结构生物化学Chapter8 Introduction to enzymology
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«What is a substrate? – A substrate is the compound that is converted into the product in an enzyme catalyzed reaction. – For the reaction catalyzed by aldolase, fructose 1,6phosphate is the substrate.
The active site of an enzyme is the place where all of the action occurs. It contains the functional groups (amino acid side chains) that bind the substrate(s) and catalyze it’s conversion to product(s).
« Enzyme catalyzed reactions are much faster than uncatalyzed reactions.
« Enzyme catalyzed reactions display saturation kinetics with respect to substrate concentration.
Active site of chymotrypsin
What does the active site do?
« The active site binds the substrates and positions them in the proper orientation for the reaction to occur.
What is Enzymes?
«Enzymes are biological catalysts that accelerate
the rates of chemical reactions.
Snail without enzyme catalyst
Snail withze both the reactions in both the forward and reverse direction
LDH LDH
3. Enzymes do not alter the equilibrium (or equilibrium constant) between substrates and products.
Chapter 8 Introduction to Enzymology
Outline
@What is enzyme? @Chemical nature of enzymes @Catalytic properties of enzymes
-Features shared by enzymes and non-enzymatic catalysts -Unique features of enzymes @Classification and nomenclature of enzymes
Transition State Theory
«Reactants are required to reach a high-energy (unstable) state referred to as the transition state. Residence may last only 10-13 to 10-14s
Enzyme catalyzed reaction
Reaction Rate =
D[Product] D(time)
[Product]
Uncatalyzed
reaction
0
0
Time
Enzyme catalyzed reactions display saturation kinetics with respect to reactant concentration
Important things to remember about enzymes just like other catalysts
1. Enzymes are not consumed or altered by the reaction they catalyze.
☺ Just as a construction worker can take a pile of lumber and build a home without being physically changed by the process, an enzyme takes substrates and converts them into products without being physically changed or consumed.
2. Enzymes catalyze both the forward and the reverse reaction.
☺ This is an important point. An enzyme does not determine which direction the reaction goes, it only increases the rate at which the reaction approaches equilibrium.
Chemical nature of enzymes
« All enzymes are proteins except some RNAs and not all proteins are enzymes
« It was assumed that all enzymes are proteins until 1982 when Thomas Cech and Sydney Altman discovered catalytic RNAs (Nobel prize in Chemistry, 1989)
What is the transition state of a reaction?
The transition state of a reaction is an unstable, high energy species that the substrates of the reaction must go through in order to form product. By lowering the energy of the transition state, enzymes increase the rate of the reaction.
General Properties of Enzymes
1. Higher reaction rates: The rates of enzyme-mediated
processes are accelerated by a factor of 108-1010. Example1: carbonic anhydrase catalyzes: CO2 +H2OH2CO3 nonenzymatic rate constant = 1.3 x 10-1 s-1 enzymatic rate constant = 1 x 106 s-1 (x 7.7 x 106) Example2: Staphylococcal nuclease catalyzes nucleic acid hydrolysis nonenzymatic rate constant = 1.7 x 10-13 s-1 enzymatic rate constant = 95 s-1 (x 5.6 x 1014)
What is the transition state?
The transition state is the most unstable species on the reaction coordinate (i.e. the species with the highest energy)
The transition state is not an intermediate species
« Enzyme catalyzed reactions are optimized for specific temperature and pH values.
Enzyme catalyzed reactions are much faster than uncatalyzed reactions
☺ At equilibrium, the ratio of substrates to products is the same regardless of whether an enzyme catalyst is present. Although the final equilibrium ratio of reactants to products is not altered by an enzyme, the rate at equilibrium is achieved is increased.
« Catalytic RNA, or ribozymes, satisfy several enzymatic criteria: substrate specificity, enhance reaction rate, and emerge from reaction unchanged
The difference between enzyme catalyzed & uncatalyzed chemical reactions
distinction!
Enzymes lower ∆G‡ but don’t affect ∆G for the reaction
General Properties of Enzymes
2. Enzymes bind substrates to their active site and stabilize the transition state of the reaction.
is related to the equilibrium ratio of [S] and [P] « The free energy of activation for a reaction (DG‡) is
related to the reaction rate « It is extremely important to appreciate this
Substrates, products, and enzymes
Enzymes catalyze the rate at which substrates are converted to product
Enzyme
Substrates
Product
Enzymes catalyze the conversion of substrates into products
«Only a limited number of molecules will possess sufficient energy to reach this transition state .
«Increasing temperature or other conditions may facilitate reactants reaching the transition state
The transition state cannot be trapped or isolated. Intermediates can be trapped or isolated.
The Transition State
« Understand the difference between DG and DG‡ « The overall free energy change for a reaction (∆G)
The active site of an enzyme is the place where all of the action occurs. It contains the functional groups (amino acid side chains) that bind the substrate(s) and catalyze it’s conversion to product(s).
« Enzyme catalyzed reactions are much faster than uncatalyzed reactions.
« Enzyme catalyzed reactions display saturation kinetics with respect to substrate concentration.
Active site of chymotrypsin
What does the active site do?
« The active site binds the substrates and positions them in the proper orientation for the reaction to occur.
What is Enzymes?
«Enzymes are biological catalysts that accelerate
the rates of chemical reactions.
Snail without enzyme catalyst
Snail withze both the reactions in both the forward and reverse direction
LDH LDH
3. Enzymes do not alter the equilibrium (or equilibrium constant) between substrates and products.
Chapter 8 Introduction to Enzymology
Outline
@What is enzyme? @Chemical nature of enzymes @Catalytic properties of enzymes
-Features shared by enzymes and non-enzymatic catalysts -Unique features of enzymes @Classification and nomenclature of enzymes
Transition State Theory
«Reactants are required to reach a high-energy (unstable) state referred to as the transition state. Residence may last only 10-13 to 10-14s
Enzyme catalyzed reaction
Reaction Rate =
D[Product] D(time)
[Product]
Uncatalyzed
reaction
0
0
Time
Enzyme catalyzed reactions display saturation kinetics with respect to reactant concentration
Important things to remember about enzymes just like other catalysts
1. Enzymes are not consumed or altered by the reaction they catalyze.
☺ Just as a construction worker can take a pile of lumber and build a home without being physically changed by the process, an enzyme takes substrates and converts them into products without being physically changed or consumed.
2. Enzymes catalyze both the forward and the reverse reaction.
☺ This is an important point. An enzyme does not determine which direction the reaction goes, it only increases the rate at which the reaction approaches equilibrium.
Chemical nature of enzymes
« All enzymes are proteins except some RNAs and not all proteins are enzymes
« It was assumed that all enzymes are proteins until 1982 when Thomas Cech and Sydney Altman discovered catalytic RNAs (Nobel prize in Chemistry, 1989)
What is the transition state of a reaction?
The transition state of a reaction is an unstable, high energy species that the substrates of the reaction must go through in order to form product. By lowering the energy of the transition state, enzymes increase the rate of the reaction.
General Properties of Enzymes
1. Higher reaction rates: The rates of enzyme-mediated
processes are accelerated by a factor of 108-1010. Example1: carbonic anhydrase catalyzes: CO2 +H2OH2CO3 nonenzymatic rate constant = 1.3 x 10-1 s-1 enzymatic rate constant = 1 x 106 s-1 (x 7.7 x 106) Example2: Staphylococcal nuclease catalyzes nucleic acid hydrolysis nonenzymatic rate constant = 1.7 x 10-13 s-1 enzymatic rate constant = 95 s-1 (x 5.6 x 1014)
What is the transition state?
The transition state is the most unstable species on the reaction coordinate (i.e. the species with the highest energy)
The transition state is not an intermediate species
« Enzyme catalyzed reactions are optimized for specific temperature and pH values.
Enzyme catalyzed reactions are much faster than uncatalyzed reactions
☺ At equilibrium, the ratio of substrates to products is the same regardless of whether an enzyme catalyst is present. Although the final equilibrium ratio of reactants to products is not altered by an enzyme, the rate at equilibrium is achieved is increased.
« Catalytic RNA, or ribozymes, satisfy several enzymatic criteria: substrate specificity, enhance reaction rate, and emerge from reaction unchanged
The difference between enzyme catalyzed & uncatalyzed chemical reactions
distinction!
Enzymes lower ∆G‡ but don’t affect ∆G for the reaction
General Properties of Enzymes
2. Enzymes bind substrates to their active site and stabilize the transition state of the reaction.
is related to the equilibrium ratio of [S] and [P] « The free energy of activation for a reaction (DG‡) is
related to the reaction rate « It is extremely important to appreciate this
Substrates, products, and enzymes
Enzymes catalyze the rate at which substrates are converted to product
Enzyme
Substrates
Product
Enzymes catalyze the conversion of substrates into products
«Only a limited number of molecules will possess sufficient energy to reach this transition state .
«Increasing temperature or other conditions may facilitate reactants reaching the transition state
The transition state cannot be trapped or isolated. Intermediates can be trapped or isolated.
The Transition State
« Understand the difference between DG and DG‡ « The overall free energy change for a reaction (∆G)