Biochemistry Lecture 3: Enzymes and Kinetics

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Questions and Answers

What effect does non-competitive inhibition have on Vmax?

Vmax is increased

Vmax is decreased (correct)

Vmax is doubled

Vmax remains unchanged

How does uncompetitive inhibition affect KM?

KM remains unchanged

KM is decreased (correct)

KM is increased

KM varies based on the concentration of the substrate

Which statement about non-competitive inhibition is true?

The substrate can bind to the enzyme-inhibitor complex, but it does not progress to product (correct)

The KM changes due to a higher substrate affinity

The inhibitor binds only to the free enzyme

The Vmax increases due to increased substrate concentration

What is the main characteristic of uncompetitive inhibition?

It binds only to the ES complex Signup and view all the answers

In competitive inhibition, how does the KM change?

KM remains unchanged Signup and view all the answers

What effect do competitive inhibitors have on the maximum velocity (Vmax) of an enzyme?

Vmax remains unchanged Signup and view all the answers

Which type of inhibitor would most likely require calculating the KI value?

Competitive inhibitor Signup and view all the answers

How do irreversible inhibitors primarily affect enzymes?

They covalently modify essential functional groups on the enzyme. Signup and view all the answers

Which of the following best describes mechanism-based irreversible inhibitors?

They hijack the normal enzyme reaction mechanism. Signup and view all the answers

Which of the following does NOT result from reversible enzyme inhibition?

Permanent loss of enzyme activity Signup and view all the answers

What is the characteristic change in Km for uncompetitive inhibitors?

Km decreases Signup and view all the answers

In enzyme regulation, what is the primary goal of modulating enzyme activity?

To facilitate metabolic pathway control Signup and view all the answers

Diisopropylfluorophosphate (DIFP) is known as what type of inhibitor?

Irreversible inhibitor Signup and view all the answers

How do enzymes respond when a product is available in excess?

They are regulated to divert resources elsewhere. Signup and view all the answers

What is the function of allosteric modulators in enzyme regulation?

They bind away from the active site causing shape changes. Signup and view all the answers

Which of the following describes reversible covalent modification?

It is a temporary change through covalent bonding. Signup and view all the answers

What is the role of proteolytic cleavage in enzyme regulation?

It converts active forms of the enzyme from pro-enzymes. Signup and view all the answers

Which statement best describes feedback regulation?

It decreases the rate of production by inhibiting upstream enzymes. Signup and view all the answers

What initiates the activation of a zymogen?

Proteolytic cleavage removing a polypeptide segment. Signup and view all the answers

What is an example of an enzyme regulated by proteolytic cleavage mentioned?

Kallikrein-related peptidases (KLKs) Signup and view all the answers

What distinguishes reversible enzyme inhibitors from irreversible ones?

Reversible inhibitors interact transiently while irreversible bind permanently. Signup and view all the answers

What does KM represent in Michaelis-Menten kinetics?

The substrate concentration at half maximal rate Signup and view all the answers

What does a Lineweaver-Burk plot visually represent?

1/V0 against 1/[S] Signup and view all the answers

How can Vmax be experimentally estimated?

Using the approach of saturation with substrate Signup and view all the answers

Which statement is true about enzyme inhibitors?

They can be both reversible and irreversible Signup and view all the answers

What happens to the initial reaction rate when [S] is much greater than KM?

The initial rate becomes independent of [S] Signup and view all the answers

Which of the following enzymes are considered irreversible inhibitors?

Enzymes that form covalent bonds with the active site Signup and view all the answers

What role do enzyme inhibitors play in medicine?

They are utilized to treat various diseases Signup and view all the answers

Which part of the Lineweaver-Burk plot corresponds to Vmax?

The y-intercept Signup and view all the answers

What characterizes competitive inhibition in enzyme activity?

The inhibitor prevents the substrate from binding to the active site. Signup and view all the answers

What is represented by the inhibitor constant KI?

The concentration of the inhibitor required to produce half maximum inhibition. Signup and view all the answers

What happens to the Michaelis-Menten constant (KM) in the presence of a competitive inhibitor?

KM increases. Signup and view all the answers

How does non-competitive inhibition differ from competitive inhibition?

It binds to a site other than the active site without affecting substrate binding. Signup and view all the answers

Which statement correctly describes the effect of a competitive inhibitor when substrate concentration is much higher than inhibitor concentration?

The reaction may still reach Vmax. Signup and view all the answers

What is the role of ethanol in the context of methanol poisoning?

It is a competitive inhibitor that prevents methanol from being metabolized. Signup and view all the answers

What is the consequence of non-competitive inhibition on the Vmax of an enzyme-catalyzed reaction?

Vmax decreases. Signup and view all the answers

Which of the following is true regarding the binding of an inhibitor in competitive inhibition?

The inhibitor binding can be overcome by increasing substrate concentration. Signup and view all the answers

Study Notes

Biochemistry Lecture Notes - Enzymes

Topic 3: Enzymes
- Lecture 7: Basic concepts – catalysis, transition states, and binding energy.
- Lecture 8: Enzyme classes and enzyme kinetics.
- Lecture 9: Enzyme inhibition and regulation.

Michaelis-Menten Kinetics

The Michaelis-Menten equation describes enzyme kinetics.
It relates reaction velocity (V₀) to substrate concentration ([S]).
V₀ = (V_max[S])/(K_M + [S])
V_max: maximum reaction velocity
K_M: Michaelis constant, substrate concentration at half-maximal velocity.
When [S] << K_M, the initial rate is proportional to [S].
When [S] >> K_M, the initial rate is independent of [S].
When [S] = K_M, the reaction rate is half the maximal rate.

Measuring K_M and V_max

V_max is an upper limit, never truly achieved.
Experimental determination:
- Measuring the initial rate of catalysis (V₀) at varying substrate concentrations ([S]).
- Using curve-fitting programs to determine K_M and V_max.
Lineweaver-Burk plot:
- A plot of 1/V₀ vs 1/[S].
- Yields a straight line with a y-intercept of 1/V_max and a slope of K_M/V_max.
- The x-intercept is -1/K_M.

Enzyme Inhibition

Molecules that interfere with enzyme catalysis, slowing or halting enzymatic transformations.
Important for pharmaceuticals.
Specific
- Reversible
  - Competitive
  - Non-competitive.
  - Uncompetitive.
Non-Specific
- Irreversible
Inhibitors vary in how they affect enzyme activity.

Competitive Inhibition

Inhibitor competes with the substrate for the enzyme's active site.
Inhibitor constant (K_I) indicates inhibitor potency.
K_M increases (apparent K_M) in the presence of an inhibitor.
Vmax remains unchanged.

Non-competitive Inhibition

The inhibitor binds to a site distinct from the active site, on either the free enzyme or the enzyme-substrate complex.
Binding of the inhibitor does not inactivate the enzyme.
KM remains unchanged.
Vmax decreases.

Uncompetitive Inhibition

Inhibitor binds to the enzyme-substrate complex but not to the free enzyme, at a site distinct from the substrate site.
Binding of the inhibitor does not inactivate the enzyme.
Both K_M and V_max decrease.

Irreversible Inhibition

Inhibitors covalently modify an essential functional group on the enzyme.
Enzyme activity is permanently lost.
Example: Diisopropylfluorophosphate (DIFP), a highly toxic sarin gas analogue that inhibits serine proteases.

Enzyme Regulation

Mechanisms cells use to control enzyme activity include allosteric regulation, reversible covalent modification (e.g., phosphorylation), proteolytic cleavage, and feedback regulation.
Allosteric Regulation*—Regulatory molecules alter enzyme activity by binding to sites other than the active site, changing enzyme shape.
Activation – active site becomes available.
Deactivation – active site becomes unavailable.
Reversible Covalent Modification: Enzyme activity is modulated by covalent modification of amino acid residues.
Proteolytic Cleavage: Enzymes are produced in an inactive form (zymogen/proenzyme), activated after cleavage.
Feedback Regulation: The end product of a metabolic pathway inhibits an upstream enzyme, limiting further product production.

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Description

Explore the fascinating world of enzymes in this biochemistry quiz. Covering key concepts such as catalysis, enzyme classes, and the Michaelis-Menten kinetics, this quiz will test your understanding of enzyme functions and regulations. Prepare to delve into enzyme inhibition and learn how to measure important parameters like Vmax and Km.