Enzyme Kinetics and Reaction Mechanisms

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Questions and Answers

What characterizes first-order reactions in terms of substrate concentration?

Reaction rate is independent of substrate concentration.

A threshold substrate concentration must be reached for the reaction to occur.

Reaction rate is directly proportional to substrate concentration. (correct)

Reaction rate decreases as substrate concentration increases.

What happens to the reaction rate when the substrate concentration reaches saturation in enzyme-catalyzed reactions?

The reaction rate drops significantly due to enzyme inhibition.

The reaction becomes a zero-order reaction with constant substrate dependency.

The reaction rate continues to increase with substrate concentration.

The reaction rate becomes independent of further increases in substrate concentration. (correct)

How does temperature affect the rate of an enzyme-catalyzed reaction?

Increases indefinitely as temperature rises.

Increases up to a peak, then decreases due to denaturation. (correct)

Remains constant regardless of temperature fluctuations.

Decreases consistently with rising temperature.

What is the effect of a high reaction rate constant (k) on the reaction?

It signifies a faster reaction rate. (A) Signup and view all the answers

Which of the following describes the behavior of allosteric enzymes?

They demonstrate a sigmoidal response to substrate concentration. (B) Signup and view all the answers

In which scenario will zero-order kinetics be observed?

When all enzyme active sites are saturated. (A) Signup and view all the answers

What is the typical temperature range for optimal enzyme activity?

30 to 45°C (D) Signup and view all the answers

What can result from increasing temperature beyond the optimum range for enzymes?

Denaturation and loss of activity. (B) Signup and view all the answers

What effect does a pH level below 4.0 or above 10.0 have on enzymes?

It causes denaturation of the enzyme. (A) Signup and view all the answers

How does temperature influence enzyme activity at low temperatures near 0°C?

It decreases the enzyme's catalytic power reversibly. (D) Signup and view all the answers

What is the significance of the Michaelis constant (Km) in enzyme catalysis?

It reflects the enzyme's affinity for the substrate. (C) Signup and view all the answers

Which statement is true about the relationship between enzyme concentration [E] and reaction velocity?

There is a linear relationship between [E] and reaction velocity. (B) Signup and view all the answers

Which of the following changes in pH may have a significant effect on enzyme activity?

A change from 5 to 9. (B) Signup and view all the answers

What does a decrease in Km indicate about the enzyme's binding to the substrate?

Strong binding and high affinity. (D) Signup and view all the answers

How do small changes in pH affect enzyme ionization and catalytic activity?

They can affect active site groups, altering enzyme activity. (C) Signup and view all the answers

Which statement correctly describes the Michaelis-Menten equation?

It describes how velocity (v) varies with substrate concentration ([S]). (D) Signup and view all the answers

What is the primary effect of irreversible inhibitors on enzyme activity?

They permanently inactivate the enzyme. (C) Signup and view all the answers

Which of the following best describes the action of diisopropylfluorophosphate (DIFP)?

It binds covalently to a serine residue in digestive enzymes. (A) Signup and view all the answers

Which enzyme is irreversibly inhibited by disulfiram?

Aldehyde dehydrogenase (D) Signup and view all the answers

What type of inhibition does allopurinol exhibit on xanthine oxidase?

Suicide inhibition (B) Signup and view all the answers

How does fluoride inhibit enzyme activity?

By removing manganese from the enzyme. (C) Signup and view all the answers

Which of the following statements about aspirin is accurate?

Aspirin inhibits prostaglandin synthesis by acetylating serine in COX. (C) Signup and view all the answers

What is the mechanism of action of statins like lovastatin?

They competitively inhibit HMG CoA reductase to lower cholesterol levels. (A) Signup and view all the answers

Which of the following components is a consequence of the irreversible inhibition of acetylcholinesterase by nerve gases?

Paralysis of vital body functions. (A) Signup and view all the answers

What type of inhibition does Omeprazole exhibit in treating gastroesophageal reflux disease?

Irreversible inhibition (C) Signup and view all the answers

Which statement accurately describes competitive inhibition?

Inhibitor resembles the substrate and competes for the active site. (C) Signup and view all the answers

How does competitive inhibition affect Km in enzyme kinetics?

It increases Km, requiring more substrate for ½ Vmax. (B) Signup and view all the answers

What is the primary mechanism by which Penicillin inhibits glycopeptides transpeptidase?

It covalently attaches to Ser in the active site. (B) Signup and view all the answers

What type of inhibition does Dicoumarol exhibit when used as an anticoagulant?

Competitive inhibition (C) Signup and view all the answers

What is observed about the Vmax when a competitive inhibitor is present?

Vmax remains unchanged. (C) Signup and view all the answers

Which of the following describes the characteristics of reversible inhibitors?

They bind through noncovalent interactions and can be displaced. (C) Signup and view all the answers

Which of the following drugs is primarily used to treat gout?

Allopurinol (C) Signup and view all the answers

In non-competitive inhibition, how does the inhibitor affect Vmax?

Vmax decreases (B) Signup and view all the answers

Which drug can be classified as a non-competitive inhibitor of angiotensin-converting enzyme (ACE)?

Captopril (C) Signup and view all the answers

How does an increase in inhibitor concentration affect the rate of an enzymatic reaction under competitive inhibition?

It decreases the rate by forming less ES complex. (D) Signup and view all the answers

Which enzyme is inhibited by 5-fluorouracil, a drug used in cancer treatment?

Thymidylate synthase (B) Signup and view all the answers

What type of inhibition is characterized by the inhibitor binding to a site other than the active site of the enzyme?

Non-competitive inhibition (B) Signup and view all the answers

Which of the following enzymes does Penicillin inhibit to exert its antibiotic effect?

Transpeptidase (A) Signup and view all the answers

What is the clinical use of Oseltamivir, also known as Tamiflu?

Influenza treatment (C) Signup and view all the answers

Which of the following correctly describes the effect of non-competitive inhibitors on Km?

Km remains unchanged (A) Signup and view all the answers

What type of effector is the substrate when it itself enhances enzyme activity?

Homotropic effector (C) Signup and view all the answers

Which enzyme is mentioned as being inhibited by a product in a feedback inhibition mechanism?

PFK-1 (A) Signup and view all the answers

Which process is generally used for the reversible regulation of many enzymes?

Covalent modification (D) Signup and view all the answers

What effect does a positive allosteric effector generally have on enzyme activity?

Increases enzyme activity (C) Signup and view all the answers

Which of the following enzymes is activated by phosphorylation?

HMG-CoA reductase kinase (B) Signup and view all the answers

Which of the following correctly defines heterotropic effectors?

Effectors that are different from the substrate (B) Signup and view all the answers

Which statement is true regarding the effects of the product on the enzyme hexokinase (HK)?

G6P inhibits HK through competitive binding (B) Signup and view all the answers

Which type of modification is NOT mentioned as a reversible covalent modification for enzyme regulation?

Methylation (D) Signup and view all the answers

Study Notes

Enzymes

Enzymes are biological catalysts, meaning they speed up chemical reactions without being consumed or changed by the reaction.
Almost all enzymes are proteins, but not all proteins are enzymes.
Some RNA molecules (ribozymes) can also act as enzymes.
Life as we know it would not exist without enzymes.
Enzymes consist of 100-300 amino acids linked into unbranched polypeptide chains.
Substrate + Enzyme → Product + Enzyme

Enzyme Nomenclature

Old System: Enzymes were initially named based on their function, often related to degradation reactions. The suffix "-ase" was added to the name of the substrate.
- Example: Arginase hydrolyzes arginine to urea and ornithine.
- Example: Urease hydrolyzes urea to ammonia and carbon dioxide.
New System (EC numbers): The International Union of Biochemistry and Molecular Biology (IUBMB) introduced a standardized system using EC numbers (Enzyme Commission numbers).
- These numbers are 4 digits long.
- The first digit indicates the class of the enzyme.
- The next two digits indicate the subclass.
- The fourth digit indicates the specific enzyme within the subclass.

Enzyme Classification

Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., lactate dehydrogenase).
Transferases: Catalyze the transfer of a functional group (e.g., methyl transferase).
Hydrolases: Catalyze the hydrolysis of a bond (e.g., urease).
Lyases: Catalyze the cleavage of a bond (e.g., decarboxylase).
Isomerases: Catalyze the isomerization of a molecule (e.g., methylmalonyl CoA mutase).
Ligases: Catalyze the joining of two molecules (e.g., pyruvate carboxylase).

Structure of Enzymes

Apoenzyme: The protein component of a holoenzyme.
Cofactors: Metal ions (e.g., Zn2+, Fe2+) that bind in a transient, dissociable manner.
Coenzymes: Organic molecules that bind to the apoenzyme and participate in catalysis. They often derive from vitamins.
- Cosubstrates are coenzymes that only associate temporarily with the enzyme.
- Prosthetic groups are coenzymes that associate with the enzyme permanently.
Holoenzyme: The complete, active enzyme consisting of the apoenzyme and cofactor/coenzyme (e.g., E + cofactor).
Active site: The specific region of the enzyme where the substrate binds. The active site has a 3-D structure resulting from specific amino acid interactions.
Isoenzymes: Different forms of an enzyme that have slightly different structures but similar functions. These variations may result from encoded by different genes and have different kinetic or regulatory properties (e.g., LDH, CK).
Zymogens (proenzymes): Inactive precursor forms converted to active enzymes through proteolytic activation, which is a mechanism for activating proteins outside cells.
- Example: Trypsinogen → Trypsin
- Example: Pepsinogen → Pepsin

Enzyme Kinetics

Transition state: A high-energy intermediate formed during the conversion of reactants to products. Enzymes lower the activation energy to speed up reactions.
Rate of reaction: The higher the enzyme concentration, the greater the reaction velocity.

Michaelis-Menten Kinetics

Michaelis-Menten equation: Describes how the initial reaction velocity (v°) varies with substrate concentration ([S]).
- v° = (Vmax[S]) / (Km + [S])
- Vmax is the maximal velocity.
- Km is the Michaelis constant.
Order of reaction: When [S] is much less than Km, reaction is first order; When [S] is much greater than Km, reaction is zero order

Lineweaver-Burk Plot

This linear plot of 1/v0 vs. 1/[S] allows for easier determination of Vmax and Km values.

Enzyme Inhibition

Irreversible inhibitors: Bind covalently to the enzyme, permanently inactivating it (e.g., DIFP, nerve gases).
Reversible inhibitors: Bind noncovalently to the enzyme and can be removed, leading to different effects depending on the type of inhibitor.
- Competitive inhibitors: Structurally similar to the substrate; compete with the substrate for binding to the active site. Increased substrate concentration can overcome competitive inhibition.
- Noncompetitive inhibitors: Bind to a site other than the active site, altering the enzyme's conformation. These inhibitors cannot be overcome with increased substrate.
- Uncompetitive inhibitors: Bind only to the enzyme-substrate complex, at a location other than the active site.

Regulation of Enzyme Activity

Allosteric regulation: Involves allosteric effectors which bind non-covalently to allosteric sites, altering the enzyme's shape and activity. The binding of one substrate can affect the binding of another substrate.
Covalent modification (e.g., phosphorylation/dephosphorylation). Addition/removal of chemical groups.
Enzyme synthesis and breakdown: Increased or decreased synthesis, or degradation.

Clinical Applications

Clinical use: measuring enzyme activity in the blood to diagnose diseases or monitor therapy.
- Elevation of specific enzymes in the blood can indicate specific disorders. Enzyme activity (e.g., AST, ALT) can help in monitoring the efficacy of therapy for liver diseases or other metabolic disorders.

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Description

This quiz covers essential concepts of enzyme kinetics, including the characteristics of first-order and zero-order reactions, the impact of substrate concentration on reaction rates, and the effects of temperature and pH on enzyme activity. Test your understanding of the Michaelis constant and allosteric enzymes in catalysis.