Tricarboxcylic Acid Cycle

Question 1

What is the citric acid cycle (TCA)?

Answer 1

This is a series of chemical reactions that occur via aerobic respiration, that generate energy via the oxidation of acetate.

It occurs in the matrix of the mitochondria.

Question 2

Picture of TCA.

Answer 2

Question 3

Picture 2 of TCA cycle.

Answer 3

Question 4

What are some of the catabolic pathways that converge on the TCA?

Answer 4

Glycolysis breaks glucose (a six-carbon-molecule) down into pyruvate (a three-carbon molecule). It moves into the mitochondria where it is converted into acetyl-CoA by decarboxylation and enters the citric acid cycle.

In protein catabolism, proteins are broken down into amino acids. The carbon backbone of these amino acids can become a source of energy by being converted to acetyl-CoA and entering into the citric acid cycle.

In fat catabolism, triglycerides are hydrolyzed to break them into fatty acids and glycerol. In the liver the glycerol can be converted into glucose via dihydroxyacetone phosphate and glyceraldehyde-3-phosphate by way of gluconeogenesis. In many tissues, especially heart tissue, fatty acids are broken down through a process known as beta oxidation, which results in acetyl-CoA, which can be used in the citric acid cycle. Beta oxidation of fatty acids with an odd number of methylene bridges produces propionyl CoA, which is then converted into succinyl-CoA and fed into the citric acid cycle.

Question 5

What does the TCA generate from one molecule of acetyl CoA?

Answer 5

• The acetyl-CoA, has been oxidized to two molecules of carbon dioxide.
• Three molecules of NAD were reduced to NADH.
• One molecule of FAD was reduced to FADH2.
• One molecule of GTP (the equivalent of ATP) was produced.

Question 6

TCA and ETC link picture.

Answer 6

Question 7

Generalized picture of the TCA.

Answer 7

Question 8

What is the Bridge Reaction and where does it occur?

Answer 8

The Bridge reaction starts in the cytosol and ends in the inner membrane of the mitochondria. It is the step in between glycolysis and the TCA Cycle where pyruvate is converted into Acetyl CoA

This reaction converts two molecules of 3-carbon pyruvate into two molecules of 2-carbon acetyl CoA.

Pyruvate decarboxylase (Pyruvate dehydrogenase) decarboxylates pyruvate in the matrix of the mitochondria. NAD⁺ acquires two e^- that break down pyruvate and release CO₂ as a result.

Question 9

What are the reactants in the Bridge Reaction?

Answer 9

2 Pyruvate, 2 coenzymeA

Question 10

What are the products in the Bridge Reaction?

Answer 10

2 acetyl CoA, 2 NADH, 2 CO2

Question 11

What happens to NAD⁺ in the Bridge Reaction?

Answer 11

Reduced to NADH

Question 12

What happens to the TCA when there is an increased need for ATP?

Answer 12

As ATP levels fall, there is a concomitment increase of ADP and AMP. Increased levels of ADP / AMP serve as + activator of pyruvate dehydrogenase complex, citrate synthase, and isocitrate dehydrogenase.

This speeds the cycle up.

Question 13

What happens to the TCA when there is a reduced need for ATP (Increase in ATP that is)?

Answer 13

When the energy levels of a cell increase, NADH and ATP levels increase. This in turn inhibits the activities of pyruvate dehydrogenase (- regulated by ATP, NADH, and acetyl CoA), citrate synthase (- regulated by NAD⁺ , succinyl CoA, and ATP), isocitrate dehydrogenase and alpha ketogluterate dehydrogenase (also - regulated by succinyl CoA)

Question 14

How is the TCA cycle regulated?

Answer 14

The rate of ATP hydrolysis regulates the rate of ATP synthesis. This in turn controls the rate of NADH oxidation in the ETC.

ALL NADH and FADH₂ produced by the TCA contribute e^- to the ETC.

Thus, the oxidation of acetyl-CoA in the TCA can only occur as fast as e^- from NADH enter the ETC (which is controlled by ATP to ADP ratio).

Isocitrate dehydrogenase, alpha ketogluterate dehydrogenase and malate dehydrogenase are inhibited by increased NADH conc.

The NADH to NAD⁺ ratio alters the concentration of oxaloacetate.

Citrate is a product inhibitor of citrate synthase.

ADP is an allosteric activator of isocitrate dehydrogenase.

Increased Ca²⁺ conc. activate isocitrate dehydrogenase, alpha ketogluterate dehydrogenase, and pyruvate dehydrogenase.

Question 15

How is citrate synthase regulated?

Answer 15

Rate controlled by oxaloacetate and citrate.

Citrate is a competitive inhibitor of oxaloacetate.

When NADH / NAD⁺ increases, the ratio of oxaloacetate to malate increases. When isocitrate dehydrogenase is activated, the concentration of citrate decreases thus relieving citrate inhibition on citrate synthase.

Thus, both increased oxaloacetate and decreased citrate levels regulate citrate synthase.

Question 16

How is isocitrate dehydrogenase regulated?

Answer 16

Isocitrate dehydrogenase is the rate limiting step in the TCA.

It is allosterically activated by ADP (cooperative binding) and inhibited by increased NADH concentration. Increased Ca²⁺ also increases its activity.

Question 17

How is alpha ketogluterate dehydrogenase regulated?

Answer 17

This is inhibited by NADH and succinyl CoA; it may also be inhibited by GTP.

It can be activated by increased Ca²⁺ .

Question 18

How are TCA intermediates regulated?

Answer 18

Decreased isocitrate dehydrogenase increases citrate concentration. This in turn stimulates citrate efflux to the cytosol.

Question 19

What are sources of acetyl CoA?

Answer 19

Beta oxidation of fatty acids

Degredation of ketone bodies (Beta hydroxybutyrate and acetoacetate)

Acetate from ethanol oxidation

Pyruvate from glycolysis

Amino acids alanine, serine, leucine, and isoleucine

Question 20

What is the pyruvate dehydrogenase complex and what does it do? How is it regulated?

Answer 20

The PDC linkes glycolysis and the TCA.

It ocxidizes pyruvate to acetyl CoA

Regulation of PDH in the brain is critical

PDC kinase, a subunit of the enzyme, phosphorylates PDC at a specific serine and inactivates PDC. PDC phosphatase, another subunit of the enzyme, removes the phosphate and activates PDC.

The kinase is inhibited by ADP and Pyruvate.

The phosphatase is activated by Ca²⁺

When acetyl CoA and NADH are bound to PDC, they stimulate the kinase and PDC is phosphorylated to the inactive form

Question 21

What is an anaplerotic reaction?

Answer 21

This is a reaction in which it replenishes the intermediates of the TCA cycle.

Question 22

What is a major anaplerotic enzyme?

Answer 22

Pyruvate decarboxylase.

It catalyzes the addition of CO₂ to pyruvate to form oxaloacetate

It contains biotin

Found in liver, brain, fat cells, kidney

Activated by acetyl CoA and inhibited by high concentrations of acyl CoA derivatives