Biochemistry: Enzyme Inhibitors

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Questions and Answers

What effect do activators have on the S-curve of enzyme affinity?

Shift it to the left (correct)

Make it linear

Shift it to the right

Eliminate the curve

Which state do inhibitors tend to bind more tightly to?

Active state

Inactive state

R state

T state (correct)

What is a characteristic result of allosteric control?

Increase in substrate availability

Decrease in product formation

Immediate enzyme activation

Change in Vm and/or Km (correct)

What is the typical timeframe for changes caused by covalent modification of enzymes?

Seconds to minutes Signup and view all the answers

Which of the following is NOT considered a typical regulator event in enzymatic regulation?

Genetic mutation Signup and view all the answers

What type of modification can hormones cause in enzyme regulation?

Induction and repression Signup and view all the answers

What is the typical effect of product inhibition on enzyme activity?

Change in Vm and/or Km Signup and view all the answers

What is one way that enzyme regulation can occur through hormonal control?

Changing the amount of enzyme available Signup and view all the answers

What characterizes a competitive inhibitor in enzyme kinetics?

It binds to the substrate-binding site. Signup and view all the answers

Which of the following statements is true regarding noncompetitive inhibition?

It affects Vmax without changing Km. Signup and view all the answers

What is an effect of uncompetitive inhibition in multi-substrate reactions?

It reduces the maximum velocity (Vmax). Signup and view all the answers

How does the inhibition from allopurinol occur?

By mimicking the transition state. Signup and view all the answers

In a Lineweaver-Burk plot, what does a hyperbolic curve represent?

A direct relationship between velocity and substrate concentration. Signup and view all the answers

How is Km affected by competitive inhibitors?

Km increases, indicating decreased affinity. Signup and view all the answers

Which of the following statements about Vmax in competitive inhibition is correct?

Vmax is unchanged with high substrate concentration. Signup and view all the answers

What distinguishes mixed inhibition from pure noncompetitive inhibition?

Mixed inhibitors can bind either the enzyme or the enzyme-substrate complex. Signup and view all the answers

What is the role of fructose-2,6-bisphosphate (F-2,6-BP) in regulating PFK-1 activity?

Activates PFK-1 activity Signup and view all the answers

Which molecule is an allosteric inhibitor of PFK-1?

ATP Signup and view all the answers

How does the phosphorylation state of PFK-2 affect its activity in the liver?

Dephosphorylated PFK-2 is active Signup and view all the answers

What determines the levels of fructose-2,6-bisphosphate (F-2,6-BP)?

PFK-2 and F-2,6-BPase Signup and view all the answers

What effect do citrate and ATP have on PFK-1?

They inhibit its activity Signup and view all the answers

In skeletal muscle, how is PFK-2 regulated?

Through substrate availability Signup and view all the answers

Which of the following statements about PFK-1 is true?

It is the rate-limiting enzyme in all tissues Signup and view all the answers

What is a consequence of arsenite's interaction with GAPDH?

It causes the enzyme to become inactive Signup and view all the answers

What is the primary consequence of a deficiency in Aldolase B?

Accumulation of fructose 1-P in the liver Signup and view all the answers

Which cofactors are essential for the Pyruvate Dehydrogenase (PDH) complex?

NAD+, CoA, Thiamine Signup and view all the answers

What is the main role of the pentose phosphate pathway (PPP)?

Provide NADPH and ribose for biosynthesis Signup and view all the answers

What dietary restrictions are recommended for individuals with hereditary fructose intolerance?

Limit high-fructose corn syrup and certain fruits Signup and view all the answers

What characterizes the catapleurotic reactions in metabolism?

They provide intermediates for biosynthesis Signup and view all the answers

Which of the following is NOT a symptom associated with fructokinase deficiency?

Severe vomiting Signup and view all the answers

In what forms is energy produced during the TCA cycle?

NADH and ATP Signup and view all the answers

What is a key step in the regulation of the TCA cycle?

Oxidation of isocitrate to alpha-ketoglutarate Signup and view all the answers

Which cofactors are essential for the functioning of the PDH complex?

CoA (B5, pantothenic acid) Signup and view all the answers

What are the known inhibitors of the PDH complex?

Acetyl-CoA Signup and view all the answers

What are potential symptoms of PDH deficiency?

Increased levels of pyruvate Signup and view all the answers

Which substance serves as an allosteric activator for the PDH complex?

Insulin Signup and view all the answers

What is a key feature of the TCA cycle's mitochondrial location?

Most reactions happen in the mitochondrial matrix Signup and view all the answers

How is energy produced in the TCA cycle?

Via the generation of GTP, NADH, and FADH2 Signup and view all the answers

What happens to oxaloacetate during the TCA cycle?

It is continually regenerated Signup and view all the answers

Which type of reactions are linked to pyruvate carboxylase deficiency?

Anapleurotic reactions Signup and view all the answers

Study Notes

Inhibitors

Inhibitors bind more tightly to an enzyme than substrates or products.
Many pharmacological examples exist, such as penicillin and allopurinol.
Allopurinol is an example of a transition state analog - it inhibits by binding to the active site of the enzyme that makes uric acid.

Reversible Inhibition

Competitive Inhibition: The inhibitor binds to the active site of an enzyme.
Noncompetitive Inhibition: The inhibitor binds to a site on the enzyme other than the active site (allosteric site).
- It can bind before or after a substrate binds.
- Pure noncompetitive inhibition occurs when the inhibitor binds to a site different from the substrate. The enzyme may have one or more substrates.
- Mixed noncompetitive inhibition occurs when the inhibitor binds outside of the substrate-binding sites.
Uncompetitive Inhibition: The inhibitor only binds to the enzyme-substrate complex, but not to the enzyme alone.
- This only happens in multi-substrate reactions.

Lineweaver-Burk Plots

Using the reciprocal of both velocity and substrate concentration, a hyperbolic curve can be transformed into a straight line.
This transformation makes it easier to compare different conditions and determine the type of inhibition.

Competitive Inhibition

Vmax remains unaffected.
- The definition of Vmax is the maximum velocity at which the enzyme works when there is an unlimited amount of substrate.
- The inhibitor has no effect if there is an unlimited amount of substrate.
Km is affected.
- The apparent Km (Km,app) is increased with the inhibitor.
- This means that the enzyme's affinity for substrate is decreased.
Example: Hemoglobin binding oxygen.

Cooperativity

Activators:
- Increase the affinity of the enzyme for the substrate.
- Tend to bind more tightly to the R state.
Inhibitors:
- Decrease the affinity of the enzyme for the substrate.
- Tend to bind more tightly to the T state.
Activators shift the sigmoid-shaped substrate saturation curve to the left.
Inhibitors shift the curve to the right.
Example: Phosphofructokinase-1 (PFK-1) in glycolysis.

Enzymatic Regulation

Regulation by Substrate Availability: Changes in the concentration of the substrate can change the reaction velocity.
- This is a fast response, occurring over seconds.
Product Inhibition: Accumulation of the product can inhibit the reaction, decreasing the apparent Vmax and/or Km.
- This is a fast response, occurring over seconds.
Allosteric Control: Binding of an effector molecule to an allosteric site can change the enzyme's activity, altering the Vmax and/or Km.
- This is also a fast response, occurring over seconds.
Covalent Modification: The covalent modification of an enzyme, such as phosphorylation, can alter its activity.
- This change occurs over seconds to minutes, or even days.
Induction or Repression: Changes in the amount of enzyme protein can be caused by hormones or metabolites, which can occur over minutes, hours, or days.

PFK-1 Regulation

PFK-1 is the rate-limiting enzyme in glycolysis.
Activators: AMP and fructose-2,6-bisphosphate (F-2,6-BP).
Inhibitors: Citrate and ATP.

F-2,6-BP

Glucose is converted to fructose-6-phosphate (F-6-P) which is then converted to fructose-2,6-bisphosphate (F-2,6-BP) by the enzyme PFK-2.
F-2,6-BP is then converted back to F-6-P by the enzyme F-2,6-bisPase.
PFK-1 is activated by F-2,6-BP.
Levels of F-2,6-BP determine how much PFK-1 activity occurs.

PFK-2 & F-2,6-BPase

PFK-2 and F2,6-BPase can be phosphorylated and dephosphorylated.
- In heart and skeletal muscle, the enzymes are regulated by the concentration of the substrate.
In the liver:
- PFK-2 is active when it is dephosphorylated and inactive when it is phosphorylated.
- F2,6-BPase is inactive when it is dephosphorylated and active when it is phosphorylated.
The liver regulates glycolysis and gluconeogenesis based on these changes.
Adipose tissue regulates glycolysis based on changes in these enzymes.

Phosphofructokinase-1

Regulated allosterically by AMP & F-2,6-BP.
There is no phosphorylation site in skeletal muscle PFK-2.
- It is regulated by the availability of the substrate.
There is a cAMP-dependent protein kinase (PKA) phosphorylation site in liver PFK-2.

Glyceraldehyde 3-Phosphate Dehydrogenase (GAPDH)

Catalyzes a reversible reaction found in both glycolysis and gluconeogenesis.
Produces the high-energy molecule 1,3-bisphosphoglycerate (1,3-BPG) and reduces NAD+ to NADH.
Inactivated by reaction with iodoacetate, iodoacetamide, arsenite, or mercury ions.
Arsenate (AsO43-) can substitute for phosphate (PO43-).
- This leads to a futile cycle, which is a potential treatment for cancer cells that rely heavily on glycolysis.

Fructose Metabolism

Fructose bypasses the rate-limiting step of glycolysis, PFK-1, making fructose metabolism less regulated.
- High fructose corn syrup consumption (about 50% fructose) is associated with fatty liver and hyperglycemia.
Fructokinase deficiency leads to fructosuria.
Aldolase B deficiency leads to hereditary fructose intolerance.

Fructokinase Deficiency

Fructosuria: A rare but benign condition (autosomal recessive).
- Individuals with this condition have high blood fructose levels.

Aldolase B Deficiency

Hereditary fructose intolerance: An autosomal recessive condition.
- Aldolase B is the rate-limiting enzyme in fructose catabolism.
- Individuals with this deficiency accumulate F-1-P in their livers.
  - This depletes liver phosphate levels, which are trapped as F-1-P.
- Symptoms are severe: diarrhea, vomiting, failure to thrive, liver and kidney damage.
- Untreated, it can lead to death.
- Dietary restrictions are necessary: Avoid fruits, fruit juices, maple syrup, etc.

PDH Complex

PDH - Pyruvate dehydrogenase complex.
Composition:
- 5 cofactors: Thiamine pyrophosphate (B1), lipoic acid, CoA (B5, pantothenic acid), FAD (B2, riboflavin), NAD+ (B3, niacin).
Role: Catalyzes the conversion of pyruvate to acetyl-CoA - the link between glycolysis and the TCA cycle.

PDH Complex Regulation

Substrate activation and product inhibition:
- Activators: Pyruvate, CoA, and NAD+.
- Inhibitors: Acetyl-CoA and NADH.
Covalent modification:
- Phosphorylation and dephosphorylation of the complex by hormones.
  - Insulin is an activator of dephosphorylation, which leads to activation of the complex, in the liver.
Allosteric regulation:
- The complex can be activated or inhibited by allosteric effectors.

PDH Deficiency

Symptoms: Increased levels of pyruvate, lactate, and alanine.
- Chronic lactic acidosis.
- Severe neurological defects that could eventually lead to death.

TCA Cycle Regulation

3 Steps:
- Citrate synthase (inhibited by ATP, NADH, and acetyl-CoA).
- Isocitrate dehydrogenase (activated by ADP and NAD+, inhibited by ATP and NADH).
- α-ketoglutarate dehydrogenase (inhibited by succinyl CoA, NADH, and ATP).
Location: Inside the mitochondrial matrix, except succinate dehydrogenase, which is located in the inner mitochondrial membrane.

TCA Cycle - Energy Production and Investment

Energy Investment:
- 1 ATP equivalent (GTP) is produced per turn of the cycle.
Energy Production:
- 3 NADH molecules are produced per turn.
- 1 FADH2 molecule is produced per turn.
- These reduced cofactors provide the electrons to the ETC for ATP generation.

Catapleurotic/Anapleurotic Reactions

Catapleurotic reactions: Reactions that remove intermediates from the TCA cycle.
Anapleurotic reactions: Reactions that replenish intermediates in the TCA cycle.
Pyruvate carboxylase deficiency: A rare genetic condition that affects the anapleurotic pathway.
- This can disrupt the proper functioning of the TCA cycle.

Pentose Phosphate Pathway (PPP)

Two Phases:
- Oxidative phase: This phase generates NADPH and the precursor for nucleotide biosynthesis (ribose 5-phosphate).
- Non-oxidative phase: This phase interconverts pentose phosphates (5-carbon sugars) to other sugars like glucose-6-phosphate.
Regulation:
- The PPP responds to different metabolic needs.
  - Increased NADPH production: When cells need to reduce oxidative stress.
  - Increased nucleotide biosynthesis: When cells need to synthesize nucleic acids.

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Description

Explore the fascinating world of enzyme inhibitors in this quiz. Learn about different types of inhibition, including competitive, noncompetitive, and uncompetitive inhibition. Test your knowledge on pharmacological examples like allopurinol and how inhibitors interact with enzymes.