that it can bind with its substrates.
Caption text
Figure 5.7 Feedback inhibition. a. In an active pathway, the first reactant (A) is able to bind to the active site of enzyme E1. b. Feedback inhibition occurs when the end product (F) of the metabolic pathway binds to the first enzyme of the pathwayโat a site other than the active site. This binding causes the active site to change its shape. Now reactant A is unable to bind to the enzymeโs active site, and the whole pathway shuts down.
enzyme inhibition: occurs when the substrate is unable to bind to the active site of an enzyme
cofactors: inorganic ion helpers required by enzymes to function properly
coenzymes: organic, nonprotein molecule helpers required by enzymes to function properly
vitamins: relatively small organic molecules that are required in trace amounts in diets for synthesis of coenzymes that affect health and fitness
Answers
1. Enzymes are needed to reduce the activation energy of biochemical reactions, thus allowing reactions to occur under conditions of the cell. A cell can convert an enzyme from an inactive form to an active form by the addition or removal of phosphate groups. Cellular enzymes are subject to feedback inhibition. The presence of cofactors and enzymes in the cell regulates enzyme activity.
2. Denaturing changes the shape of the enzyme and its active site, altering the fit of the reactant with the enzyme.
3. How well the enzyme interacts with the reactants and the rate at which the product is formed is determined by its three-dimensional shape and its active site.
