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Citric Acid Cycle Overview
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Citric Acid Cycle Overview

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Questions and Answers

What reaction does GTP participate in to form ATP?

GTP can donate its phosphoryl group to ADP to form ATP.

What role does succinate dehydrogenase play in the citric acid cycle?

Succinate dehydrogenase catalyzes the oxidation of succinate to fumarate.

How does malonate affect the citric acid cycle?

Malonate is a competitive inhibitor of succinate dehydrogenase, blocking the citric acid cycle.

What products are generated from one turn of the citric acid cycle?

<p>One turn produces three NADH, one FADH2, one GTP (or ATP), and two CO2.</p> Signup and view all the answers

What is the impact of arsenic poisoning on the citric acid cycle?

<p>Arsenic inhibits enzymes that require lipoic acid, leading to pyruvate accumulation.</p> Signup and view all the answers

Which amino acids can α-ketoglutarate and oxaloacetate serve as precursors for?

<p>They serve as precursors for the amino acids aspartate and glutamate.</p> Signup and view all the answers

What is the significance of fumarase in the citric acid cycle?

<p>Fumarase catalyzes the reversible hydration of fumarate to L-malate.</p> Signup and view all the answers

How is L-malate oxidized in the citric acid cycle?

<p>L-malate is oxidized to oxaloacetate by NAD+-linked L-malate dehydrogenase.</p> Signup and view all the answers

What is the primary function of the citric acid cycle in cellular respiration?

<p>The primary function is to oxidize acetyl residues to CO2 while conserving energy in reduced electron carriers.</p> Signup and view all the answers

What are some symptoms of PDH deficiency?

<p>Symptoms include neurodegeneration, muscle spasticity, and in neonatal forms, early death.</p> Signup and view all the answers

Which two reduced electron carriers are generated during the citric acid cycle?

<p>The two reduced electron carriers are NADH and FADH2.</p> Signup and view all the answers

Why is the E1 defect classified as X-linked dominant?

<p>The E1 defect is classified as X-linked dominant because it affects both males and females, although it is X-linked.</p> Signup and view all the answers

How is pyruvate transported to the mitochondria for the citric acid cycle?

<p>Pyruvate is transported across the inner mitochondrial membrane via a pyruvate/H+ symport.</p> Signup and view all the answers

What dietary measures can help reduce symptoms of PDH deficiency?

<p>Dietary restriction of carbohydrate and supplementation with TPP may help reduce symptoms.</p> Signup and view all the answers

What is produced alongside acetyl-CoA during the conversion of pyruvate?

<p>Carbon dioxide (CO2) is produced alongside acetyl-CoA.</p> Signup and view all the answers

What is the first step in the Citric Acid Cycle?

<p>The first step is the formation of citrate from acetyl-CoA and oxaloacetate, catalyzed by citrate synthase.</p> Signup and view all the answers

What are the three distinct enzymes that make up the pyruvate dehydrogenase complex?

<p>The three enzymes are pyruvate dehydrogenase (E1), dihydrolipoyl transacetylase (E2), and dihydrolipoyl dehydrogenase (E3).</p> Signup and view all the answers

How does aconitase affect the Citric Acid Cycle?

<p>Aconitase is responsible for isomerizing citrate to isocitrate, and it is inhibited by fluoroacetate.</p> Signup and view all the answers

Why is the citric acid cycle considered a common pathway in metabolism?

<p>It is a common pathway because carbohydrates, lipids, and proteins are metabolized to acetyl-CoA or cycle intermediates.</p> Signup and view all the answers

What role does the citric acid cycle play in gluconeogenesis?

<p>The citric acid cycle has a central role in gluconeogenesis by providing intermediates for glucose synthesis.</p> Signup and view all the answers

What role does isocitrate dehydrogenase play in the TCA cycle?

<p>Isocitrate dehydrogenase catalyzes the oxidative decarboxylation of isocitrate to form α-ketoglutarate and CO2.</p> Signup and view all the answers

What type of reaction does the pyruvate dehydrogenase complex catalyze?

<p>The pyruvate dehydrogenase complex catalyzes an oxidative decarboxylation reaction.</p> Signup and view all the answers

What is produced when α-ketoglutarate is converted to succinyl-CoA?

<p>The conversion produces succinyl-CoA and CO2.</p> Signup and view all the answers

What is coupled with the conversion of succinyl-CoA to succinate?

<p>The conversion is coupled to the phosphorylation of GDP to GTP.</p> Signup and view all the answers

What are the five coenzymes required by the Pyruvate Dehydrogenase Complex?

<p>Thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A, nicotinamide adenine dinucleotide (NAD), and lipoate.</p> Signup and view all the answers

What serious condition can arise from thiamine deficiency?

<p>Beriberi, characterized by loss of neural function.</p> Signup and view all the answers

What is the role of PDH kinase in the regulation of the PDH complex?

<p>PDH kinase phosphorylates and inactivates E1 of the PDH complex.</p> Signup and view all the answers

How does calcium affect PDH phosphatase?

<p>Calcium activates PDH phosphatase.</p> Signup and view all the answers

What condition is associated with a deficiency of the E1 component of the PDH complex?

<p>Congenital lactic acidosis.</p> Signup and view all the answers

What happens to brain function in cases of pyruvate dehydrogenase deficiency?

<p>Brain function deteriorates due to inadequate TCA cycle activity.</p> Signup and view all the answers

What are the allosteric activators of the pyruvate dehydrogenase complex?

<p>AMP, CoA, and NAD+.</p> Signup and view all the answers

What is the effect of elevated pyruvate levels in the blood?

<p>It indicates defects in pyruvate oxidation.</p> Signup and view all the answers

Study Notes

Citric Acid Cycle Overview

  • Also known as Krebs cycle or tricarboxylic acid cycle (TCA), it is a crucial part of cellular respiration occurring in mitochondria.
  • Involves oxidation of acetyl-CoA, producing CO2 and conserving energy as NADH and FADH2.
  • Functions as a common pathway for aerobic oxidation of carbohydrates, lipids, and proteins.

Transport and Production of Acetyl-CoA

  • Pyruvate from glycolysis is transported into mitochondria via a pyruvate/H+ symport.
  • Pyruvate is oxidized to acetyl-CoA and CO2 by the pyruvate dehydrogenase (PDH) complex.
  • The process involves oxidative decarboxylation, removing the carboxyl group from pyruvate.

Components of the Pyruvate Dehydrogenase Complex

  • Comprises three enzymes: E1 (pyruvate dehydrogenase), E2 (dihydrolipoyl transacetylase), E3 (dihydrolipoyl dehydrogenase).
  • Requires five coenzymes: thiamine pyrophosphate (TPP), flavin adenine dinucleotide (FAD), coenzyme A, nicotinamide adenine dinucleotide (NAD), and lipoate.
  • Thiamine deficiency can lead to serious neurological issues (e.g., Beriberi).

Regulation of Acetyl-CoA Production

  • Allosteric activation occurs when AMP, CoA, and NAD+ levels rise.
  • PDH complex can be inactivated by phosphorylation of E1 by PDH kinase, which is activated by ATP, acetyl-CoA, and NADH.
  • Calcium ions activate PDH phosphatase, reversing E1 inactivation.

Pyruvate Dehydrogenase Deficiency

  • E1 deficiency linked to congenital lactic acidosis causes pyruvate accumulation and subsequent lactic acid buildup.
  • Neural effects include neurodegeneration and muscle spasticity; neonatal forms can be fatal.
  • Condition is X-linked dominant and lacks proven treatments, though dietary changes may help.

Reactions of the Citric Acid Cycle

  • Eight steps involved, starting with acetyl-CoA combining with oxaloacetate to form citrate.
  • Citrate is converted to isocitrate, followed by oxidative decarboxylation to form α-ketoglutarate.
  • Further steps convert α-ketoglutarate to succinyl-CoA, then to succinate, followed by oxidation to fumarate, hydration to malate, and oxidation back to oxaloacetate.

Energy Yield of the Citric Acid Cycle

  • One turn of the cycle yields 3 NADH, 1 FADH2, 1 ATP (or GTP), and 2 CO2.
  • Total energy yield from one acetyl-CoA can reach 12 ATP equivalents.

Arsenic Poisoning

  • Arsenic inhibits enzymes dependent on lipoic acid, notably impacting the PDH complex.
  • Leads to pyruvate accumulation, similar in effect to PDH deficiency, causing neurological damage.

Biosynthetic Importance of Cycle Intermediates

  • α-Ketoglutarate and oxaloacetate serve as precursors for amino acids through transamination reactions.

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Description

Explore the essential processes of the citric acid cycle, also known as the Krebs cycle. This quiz focuses on its significance in cellular respiration, detailing how acetyl-CoA is oxidized to release energy captured by NADH and FADH2. Test your knowledge of this vital metabolic pathway.

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