Enzymes and Catalysis Quiz

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Questions and Answers

What defines the specificity of enzymes?

They are highly specific in the reaction catalyzed and choice of substrates. (correct)

They catalyze multiple reactions simultaneously.

They produce by-products in significant amounts.

They can work with any type of substrate equally.

Which enzyme is known for cutting the peptide bond on the carboxyl side of basic amino acids?

Trypsin (correct)

Elastase

Aminopeptidase

Chymotrypsin

What are isoenzymes?

Inactive precursors of enzymes.

Multiple forms of an enzyme catalyzing the same reaction. (correct)

Multiple forms of an enzyme that catalyze different reactions.

Active forms of enzymes only.

What role do enzymes play in the transition state of a reaction?

They actively stabilize the transition state. Signup and view all the answers

What describes proenzymes?

Inactive precursors that require cleavage of a peptide bond to become active. Signup and view all the answers

Which of the following statements is true concerning enzymes and free energy?

Enzymes lower the activation energy of reactions. Signup and view all the answers

In which model is the enzyme's active site perfectly shaped to fit the substrate?

Lock and Key Model Signup and view all the answers

Which type of enzyme often functions as a multi-subunit structure?

Allosteric enzyme Signup and view all the answers

Which of the following is NOT a factor necessary for enzyme catalysis?

High Temperature Signup and view all the answers

What is the difference in free energy known as when referring to the transition state?

Free Energy of Activation Signup and view all the answers

What is the effect of pH on enzyme activity?

Enzymes have the highest activity at a pH of 7.2. Signup and view all the answers

At what temperature do human enzymes peak in activity?

98.6 degrees Fahrenheit Signup and view all the answers

What happens to enzymes if the pH is significantly deviated from their optimal level?

They can denature, leading to decreased activity. Signup and view all the answers

How does temperature influence enzyme activity?

Increased temperature enhances collisions, increasing activity up to a peak. Signup and view all the answers

What would likely happen to enzyme activity if body temperature rises significantly above 98.6 degrees Fahrenheit?

Enzyme activity could begin to decrease due to denaturation. Signup and view all the answers

What is a characteristic feature of allosteric enzymes?

They show cooperative binding. Signup and view all the answers

How do activators affect allosteric enzymes?

They bind more tightly to the R state. Signup and view all the answers

Which statement about feedback inhibition is true?

It is a reversible process that does not affect the enzyme permanently. Signup and view all the answers

What shape does the velocity vs. substrate concentration curve for allosteric enzymes typically exhibit?

Sigmoidal Signup and view all the answers

What is an effect of covalent modification on enzymes?

It can lead to instantaneous changes in enzyme activity. Signup and view all the answers

How does physostigmine alleviate the symptoms related to neurotransmitter signaling defects?

Increases the acetylcholine concentration in the neuromuscular junctions Signup and view all the answers

What best describes the effect of aspirin on COX enzymes?

Irreversible inhibitor Signup and view all the answers

What is the mechanism of action of methotrexate on dihydrofolate reductase (DHFR)?

The maximum reaction rate for TH4 formation is decreased by Methotrexate Signup and view all the answers

What mechanism do virtually all enzymes use to facilitate reactions?

Stable binding to the transition state Signup and view all the answers

What is the expected effect of methotrexate on the synthesis of tetrahydrofolate?

Decreases the synthesis rate of tetrahydrofolate Signup and view all the answers

What happens to enzyme activity with a further increase in temperature after an optimal point?

It decreases due to denaturation. Signup and view all the answers

How does the rate of an enzymatic reaction change with a 10-degree increase in temperature?

It doubles. Signup and view all the answers

Which factors influence the rate of an enzymatic reaction?

pH, temperature, substrate concentration, and enzyme concentration. Signup and view all the answers

What effect does increasing substrate concentration have on the initial rate of an enzymatic reaction?

It increases the rate. Signup and view all the answers

What is enzyme kinetics primarily concerned with?

How fast the reaction takes place. Signup and view all the answers

If the temperature is kept constant, what remains the most influential aspect on reaction rate?

Enzyme concentration. Signup and view all the answers

What is the consequence of enzyme denaturation?

Decreased enzyme activity. Signup and view all the answers

What does a plot of substrate concentration versus enzyme activity typically show?

Linear correlation until saturation. Signup and view all the answers

Study Notes

Introduction to Enzymes

Enzymes are protein catalysts that increase the rate of reactions without being altered in the process.
All enzymes are proteins, with a notable exception being ribozymes, which are catalytic RNA molecules involved in RNA processing.

Learning Objectives

Explain how substrates are enzymatically transformed into products, including transition states and activation energy. Use energy diagrams to illustrate the process.
Compare and contrast the lock-and-key and induced-fit models of enzyme-substrate interactions.
Differentiate between cofactors, coenzymes, and isozymes.
Describe how functional groups, amino acid side chains, coenzymes, and metal ions contribute to enzymatic reactions.
Describe how factors like pH, temperature, and enzyme/substrate concentrations affect reaction rates.
Compare and contrast Michaelis-Menten and Lineweaver-Burk plots, including the roles of Vmax and Km in enzyme inhibition.
Explain how different factors impact enzyme activity. Explain KD vs. Km comparisons. Compare different methods of enzyme regulation
Explain the importance of regulatory enzymes, including various methods for regulation (feedback inhibition, allosteric regulation, covalent modification, and hormonal regulation), in metabolic pathways.
Compare the catalytic mechanisms of allosteric and Michaelis-Menten enzymes.

Enzyme Substrates and Transition States

The combination of an enzyme and its substrate forms an enzyme-substrate complex.
The rate of an enzyme-catalyzed reaction is directly proportional to the amount of enzyme-substrate complex.
The transition state is a highly unstable intermediate between substrate and product.
Enzymes stabilize transition states, lowering activation energy, and thus increasing the reaction rate.

Catalytic Efficiency

Uncatalyzed reactions are slow, requiring high activation energy.
Enzymes lower the free energy of activation, increasing the reaction rate.
Enzymes do not alter the free energy of reactants/products.
Enzymes are not altered during the reaction.

Free Energy of Activation

The free energy of activation is the difference in free energy between the transition state and the reactants.
Enzymes increase the rate of exergonic reactions by lowering the activation energy.

Types of Enzyme-Substrate Interactions

"Lock and Key" model: substrate fits precisely into the active site.
"Induced Fit" model: enzyme active site changes shape to fit the substrate.

Factors for Enzyme Catalysis

Active sites: pockets on the enzyme with specific shapes and chemical properties.
Cofactors: non-protein molecules. Can be metal ions or coenzymes.
Coenzymes: organic molecules derived from vitamins.
Prosthetic groups: tightly bound coenzymes.
Compartmentalization: enzymes are localized in specific organelles, isolating reaction substrates.
Specificity: enzymes are highly specific for their substrates and reactions.

Reaction in the Active Site

Enzymes contain an active site where substrates bind and reactions occur.
The active site is specific for the substrate, creating a three-dimensional structure matching the substrate.
Amino acids in the active site form weak bonds with the substrate, which helps catalyze the reaction.
A transition complex (intermediate between substrate and product) is created.
Products are released, and the enzyme returns to its original state.

Cofactors

Inorganic cofactors: include metal ions (e.g., Mg2+, Fe2+, Zn2+).
Organic cofactors (coenzymes): derived from vitamins (e.g., NAD+, FAD, Coenzyme A).
Prosthetic groups: tightly bound to the enzyme.
Co-substrates: temporarily bind to the enzyme.

Enzyme Kinetics

Kinetics measures the speed of a reaction.
The rate of an enzyme-catalyzed reaction is affected by pH, temperature, enzyme concentration, and substrate concentration.
Michaelis-Menten equation describes the relationship between reaction rates and substrate concentration. Vmax and Km are important constants.

KM and KD

KM: Michaelis-Menten constant; numerically equivalent to substrate concentration when reaction velocity equals half Vmax; measures the impact of substrate concentration on reaction speeds, and can be used as an indirect measure of affinity
KD: Dissociation constant: thermodynamic constant, measures the affinity of ligand to binding site; concentration at which 50% ligand dissociates from the enzyme; doesn't reflect reaction speed.

Important Information from Michaelis-Menten Kinetics

Comparison of Isoenzymes (e.g., hexokinase and glucokinase)
The role of the enzymes in transferring a phosphate from ATP to glucose

Hexokinase

Found in all cells, but little in the liver
Has broad specificity (glucose or fructose)
High affinity, so low substrate concentration (e.g., low blood glucose) is enough to get a reaction.

Glucokinase

Found primarily in liver and pancreatic cells.
Only acts on glucose
Low affinity, requiring higher substrate concentrations to activate.
Essential to remove excess glucose from the bloodstream after eating (high blood glucose).

Enzyme Regulation

Regulation by other molecules that either promote or inhibit activity.
Activators, inhibitors. Diverse pathways and molecules that block, promote, or otherwise affect enzyme function.
Examples:
- Substrate availability: The velocity of reaction changes with substrate availability.
- Product inhibition: Product inhibition can impact Vmax and/or Km
- Allosteric control: allosteric effectors (or activators/inhibitors) binds at a site other than the active site; these effectors can change the enzyme's conformation
- Covalent modification: another enzyme adds a phosphate group (or otherwise covalently alters the enzyme); e.g., phosphorylating/dephosphorylating a protein
- Induction/repression: alters the amount of an enzyme, typically via changes to gene expression to produce an enzyme

Regulatory Enzymes

Catalyze irreversible reactions and are at committed steps in metabolic pathways
Important for precisely controlling metabolic flux.
Examples: Phosphofructokinase (a key enzyme in glycolysis)

Enzyme Inhibition

Types of inhibition include:
- Competitive: inhibitor resembles substrate, binding reversibly at the active site.
- Noncompetitive: inhibitor binds to an enzyme site different from the active site (changing the conformation), altering Vmax, but not Km.
- Uncompetitive: inhibitor binds only to the enzyme-substrate complex, altering both Km and Vmax.
- Irreversible: inhibitor binds covalently to the enzyme (often at the active site). The inhibitor permanently inactivates the enzyme

Irreversible (Suicide) Inhibitors

Bind covalently to the enzyme, permanently inactivating it
Useful in new drugs and therapies

Enzyme Examples

Use of inhibitors (for example lovastatin, aspirin, and penicillin)
Regulation of enzymes such as HMG-CoA reductase, cyclooxygenase(COX), dihydrofolate reductase (DHFR)

Nutrition and Enzymes

Endogenous enzymes (produced by the body) include those for digestion (proteases, lipases, amylases), and autolytic enzymes.
Exogenous enzymes (from other sources) are often inactivated by high temperatures (e.g., cooked foods vs. raw foods), and are found in raw food sources.

Study Questions

Clinical scenarios and examples of how drugs affect enzyme functioning

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Description

Test your knowledge on the specificity, function, and characteristics of enzymes through a series of challenging questions. This quiz covers topics such as enzyme activity, transition states, proenzymes, and the impact of pH and temperature on enzyme function.