Transition Stage of Cellular Respiration Quiz

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Questions and Answers

What is the primary function of pyruvate dehydrogenase complex (PDH complex) in cellular respiration?

To produce lactic acid from NADH

To phosphorylate glucose derivatives

To convert pyruvate into glucose

To convert pyruvate into acetyl-CoA (correct)

What cofactor does enzyme 1 of the PDH complex utilize?

Thiamine pyrophosphate (TPP) (correct)

Nicotinamide adenine dinucleotide (NAD+)

Lipoic acid

Flavin adenine dinucleotide (FAD)

Which of the following substances stimulates pyruvate dehydrogenase kinase (PDK), inhibiting the PDH complex?

ADP

Pyruvate

NADH (correct)

Calcium

What is the product formed when NADH reduces pyruvate in the absence of oxygen?

Lactic acid Signup and view all the answers

How does insulin affect the activity of the PDH complex?

It inhibits PDK activity Signup and view all the answers

What occurs during step 1 of the PDH complex reaction?

Pyruvate reacts with TPP releasing CO2 Signup and view all the answers

What consequence can result from a thiamine deficiency?

Occurrence of Wernicke-Korsakoff syndrome Signup and view all the answers

Which enzyme in the PDH complex contains FAD as a cofactor?

Dihydrolipoamide dehydrogenase Signup and view all the answers

What effect does high ATP levels have on PDH complex activity?

Inhibits PDH complex activity Signup and view all the answers

What dietary habit is associated with thiamine deficiency and may lead to beriberi?

White rice diet Signup and view all the answers

What are the outputs of the Pyruvate Dehydrogenase Complex reaction?

Acetyl-CoA, CO2, FADH2, NADH Signup and view all the answers

Which compound is generated first when Coenzyme A reacts with the acetyl group?

Acetyl-CoA Signup and view all the answers

Which of the following inhibits the Pyruvate Dehydrogenase Complex?

High NADH Signup and view all the answers

What role does FAD play in the Pyruvate Dehydrogenase Complex?

It reduces lipoate. Signup and view all the answers

Which enzyme in the Pyruvate Dehydrogenase Complex is responsible for removing CO2?

Pyruvate Dehydrogenase (E1) Signup and view all the answers

Which symptom is associated with Wernicke-Korsakoff Syndrome?

Confabulation Signup and view all the answers

Which of the following is NOT a condition caused by thiamine deficiency?

High blood sugar Signup and view all the answers

Which factors stimulate the activity of the Pyruvate Dehydrogenase Complex?

Insulin and calcium Signup and view all the answers

What is the main consequence of a deficiency in thiamine?

Neurological complications Signup and view all the answers

How does NAD+ interact in the reaction sequence involving FADH2?

It picks up hydrides from FADH2 to form NADH. Signup and view all the answers

Study Notes

Transition Stage of Cellular Respiration

Pyruvate, a 3-carbon molecule, is the end product of glycolysis.
Two pyruvate molecules are generated from one glucose molecule.
Pyruvate enters the mitochondria and is converted to acetyl-CoA in the presence of oxygen.
NADH from glycolysis carries electrons to the electron transport chain in the presence of oxygen.
In the absence of oxygen, NADH reduces pyruvate to lactic acid.

Pyruvate Dehydrogenase Complex

Pyruvate dehydrogenase complex (PDH complex) consists of 3 enzymes:
- Enzyme 1 (Pyruvate dehydrogenase): contains thiamine pyrophosphate (TPP) as a cofactor.
- Enzyme 2 (Dihydrolipoamide acetyltransferase): contains lipoic acid with disulfide bonds.
- Enzyme 3 (Dihydrolipoamide dehydrogenase): contains FAD and NAD+.

Regulation of the Pyruvate Dehydrogenase Complex

PDH complex activity is tightly regulated via phosphorylation and dephosphorylation.
Pyruvate dehydrogenase kinase (PDK) phosphorylates enzyme 1, inhibiting PDH complex activity.
Pyruvate dehydrogenase phosphatase (PDP) dephosphorylates enzyme 1, activating the PDH complex.

Factors that Regulate PDH Complex Activity

High energy levels: ATP, acetyl-CoA, NADH stimulate PDK, inhibiting the PDH complex.
Low energy levels: ADP, pyruvate, calcium inhibit PDK, activating the PDH complex.
Hormonal Regulation: Insulin activates PDP, promoting dephosphorylation and activation of the PDH complex.

Clinical Relevance

Thiamine Deficiency: A deficiency in thiamine causes Beriberi, a neurological disorder characterized by pain, paralysis, and edema.
Alcoholism: Alcohol consumption contributes to thiamine deficiency due to its lack of nutritional value, causing beriberi.
White Rice Diet: A diet solely based on white rice lacks sufficient thiamine, also causing beriberi.
Wernicke-Korsakoff syndrome: A disorder associated with thiamine deficiency and alcohol abuse, characterized by neurological abnormalities including memory impairment, confusion, and eye movement disorders. Confabulation is a feature of Wernicke-Korsakoff syndrome.

Steps in Pyruvate Dehydrogenase Complex Reaction

Step 1: Pyruvate reacts with TPP in enzyme 1, releasing CO2 and forming a two-carbon acetyl group bound to TPP.
Step 2: The acetyl group is transferred from TPP to lipoic acid in enzyme 2, breaking the disulfide bond, forming acetyl-lipoate.
Step 3: Coenzyme A reacts with the acetyl group on lipoate, forming acetyl-CoA.
Step 4: The reduced lipoate releases hydrides, which are picked up by FAD, forming FADH2.
Step 5: NAD+ accepts the hydrides from FADH2, generating NADH.

Summary of Pyruvate Dehydrogenase Complex Reaction

Inputs: Pyruvate, TPP, lipoic acid, coenzyme A, FAD, NAD+
Outputs: Acetyl-CoA, CO2, FADH2, NADH

Pyruvate Dehydrogenase Complex (PDC)

Function: Converts pyruvate to acetyl-CoA, producing CO2 and NADH.
Regulation:
- Inhibition: High ATP, high NADH, high acetyl-CoA, and low ADP
- Stimulation: Low ATP, high ADP, high pyruvate, calcium and insulin (activates PDP)
Enzymes:
- Pyruvate Dehydrogenase (E1): Uses TPP to remove CO2.
- Dihydrolipoamide Acetyltransferase (E2): Contains lipoic acid, disulfides, and thiols.
- Dihydrolipoamide Dehydrogenase (E3): Contains FAD and NAD.
Deficiencies:
- Thiamine Deficiency: Leads to Wernicke-Korsakoff Syndrome (psychological issues like confabulation) and Beriberi (neurological issues like pain, paralysis, and swelling).
- Beriberi Causes: Genetic predisposition, alcohol abuse, high white rice consumption.

Confabulation

Definition: Fabricating information to fill in memory gaps.
Wernicke-Korsakoff Syndrome: Characterized by confabulation rather than memory recall.

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Description

Test your knowledge on the transition stage of cellular respiration, focusing on the conversion of pyruvate to acetyl-CoA and the role of the Pyruvate Dehydrogenase Complex. Explore the enzymes involved, their regulation, and the importance of oxygen in this crucial metabolic process.