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Questions and Answers
What happens to the reaction rate when substrate concentration increases beyond Vmax?
What happens to the reaction rate when substrate concentration increases beyond Vmax?
What is the significance of the Michaelis constant (Km)?
What is the significance of the Michaelis constant (Km)?
How does temperature affect enzyme activity?
How does temperature affect enzyme activity?
How does a change in pH affect enzyme activity?
How does a change in pH affect enzyme activity?
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What effect does increasing coenzyme concentration have on reaction rates?
What effect does increasing coenzyme concentration have on reaction rates?
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What is the role of ion activators in enzymatic reactions?
What is the role of ion activators in enzymatic reactions?
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What typically occurs to the rate of an enzymatic reaction over time?
What typically occurs to the rate of an enzymatic reaction over time?
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Which of the following describes enzymes with low Km values?
Which of the following describes enzymes with low Km values?
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What is formed when a substrate binds to an enzyme at its active site?
What is formed when a substrate binds to an enzyme at its active site?
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How do enzymes affect the activation energy of a chemical reaction?
How do enzymes affect the activation energy of a chemical reaction?
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What role do coenzymes play in enzyme function?
What role do coenzymes play in enzyme function?
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Which factor is NOT considered to affect the rate of enzyme action?
Which factor is NOT considered to affect the rate of enzyme action?
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What is the term for the enzyme and non-protein part combined?
What is the term for the enzyme and non-protein part combined?
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How does temperature typically affect enzyme activity?
How does temperature typically affect enzyme activity?
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Which of the following statements about the enzyme-substrate complex is correct?
Which of the following statements about the enzyme-substrate complex is correct?
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Which condition can inhibit enzyme activity?
Which condition can inhibit enzyme activity?
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Study Notes
Enzymes
- Enzymes are biological catalysts.
- They are organic, thermo-labile catalysts.
- They speed up chemical reactions without being used up in the reaction.
Chemical Nature of Enzymes
- Most enzymes are proteins, except ribozymes which are RNA.
- Protein enzymes are classified into two types:
- Simple protein enzymes: Composed only of protein.
- Complex (conjugated) protein enzymes: Composed of a protein part (apoenzyme) and a non-protein part (cofactor).
Cofactors
- Cofactors are non-protein molecules that help enzymes function.
- Cofactors can be either coenzymes or prosthetic groups.
- Coenzymes: Organic, thermo-labile, loosely attached to the enzyme.
- Often derived from vitamins B.
- Examples include FAD and NAD.
- Prosthetic groups: Inorganic, thermo-stable, firmly attached to the enzyme.
- Often metal ions.
- Examples include Ca and Zn.
- The whole enzyme with its cofactor is called a holoenzyme.
- Coenzymes: Organic, thermo-labile, loosely attached to the enzyme.
Enzyme Vocabulary
- Substrate: Reactant that binds to the enzyme.
- Enzyme-substrate complex: Temporary association between enzyme and substrate.
- Product: End result of the reaction.
- Active site: Enzyme's catalytic site, where substrate binds.
Active Site
- A specific region of the enzyme.
- It binds to the substrate.
- Formed by amino acid sequences in a polypeptide chain.
Mechanism of Enzyme Action
- Substrate (S) binds to the enzyme's active site.
- This forms an enzyme-substrate complex (ES).
- The complex is stabilized and broken down, releasing products (P) and the unchanged enzyme (E).
Enzyme Action
- Enzymes speed up reactions by lowering the activation energy.
- Activation energy is the energy needed to start a reaction.
- Enzymes decrease the energy barrier between reactants and products.
Factors Affecting Enzyme Rate
- Enzyme concentration: Reaction rate increases with higher enzyme concentration, up to a limit.
- Substrate concentration: Reaction rates increase with rising substrate concentration, up to a maximal rate.
- Temperature: Rate increases with rising temp. until optimal temperature, after which rate decreases due to denaturation of the enzyme
- Optimal temperature 37-40°C for many enzymes
- pH: Each enzyme has an optimal pH at which its activity is maximum.
- Concentration of coenzymes: Increased coenzyme concentration increases reaction rate in conjugated enzymes requiring coenzymes.
- Concentration of ion activators: metal ion activators increase the reaction rate
- Time: Rate decreases with time due to substrate depletion and product build-up and pH change.
- Enzyme inhibitors: presence decreases or stops enzyme activity.
Michaelis Constant (Km)
- The substrate concentration that yields half the maximum velocity (Vmax) of an enzyme-catalyzed reaction.
Enzyme Affinity
- Enzymes with low Km have high affinity for the substrate; they reach maximum velocity at low substrate concentrations.
- High Km enzymes have low affinity, reacting at maximum velocity only at high substrate concentration.
Effects of Temperature
- Reaction rate increases with temperature increase to an optimum, then decreases due to enzyme denaturation.
Effects of pH
- Each enzyme has an optimum pH where its activity is highest.
- Deviation from the optimum pH results in decreased activity and the enzyme may denature.
Concentration of Coenzymes
- In conjugated enzymes, increasing the coenzyme concentration increases the reaction rate.
Concentration of Ion Activators
- Metal ions activate some enzymes, increasing the reaction rate.
Effect of Time
- Reaction rate decreases over time, due to substrate depletion, product buildup and pH change.
Enzyme Inhibitors
- Inhibitors decrease or block enzyme activity.
- Competitive inhibitors bind to the active site, competing with the substrate.
- Non-competitive inhibitors bind to an allosteric site, changing the enzyme's shape and thus preventing the substrate from binding.
Biomedical Importance of Enzymes
- Enzymes play crucial roles in countless biological processes.
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