Biochemistry Lecture 9: Enzymes

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Questions and Answers

What is the primary role of enzymes in biological reactions?

To catalyze biological reactions. (correct)

To act as substrates in the reaction.

To decompose substrates into simpler compounds.

To provide energy for the reactions.

Which statement correctly describes the turnover number, Kcat?

It represents the number of substrate molecules converted to product per enzyme molecule per second. (correct)

It measures the efficiency of substrates in a reaction.

It is the number of enzyme molecules in a reaction.

It indicates the time taken for the reaction to occur.

Which class of enzymes is responsible for catalyzing oxidation-reduction reactions?

Oxidoreductases (correct)

Ligases

Hydrolases

Transferases

What is the function of coenzymes in enzyme reactions?

They assist in the transfer of chemical groups between molecules. Signup and view all the answers

What occurs during the formation of the enzyme-substrate complex?

The enzyme and substrate bind temporarily to form a complex. Signup and view all the answers

Which of the following correctly describes the role of cofactors in enzyme function?

They may be metal ions or organic molecules that assist in catalysis. Signup and view all the answers

What indicates that an enzyme has reached its maximal velocity (Vmax)?

All binding sites on the enzyme molecules are saturated with substrate. Signup and view all the answers

Which of the following statements about the active site of an enzyme is true?

It contains amino acid side chains that are complementary to the substrate. Signup and view all the answers

Which theory proposes that the enzyme's active site is specifically shaped to fit the substrate?

Lock-and-key theory Signup and view all the answers

What does the Michaelis-Menten equation describe in enzyme kinetics?

The rate of enzyme activity relative to substrate concentration. Signup and view all the answers

How do substrates bind to an enzyme's active site?

Through a combination of bonds including hydrogen and van der Waals forces Signup and view all the answers

What effect does temperature have on enzyme reactions?

Increase in temperature usually increases the reaction velocity. Signup and view all the answers

What characterizes the transition state of a chemical reaction?

Bonds are forming and the molecular geometry is changing. Signup and view all the answers

What is the primary role of the active site in an enzyme?

To modify the reaction mechanism and activation energy Signup and view all the answers

Which factor does NOT affect the velocity of an enzyme-catalyzed reaction?

Availability of products Signup and view all the answers

What typically happens to the enzyme after the product is released?

It returns to its initial unbound state. Signup and view all the answers

What is the effect of further increasing temperature beyond the optimal range for mammalian enzymes?

It denatures the enzyme. Signup and view all the answers

What does the Michaelis constant (Km) indicate about the interaction between an enzyme and its substrate?

It reflects the affinity of the enzyme for the substrate. Signup and view all the answers

How does enzyme concentration affect the rate of reaction?

Rate of reaction is proportional to enzyme concentration. Signup and view all the answers

What is the relationship between turnover number (Kcat) and the efficiency of an enzyme?

Higher Kcat values indicate more substrate converted per unit time. Signup and view all the answers

Which of the following conditions must remain constant to model an enzymatic reaction accurately?

Temperature, pH, and ionic strength. Signup and view all the answers

What can be inferred about an enzyme with a Km of $10^{-5}M$ compared to one with a Km of $10^{-7}M$?

The second enzyme requires a higher substrate concentration to reach half-maximal velocity. Signup and view all the answers

Which statement is true regarding steady-state assumption in enzymatic reactions?

The rate of formation of enzyme-substrate complex is equal to its breakdown. Signup and view all the answers

What is the role of enzyme inhibitors in enzymatic reactions?

They diminish the velocity of an enzyme-catalyzed reaction. Signup and view all the answers

Study Notes

Lecture 9: Enzymes

Enzymes are specialized proteins that catalyze biological reactions.
The active site of an enzyme contains amino acid side chains that create a three-dimensional surface complementary to the substrate.
Enzyme + substrate → enzyme-substrate complex (ES)
enzyme-product complex (EP) → enzyme + product
Enzyme-catalyzed reactions are highly efficient, transforming 100-1000 substrate molecules into product each second.
Turnover number (Kcat) is the number of substrate molecules converted to product per enzyme molecule per second.

Lecture Outline

Properties of enzymes
Classification of enzymes
Cofactors, co-substrates, coenzymes
Enzyme mechanism
Active site of an enzyme
Factors affecting reaction velocity
Michaelis-Menten equation & Lineweaver-Burk plot
Enzyme inhibitors

Enzyme Classification

International Union of Biochemistry and Molecular Biology (IUBMB) classifies enzymes into 6 classes based on the chemical reaction they catalyze.
1. Oxidoreductases: Catalyze oxidation-reduction reactions (e.g., lactate dehydrogenase).
1. Transferases: Transfer a chemical group (containing C-, N-, or P-) from one molecule to another (e.g., kinases).
1. Hydrolases: Catalyze hydrolysis reactions by adding water to cleave chemical bonds (e.g., urease).
1. Lyases: Catalyze the cleavage of chemical bonds (C-C, C-N, C-O, C-S, P-O) to form new double bonds.
1. Isomerases: Rearrange atoms within a molecule to create isomers (e.g., isomerases).
1. Ligases: Catalyze the joining of two large molecules by forming a new chemical bond (e.g., ligases).

Coenzymes

Coenzymes are organic, non-protein molecules that participate in enzymatic reactions but are not part of the enzyme itself.
They can be tightly bound or covalently attached to the enzyme.
Some coenzymes are modified during a reaction; others must participate in another reaction to return to their original state.
Examples include NAD, NADP, FAD, coenzyme A, biotin.

Enzyme Mechanism: Binding of Substrate

Two theories for substrate binding:
- Lock-and-key theory: The active site of the enzyme is a rigid structure that fits the substrate like a key fits a lock.
- Induced fit theory: The active site of the enzyme changes shape slightly upon substrate binding, to form a stable complex, allowing for a tighter and more efficient fit

Active Site of Enzymes

Amino acid residues in the active site recognize and bind substrates.
Interactions between enzyme and substrate include hydrogen bonds, hydrophobic interactions, temporary covalent interactions (van der Waals).
Active site residues facilitate reaction by acting as donors or acceptors of protons or other groups on the substrate.

Transition State

Enzymes lower the activation energy of a reaction by stabilizing the transition state.
The transition state is the highest energy point in a reaction, where bonds are breaking and forming.
Enzymes help stabilize the transition state, making it easier for reactants to achieve this point and proceed to products.

Factor Affecting Reaction Velocity

Substrate concentration
Temperature
pH

Michaelis-Menten Equation

Enzyme reversibly combines with its substrate to form an ES complex.
Mathematical model describing reaction velocity (rate of enzymatic reaction) as a function of substrate concentration
Vmax: maximum velocity at saturation.
Km: Michaelis constant; substrate concentration at which velocity is half Vmax.

Lineweaver-Burk Plot

Double reciprocal plot of the Michaelis-Menten equation.
Useful for determining Km and Vmax
Important for visualizing how enzyme inhibitors affect the reaction velocity.

Enzyme Inhibitors

Inhibitors are molecules that reduce or block enzyme activity.
- Irreversible inhibitors: Covalently attach to the enzyme, permanently altering its structure (e.g. nerve gas and pesticides, penicillin)
- Reversible inhibitors: Bind non-covalently, allowing the enzyme and inhibitor to dissociate (e.g. competitive, noncompetitive, uncompetitive inhibitors, statins, methotrexate)

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Description

Explore the fascinating world of enzymes in this comprehensive quiz. From the properties and classification of enzymes to the intricacies of enzyme mechanisms, this quiz covers everything you need to know about how enzymes catalyze biological reactions. Test your knowledge on enzyme kinetics, inhibitors, and the critical role of active sites.