Enzymes and Regulation Concepts

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Questions and Answers

What effect does a competitive inhibitor have on the Km value of an enzyme?

Km value remains unchanged

Km value increases (correct)

Km value is not relevant to competitive inhibition

Km value decreases

Which of the following statements is true regarding non-competitive inhibitors?

They can be outcompeted by increasing substrate concentration.

They have a structural resemblance to the substrate.

They bind to the active site of the enzyme.

They do not affect the Km value of the enzyme. (correct)

What happens to Vmax in the presence of a competitive inhibitor?

Vmax is not affected by substrate concentration

Vmax remains unchanged (correct)

Vmax increases

Vmax decreases

Which medication serves as an antidote for heavy metal poisoning through non-competitive inhibition?

BAL (Dimercaprol) Signup and view all the answers

What is a characteristic of competitive inhibitors?

Structural similarity to the substrate is required. Signup and view all the answers

What does a high Km value indicate about an enzyme's affinity for its substrate?

Low affinity for the substrate Signup and view all the answers

What is the purpose of a Lineweaver-Burk plot?

To convert hyperbolic data into linear form Signup and view all the answers

Which type of inhibitor competes with the substrate for the active site?

Competitive inhibitor Signup and view all the answers

What characteristic does a non-competitive inhibitor have?

Alters the shape of the active site Signup and view all the answers

Which of the following is true about irreversible inhibitors?

They bind to the active site permanently Signup and view all the answers

What is the role of enzymes in chemical reactions?

To accelerate reactions without being consumed Signup and view all the answers

Which factor does NOT affect enzyme activity?

Color of the substrate Signup and view all the answers

What is the meaning of 'Km' in the Michaelis-Menten Equation?

The substrate concentration at half Vmax Signup and view all the answers

How does allosteric regulation affect enzyme activity?

By modifying the enzyme's active site shape Signup and view all the answers

What happens to proenzymes to activate them?

A small part of their polypeptide chain is removed Signup and view all the answers

Which of the following best describes the enzyme-substrate complex (ES)?

A temporary association between an enzyme and its substrate Signup and view all the answers

What is a consequence of covalent modification of an enzyme?

Changes the enzyme's active site affecting its function Signup and view all the answers

What is the primary purpose of the Michaelis-Menten model?

To show how reaction velocity varies with substrate concentration Signup and view all the answers

What is the primary action of uncompetitive inhibitors?

They bind to the enzyme-substrate complex only. Signup and view all the answers

How do uncompetitive inhibitors affect Vmax and Km?

Decrease both Vmax and Km. Signup and view all the answers

What characterizes irreversible inhibitors?

They bind tightly, often covalently, to enzymes. Signup and view all the answers

What is the outcome of irreversible inhibition in terms of enzyme kinetics?

Vmax decreases, Km is unchanged. Signup and view all the answers

What role does acetylcholinesterase (AChE) play in the nervous system?

It hydrolyzes acetylcholine to terminate signals. Signup and view all the answers

Which of the following is NOT a type of cholinesterase mentioned?

Butyrylcholinesterase. Signup and view all the answers

Inhibition by organophosphates is classified as which type of inhibition?

Irreversible inhibition. Signup and view all the answers

What happens to acetylcholine after it is hydrolyzed by acetylcholinesterase?

It is converted back into choline and acetic acid. Signup and view all the answers

Study Notes

Enzymes

Enzymes are specific molecules that act on one or few types of molecules (substrates) to create a molecular product.
They are biological catalysts, capable of accelerating chemical reactions without being part of the final products or being consumed in the process.
The binding of a substrate forces the enzyme to change conformation (like oxygen binding to hemoglobin).
This change decreases the energy of activation needed to transition the substrate into an unstable state.
This allows the reaction to proceed and form a product, regenerating the enzyme.

Enzyme Regulation

Covalent modification: Enzymes are regulated by adding or removing chemical groups, often through phosphorylation.
Allosteric regulation/Feedback inhibition: The end product of a metabolic reaction can inhibit the first or regulatory enzyme in the sequence if it is in excess.
Proenzymes: These are inactive forms of enzymes that can be activated by removing a small part of their polypeptide chain.

Factors Affecting Enzyme Activity

pH
Temperature
Substrate concentration
Enzyme concentration
The presence of inhibitors or activators

Michaelis-Menten Hypothesis

This model describes an enzyme reversibly combining with its substrate to form an ES (enzyme-substrate) complex, which then forms a product and regenerates the free enzyme.
Variables:
- [E]: free enzyme molecules
- [S]: free substrate molecules
- [ES]: enzyme-substrate complexes
- [P]: free product molecules
Parameters:
- kf, kr, kcat: reaction rates

Michaelis-Menten Equation

Describes how the reaction velocity varies with [S].
Potency (Km): Reflects the enzyme's affinity to the substrate.
- High Km: low affinity
- Low Km: high affinity
Velocity of the reaction: Proportional to enzyme concentration when [S] is not limiting.

Lineweaver-Burk Plot

This plot is a double reciprocal plot that changes the hyperbolic curve of the Michaelis-Menten model into a straight line.
The intercept on the x-axis is -1/Km and the intercept on the y-axis is 1/Vmax.

Enzyme Inhibitors

Molecules or factors that interact with enzymes and prevent them from working.
Nonspecific Inhibitors: Affect all enzymes in the same way (e.g., acids, alcohol).
Specific Inhibitors: Exert their effects on a specific enzyme.
- Reversible: Inhibitor can dissociate from the enzyme and catalytic activity is regained.
  - Competitive: Resemble the enzyme's substrate and compete for the active site.
  - Non-competitive: Do not resemble the substrate, bind to a different site and cause conformational changes in the enzyme.
  - Uncompetitive: Bind only to the ES complex.
- Irreversible: Inhibitor combines with the functional groups of the amino acids in the active site irreversibly.

Competitive Inhibition

The inhibitor is structurally similar to the substrate.
The inhibitor binds to the active site, competing with the substrate.
Increased substrate concentration can overcome inhibition.
Km increases: Affinity of the enzyme to the substrate is decreased.
Vmax remains unchanged: Increasing [S] displaces the inhibitor and allows the reaction to reach Vmax.

Competitive Inhibition: Statins

Clinically useful competitive inhibitors like statins (drugs used to lower cholesterol) are examples of this type of inhibition.

Non-competitive Inhibition

Inhibitor binds to a site other than the active site.
Inhibitor has no structural resemblance to the substrate.
Increasing substrate concentration does not relieve inhibition.
Reaction is slowed down but not halted.
Km value is unchanged.
Vmax decreases.

Non-competitive Inhibition: Clinical Examples

BAL (British Anti-lewisite): Used as an antidote for heavy metal poisoning.
- Heavy metals act as enzyme poisons by reacting with SH groups.
- BAL has several SH groups that react with the heavy metal ions, reducing their poisonous effects.

Uncompetitive Inhibition

Inhibitor binds only to the ES complex, not the free enzyme.
The enzyme becomes catalytically inactive.
Inhibition cannot be reversed by increasing substrate concentration.
Decrease in both Vmax and Km.
Example: Placental alkaline phosphatase (ALP) inhibited by phenylalanine.

Irreversible Inhibition

Inhibitor binds very tightly to the enzyme, usually through covalent or non-covalent bonds, forming a very stable complex.
Inhibition is considered irreversible because the inhibitor dissociates very slowly from the enzyme complex.
Examples: Organophosphates (OP) poisoning and oxidizing agents.

Irreversible Inhibition: Kinetics

Similar to non-competitive inhibition in terms of kinetics:
- Vmax decreased
- Km unchanged

Cholinesterases

Enzymes involved in the proper functioning of the nervous system in humans, vertebrates, and insects.
- Function: Break down acetylcholine into choline and acetic acid.
- Role in nerve transmission: Signals carried by acetylcholine are terminated by acetylcholinesterase (AChE). This prevents the neuron from remaining in an activated state.

Types of Cholinesterases

Acetylcholinesterase (AChE): Most common type, also known as cholinesterase I or erythrocyte cholinesterase.
- Found in neural synapses, neuromuscular junctions, central nervous system, and red blood cell membranes.

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Description

Explore the fundamental concepts of enzymes and their regulation mechanisms in biochemical processes. This quiz covers enzyme specificity, catalytic functions, and various forms of regulation such as covalent modification and allosteric inhibition. Test your understanding of how enzymes facilitate and control metabolic reactions.