Enzymes

Question 1

Key Characteristics of Enzymes

• ___ ___ ___ - 10_ to 10__ greater than uncatalyzed reactions.
• ____ ____ ___– _______ relevant conditions.
• ____ ___ ___ - Precise interaction of substrate and enzyme (enzyme-substrate complex).
• ____ __ ______ – ____ control, ____ modifications, ____ of enzyme (synthesis and degradation)
• •Accelerate the___ ____ _____, not the ____ of product and reactants in equilibrium.

Answer 1

• Higher reactions rates - 106 to 1012 greater than uncatalyzed reactions.
• Milder reaction conditions – physiologically relevant conditions.
• Greater reaction specificity - Precise interaction of substrate and enzyme (enzyme-substrate complex).
• Capacity for regulation – allosteric control, covalent modifications, turnover of enzyme (synthesis and degradation)
• Accelerate the rate reaching equilibrium, not the composition of product and reactants in equilibrium.

Question 2

Cofactors of Enzymes

____ + ____ = _____

Coenzymes that are tightly bound are ____ ____,

while coenzymes loosely bound are known as ________

Answer 2

Apoenzyme + cofactor = holoenzyme

Coenzymes that are tightly bound are prosthetic group, while coenzymes loosely bound are known as co-substrates

Question 3

Classification of Enzymes

1) O______: ____
2) T______: ___ ____
3) H______: ___ ___ (transfer ___ ___ to ___)
4) L______: ________/_____ of ____ to form ___ ___
5) I______: ________(____ ____ ___)
6) L______: Ligation of _substrates at the expense of ____ ____

Answer 3

1) Oxidoreductastes: Redox
2) Transferases: group transfer
3) Hydrolases: hydrolysis rxns (transfer fcnal group to H20)
4) Lyases: addition/removal of groups to form double bonds
5) Isomerases: Isomerization (intramolecular group transfer)
6) Ligases: Ligation of 2 substrates at the expense of ATP hydrolysis

Question 4

Active Site of an Enzyme

• •The active site is a ___ ___ or____ formed by side chains of different amino acids from different parts of the sequence.
• •Substrates bind to the crevice by ___ _____ interactions. Formation of ___________complex.
• •Specificity of the enzyme is determined by the ___ ____ of ___ ___ in the active site.
• •The _____ of the bound substrate molecules and the _____ of these molecules towards one another affects the ____ ____

Answer 4

• The active site is a 3-D cleft or crevice formed by side chains of different amino acids from different parts of the sequence.
• Substrates bind to the crevice by weak non-covalent interactions. Formation of Enzyme-substrate complex.
• Specificity of the enzyme is determined by the exact orientation of the side chains in the active site.
• The proximity of the bound substrate molecules and the orientation of these molecules towards one another affects the catalytic power.

Question 5

Enzyme Specificity

•Specificity of the enzyme is determined by the exact _____ of the ____ ___ in the active site that interact with the substrate.
•Two theories to explain the specificity of enzymes
______
______

Answer 5

•Specificity of the enzyme is determined by the exact orientation of the side groups in the active site that interact with the substrate.
•Two theories to explain the specificity of enzymes
–Lock and key
–Induced fit models
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Question 6

Enzyme Specificity

Lock and Key model: Substrate interacts with the enzyme by ___ ___ ___ interactions. ____ has a ____ ____to fit into the active site, analogous to a lock and key.

Induced fit model: The ____ ___ of the enzyme takes on a____ ____ to the ___ ___, once the substrate is bound. This a ____ process. (More accepted model)

Answer 6

Lock and Key model: Substrate interacts with the enzyme by short range weak interactions. Substrate has a matching shape to fit into the active site, analogous to a lock and key.

Induced fit model: The active site of the enzyme takes on a complementary shape to the transition state, once the substrate is bound. This a dynamic process. (More accepted model)

Question 7

How Enzymes Operate

• The free-energy difference (ΔG) between the products and reactants determines the _____ of the reaction.
• Energy required to ____ the conversion of reactants to products determines the ____of the reaction. (____ ____)

Answer 7

•The free-energy difference (ΔG) between the products and reactants determines the spontaneity of the reaction.
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•Energy required to initiate the conversion of reactants to products determines the rate of the reaction. (Activation energy ΔG‡)

Question 8

Transition State

• When substrates are converted to products, they go through a state known as the transition state.
• The transition state has a ___ ___ ___ than the products or substrates.
• This is due to the bonds being ___ ___ in the ___ or due to ___ and ____ ___ ____
• The difference in energy between the substrate and the transition state is known as the ___ _____
• The final change of the energy DG is _____ by the ____ ___ since this energy is____ when the ____are formed.

Answer 8

• When substrates are converted to products, they go through a state known as the transition state.
• The transition state has a higher free energy than the products or substrates.
• This is due to the bonds being maximally strained in the substrate or due to unstable and strained electronic configurations.
• The difference in energy between the substrate and the transition state is known as the activation energy.
• The final change of the energy DG is unaffected by the activation energy since this energy is returned when the products are formed.

Question 9

Activation Energy ΔG‡

•Enzymes ____ the activation energy without changing the ____ for the reaction.
•This allows more molecules to ______________.
•The transition stage is stabilized by progressive formation of ___ and ____ bonds. (____ ____ and ____ ___)
•The transition state can either _____________ or __________
•

Answer 9

•Enzymes lower the activation energy without changing the DG for the reaction.
•This allows more molecules to reach the transition state.
•The transition stage is stabilized by progressive formation of weak and tighter bonds. (electrostatic interactions and by covalent intermediates)
•The transition state can either collapse back to the substrate or form the product.
•

Question 10

The Relationship between Km and Vmax

• The velocity of a reaction is dependent on the ___________
• Known as the Michaelis-Menten Equation

Where

Vmax=

Km=

Answer 10

• The velocity of a reaction is dependent on the concentration of the substrate.
• Known as the Michaelis-Menten Equation

Where

Vmax=maximal velocity

Km= Michaelis constant=[substrate] at which rate is ½ the maximal rate

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Question 11

The Relationship between Km and Vmax

• At very low substrate concentration, [S] is ____than KM, V0 =________; the rate is_________________.
• At high substrate concentration, when [S] is____ than KM, V0 = ____; the rate is ____ and ____ of _____ ____
• When [S] = KM, then V0 = Vmax/2. Thus, KM is equal to the substrate concentration at which the reaction rate is half its maximal value.

•A Higher KM implies the substrate has a ____ affinity for the enzyme, while a low KM implies a ____ affinity for the enzyme.
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Answer 11

•At very low substrate concentration, [S] is less than KM, V0 = (Vmax/KM)[S]; the rate is directly proportional to the substrate concentration .
•At high substrate concentration, when [S] is greater than KM, V0 = Vmax; the rate is maximal, independent of substrate concentration.
•When [S] = KM, then V0 = Vmax/2. Thus, KM is equal to the substrate concentration at which the reaction rate is half its maximal value.
•A Higher KM implies the substrate has a low affinity for the enzyme, while a low KM implies a higher affinity for the enzyme.
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Question 12

Lineweaver-Burk or Double Reciprocal Plots

•The Vmax can be accurately determined using the double reciprocal plots.

•Useful for distinguishing between ____/ _____inhibition.
•
Equation

Intercepts

Slope

Answer 12

•The Vmax can be accurately determined using the double reciprocal plots.•Useful for distinguishing between competitive and non competitive inhibition.
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Question 13

Enzyme Inhibition

•Enzyme inhibition can be either___ or _____
•
•An irreversible inhibitor dissociates ___ ___ from an enzyme because it is tightly bound to the enzyme by___ or ____interactions.
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•Reversible inhibition can be classified as ___ ___ or ____
•

Answer 13

•Enzyme inhibition can be either reversible or irreversible.
•
•An irreversible inhibitor dissociates very slowly from an enzyme because it is tightly bound to the enzyme by covalent or non-covalent interactions.
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•Reversible inhibition can be classified as competitive, uncompetitive or non-competitive inhibition.•
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Question 14

Competitive, non-competitive and uncompetitive inhibition

•Competitive inhibitor ____ with the ___ for the _____. Increasing the overall concentration of the substrate ___________
•Non-competitive inhibitors ____ ___ _____ for the substrate binding site. Increasing the substrate concentration has can _____________.
•Uncompetitive inhibitors bind to the________________. Overall it _____ the Vmax and the Km of the enzyme.
•

Answer 14

•Competitive inhibitor competes with the substrate for the active site. Increasing the overall concentration of the substrate can overcome the inhibition.
•Non-competitive inhibitors do not compete for the substrate binding site. Increasing the substrate concentration has can not effect on the inhibition.
•Uncompetitive inhibitors bind to the enzyme-substrate complex. Overall it decreases the Vmax and the Km of the enzyme.
•

Question 15

Effects of Competitive and Non-competitive Inhibitors on the Km and Vmax

• Competitive inhibition – The Km____ in the presence of the inhibitor, while the Vmax _____.
• Non-competitive inhibition – The Km _____, while the Vmax _____ in the presence of the inhibitor.

Answer 15

• Competitive inhibition – The Km increases in the presence of the inhibitor, while the Vmax remains the same.
• Non-competitive inhibition – The Km remains the same, while the Vmax decreases in the presence of the inhibitor.

Question 16

Example of a Competitive inhibitor
_____ is a structural analog of _______, a ____ for the enzyme ____, which plays a role in the biosynthesis of ______
•Inhibits____ ____ ____.
•Used to ___ ___

Answer 16

• Methotrexate is a structural analog of tetrahydrofolate, a coenzyme for the enzyme dihydrofolate reductase, which plays a role in the biosynthesis of nitrogenous bases.
• Inhibits nucleotide base synthesis.
• Used to treat cancer.

Question 17

Example of a Non-competitive inhibitor

Metal ions ___ ____ ___, can act as non-competitive inhibitors by binding reversibly to the____groups of ___ that form disulfide bridges. This disrupts the 3-D structure of the enzyme. Enzyme____

Answer 17

Metal ions copper, mercury and silver, can act as non-competitive inhibitors by binding reversibly to the –SH groups of cysteine that form disulfide bridges. This disrupts the 3-D structure of the enzyme. Enzyme Ferrochelatase.

Question 18

Allosteric Enzymes

• Enzymes consisting __ or more subunits that exhibit _____
• Substrate binding to one subunit facilitates______________ (Several active sites)
• Does not obey _______
• Displays______ rather than hyperbola plots.
• Allosteric enzymes are key _____ of____ ____

Answer 18

• Enzymes consisting two or more subunits that exhibit co-operativity.
• Substrate binding to one subunit facilitates binding of substrates to other subunits. (Several active sites)
• Does not obey Michaelis-Menten kinetics.
• Displays sigmoid plots rather than hyperbola plots.
• Allosteric enzymes are key regulators of metabolic pathways

Question 19

Summary

Answer 19

•Enzymes are biological catalysts.
•An Enzymes does not change the DG , but lowers the activation energy of a reaction.
•The specificity of an enzyme is explained by the lock and key and induced fit models.
•Enzyme inhibition can either be reversible or irreversible.
•Reversible inhibition can be classified as competitive, uncompetitive or non-competitive inhibition.
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