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VocabularyClick on each word below to go to the definition.
activation energy
noncompetitive inhibitor
active siteallosteric regulationcatalystcoenzymecofactorcompetitive inhibitor
enzymefeedback inhibitioninduced fit
optimal pHoptimal temperaturesubstrate
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Enzymes Unit Topics
· Enzymes, Catalytic Cycle
· Temperature, pH, Inhibition
Click on the topic to go to that section
· Allosteric Regulation, Feedback Inhibition
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A catalyst is a substance which speeds up chemical reactions without being changed by the reaction.
When a catalyst is present, less energy is needed to start a chemical reaction. When a catalyst is present, the speed of a
chemical reaction is faster.
The catalyst remains unchanged at the end of the reaction.
Catalysts
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Enzymes are catalysts in biological systems.
In early cells, enzymes were made of ____________.
In modern cells, enzymes are a type of ___________.
Enzymes
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Common Enzymes
Lactase is an enzyme that helps us to digest dairy products.
People with lactose intolerance have trouble digesting dairy because they lack this enzyme, but they are able to take a lactaid pill that contains the enzyme lactase.
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Common Enzymes
Amylase is an enzyme found in human saliva. It begins the chemical process of digestion.
Typically, you can identify an enzyme by its -ase ending.
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Enzyme Substrate Complex
Enzymes help chemical reactions occur by providing a space called an active site for the substrates (reactants), to bind.
Substrate enteringactive site of enzyme
Enzyme/substratecomplex
Enzyme/productscomplex
Products leavingactive site of enzyme
Active site
substrate
Click here to see a video on Enzymes
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Induced Fit
Just as a key can only open a specific lock, each enzyme has its own unique shape, so each enzyme is specific to certain substrates.
As the substrates enter the active site, the enzyme's shape changes just a little in order to create a better fit, called an induced fit.
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Catalytic Cycle of an Enzyme
An enzyme is capable of being used again and again to allow more of the same reactions to occur.
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1 Which of the following acts as a catalyst in the body?
A CarbohydratesB Nucleic AcidsC LipidsD EnzymesE Water
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2 Another name for protein reactants is _____.
A productsB substratesC active sitesD enzymes
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3 Enzymes bind only to certain substrates.
TrueFalse
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4 An enzyme can only be used for one reaction and then it will not work again.
TrueFalse
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Activation Energy
Enzymes work by decreasing the minimum amount of energy required for reaction. This is called the activation energy, Ea.
Just as a ball cannot get over a hill if it does not roll up the hill with enough energy, a reaction cannot occur unless the molecules possess sufficient energy to get over the activation energy barrier.
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Potential Energy DiagramsThis activation energy is usually needed to break bonds in the substrate.
initial energy
final energy
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CatalystsEnzymes increase the rate of a reaction by decreasing the activation energy of the reaction.
This graph shows the decomposition of a sugar both with and without a catalyst.
Notice that the initial energy of reactants and the final energy of the products are unchanged by the catalyst.
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5 Activation energy is _____.
A the heat released in a reaction
B the energy given off when reactants collide
C generally very high for a reaction that takes place rapidly
D an energy barrier between reactants and products
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6 What happens to a catalyst in a reaction?
A It is unchanged.B It is incorporated into the products.C It is incorporated into the reactants.D It evaporates away.
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7 Why does a catalyst cause a reaction to proceed faster?
A Only because there are more collisions per second.
B Only because collision occur with greater energy.
C Only because the activation energy is lowered.
D There are more frequent collisions and they are of greater energy.
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8 If a catalyst is used in a reaction _____.
A the energy of activation increasesB different reaction products are obtainedC the reaction rate increasesD it evaporates away
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Enzymes Have Optimal Environments
Since enzymes are proteins and proteins are sensitive to their environments, enzymes are also sensitive to their environments.
Factors Affecting Enzyme Activity
Temperature
pH
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Effect of Temperature on EnzymesIn general, increasing the temperature of a system increases the reaction rate because the substrates are able to move faster and have more collisions with the active sites of the enzymes.
This is true only up to certain temperatures for each type of enzyme! We call this temperature the optimal temperature.
The optimal temperature is different for each type of enzyme.
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Past the optimal temperature, the enzyme begins to denature or lose its shape, which changes the shape of the active site.
Effect of Temperature on Enzymes
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Optimal Temperature and Fever
The optimal temperature for most bacterial enzymes is less than 98o F, so by raising body temperature above that, the immune system attempts to denature the bacteria's enzymes and stop the infection.
Fever is the elevation of body temperature above normal (In humans 98.6o F). Typically, fevers develop in response to bacterial or viral infection.
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Common Enzymes
Laundry detergents contain enzymes that help break up and remove stains
from your clothing.
Why might you want to use a different laundry detergent when you wash clothes in hot water instead of
in cold water?
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Effect of pH on Enzymes
pH level can also cause a denaturing of the enzyme.
Most biological solutions have pH values between 6-8.In
crea
sing
ly
Aci
dic
[H+]
> [O
H–]
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(optimal pH = 4.2)The optimal pH for most enzymes is between 6-8, but again the optimal pH is different for each type of enzyme.
Effect of pH on Enzymes
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9 What is the optimal temperature for this enzyme?
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10 What is the optimal pH of this enzyme?
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11 Which enzyme has a lower optimal temperature?
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12 At which temperature do both enzymes have an equal rate of reaction?
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13 Which description best explains the temperature effects shown in the graph?
A Each enzyme will function at room temperature.
B Both enzymes are inactive at the freezing point.
C Each enzyme has its own optimal temperature range.
D Both enzymes have the same optimal temperature range.
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14 Based on this information which environment can you conclude as being more acidic?
A StomachB Liver
Liver Enzyme
Stomach Enzyme
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15 Which enzyme would you choose to use in a basic environment?
A B C
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16 The active site of an enzyme I. is the part where a substrate can fit II. can be used again and again III. is not affected by environmental factors
A I only
B II only
C III only
D I and II
E I and III
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CofactorsSometimes enzymes need a helper to bind at the active site to make the enzyme active. These helpers are called cofactors.
If the cofactors are organic molecules, then they are called coenzymes. Vitamins are a type of coenzyme.
Enzyme
coenzyme substrate
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Enzyme Inhibitors
Certain chemicals work to stop or inhibit the enzymes. These chemicals are called enzyme inhibitors.
Types of Inhibitors
· Competitive Inhibitors· Noncompetitive Inhibitors
click here for an animation about inhibition
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Competitive inhibitors are similar in shape to the substrates.
They are able to block the substrates from binding to the active site by binding to the active site themselves.
Competitive inhibitor
Competitive Inhibitors
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Competitive Inhibitors: How They Are Stopped
To stop competitive inhibition, the concentration of the substrates needs to be increased so that they outnumber the inhibitors.
This way, the substrates are more likely to bind to the active site before an inhibitor does.
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Noncompetitive Inhibitors:How they work
Noncompetitive inhibitors bind to a separate part of the enzyme and cause the enzyme to change shape.
When the enzyme changes shape, the substrate is no longer able to bind to the active site because the active site also changes shape.
This type of inhibition is sometimes irreversible.
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17 Organic molecules that aid in the action of the enzyme are called ____.A products
B coenzymes
C substrates
D helpers
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18 Which type of inhibitor binds at the active site?
A Competitive Inhibitor
B Noncompetitive Inhibitor
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19 Noncompetitive inhibitors are similar in shape to the substrates that bind at the active site of an enzyme.
True
False
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20 If an enzyme has been inhibited noncompetitively, _______.
A the enzyme is able to increase its activity
B increasing substrate concentration will increase inhibition
C the active site will be occupied by the inhibitor molecule
D the active site will change shape
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21 Noncompetitive inhibition is sometimes irreversible.
True
False
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Allosteric Regulation
In most natural processes it is necessary to regulate enzyme activity. This regulation can either be to inhibit or to stimulate activity.
Allosteric regulation is a type of noncompetitive inhibition that is reversible.
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Allosteric Activation and Inhibition
Enzymes that undergo allosteric regulation are usually made of multiple subunits or polypeptide chains. All of the subunits together form a complex that can be in an active or inactive position.
Where the subunits connect, there is an allosteric site, or a site that allows an activator or inhibitor molecule to bind.
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Allosteric Inhibitors
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If an inhibitor binds to the enzyme, the subunits are stabilized in an inactive form. This means the enzymes changes shape and the active sites are not open for substrates to bind to.
Allosteric Activators bind to the enzyme, stabilizing the subunits in an active form. This means the enzymes change shape and the active sites are available for substrates to bind to.
Allosteric Activators
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Feedback Inhibition
In certain processes the products from one enzyme act as the substrates for a second enzyme and and the second enzyme's products are a substrate for a third enzyme etc.
When this happens, the products from the last enzyme in the path can allosterically inhibit the first enzyme in the path until it is necessary for more of the products to be made again and then the inhibitor leaves.
This is called feedback inhibition.
Click here to see a video of Feedback Inhibition
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enzyme 2
enzyme 1
End Product
Intermediate A
Initial Substrate
Intermediate B
enzyme 3
End Product + X
Feedback Inhibition
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22 An allosteric site on an enzyme is
A not made of protein
B involved in feedback inhibition
C the same as the active site
D where the products leave the enzyme
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23 Which of the following is not part of allosteric regulation?
A other substrate molecules compete for the active site
B regulatory molecules bind to a site separate from the active site
C inhibitors and activators may compete with one another
D a naturally occuring molecule stabilizes an active conformation
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24 Allosteric regulation is similar to noncompetitive inhibition except that it is _____.
A always reversible
B sometimes irreversible
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25 In allosteric regulation the sites where the inhibitors and activators are able to bind are called _____.
A Active Site
B Substrates
C Allosteric Site
D Cofactors
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26 In allosteric regulation both an inhibitor and an activator can bind to one substrate complex at the same time.
True
False
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27 Feedback inhibition is a type of _____.
A competitive inhibition
B product
C allosteric regulation
D enzyme
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