BCH 3120 Lecture Notes - Lecture 3: Citric Acid Cycle, Oxidative Phosphorylation, Oxaloacetic Acid

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January 16, 2017

TCA Cycle

Tricarboxylic acid cycle (TCA) aka citric acid cycle aka Krebs cycle

Occurs in the mitochondria in eukaryotes

-Required metabolites must be able to pass through the mitochondrial membrane in order for TCA to occur

-Intermediates are made in the mitochondria or are transported there

Generates electrons for oxidative phosphorylation

Produces intermediates that are used in biosynthesis

-Select few intermediates can be transported across the mitochondrial membrane

Converts acetyl-CoA into CO2 and generates 4 pairs of electrons (3 will be transferred to NADH, 1 will be transferred

to FADH2)

-Acetyl-CoA + 3 NAD+ + FAD + GDP + Pi —> 2 CO2 + HSCoA + 3 NADH + FADH2 + GTP

Requires aerobic conditions: requires oxygen in order for NADH and FADH2 to be recycled back into NAD+ and FAD

-NB: oxygen is not actually involved in the TCA cycle pathway, but is a key part of oxidative phosphorylation

(recycling)

•4 pairs of electrons (from NADH and FADH2) allows for the production of ~10 ATP

Acetyl-CoA will be added to oxaloacetate (4C molecule) to create citrate (6C molecule)

2 C’s will then be lost as CO2 and oxaloacetate will be regenerated

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-Takes acetyl-CoA from multiple sources

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January 16, 2017

•Beta-oxidation of fatty acids

•Amino acid catabolism

•Pyruvate from glycolysis: pyruvate will be transported into the mitochondria

-Pyruvate dehydrogenase converts pyruvate into acetyl-CoA

•Requires NAD+ (will be converted to NADH)

•Catalyzed by 3 enzyme complex (E1, E2, and E3)

8 enzymes involved in the TCA cycle

Pyruvate Dehydrogenase

2 pyruvate + 2 NAD+ + 2 CoA-SH —> 2 acetyl-CoA + 2 NADH + 2 CO2

Converts pyruvate into acetyl-CoA

Requires NAD+ - will be converted to NADH

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Catalyzed by 3 enzyme complex (E1, E2, and E3)

-E1: takes pyruvate and does a decarboxylation, requires TPP

•TPP provides the electron delocalization required to stabilize the carbanion intermediate

•Irreversible

-E2: lipoic acid attached to Lys, has a disulﬁde bond

•Creates a thioester product

•Produces Acetyl-CoA

•Reversible: can be product inhibited

-E3: recycles lipoic acid (reoxidizes it)

•Produces NADH from NAD+

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January 16, 2017

•Regulated by FAD

•Reversible: can be product inhibited

Very tightly regulated

-Can be regulated by product inhibition: by acetyl-CoA and NADH acting on E2 and E3

-Can be regulated by covalent modiﬁcation: phosphorylation

•

•Pyruvate dehydrogenase kinase can phosphorylate E1 to make it inactive

-Pyruvate dehydrogenase kinase is activated by Acetyl-CoA and NADH

-Inhibited by ADP and pyruvate

•Pyruvate dehydrogenase phosphatase removes the phosphate group to reactivate E1

•Ca2 inhibits PDH kinase and activates PDH phosphatase

Reaction 1: Citrate Synthase

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Acetyl-CoA + oxaloacetate + H2O —> citrate + CoA-SH + H+

Occurs in the mitochondria

Uses acetyl-CoA and oxaloacetate to make citrate

Deprotonates acetyl-CoA to make an enolate intermediate, which will form an aldol product (with a thirster bond),

which will be hydrolyzed to form citrate

-Enolate intermediate is high energy

-Hydrolysis of the thioester releases a lot of energy, which counteracts the energy required to make the high energy

enolate

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Document Summary

Tricarboxylic acid cycle (tca) aka citric acid cycle aka krebs cycle. Required metabolites must be able to pass through the mitochondrial membrane in order for tca to occur. Intermediates are made in the mitochondria or are transported there. Select few intermediates can be transported across the mitochondrial membrane. Converts acetyl-coa into co2 and generates 4 pairs of electrons (3 will be transferred to nadh, 1 will be transferred to fadh2) Acetyl-coa + 3 nad+ + fad + gdp + pi > 2 co2 + hscoa + 3 nadh + fadh2 + gtp. Requires aerobic conditions: requires oxygen in order for nadh and fadh2 to be recycled back into nad+ and fad. Nb: oxygen is not actually involved in the tca cycle pathway, but is a key part of oxidative phosphorylation (recycling: 4 pairs of electrons (from nadh and fadh2) allows for the production of ~10 atp. Acetyl-coa will be added to oxaloacetate (4c molecule) to create citrate (6c molecule)

Related Questions

Why does an electron have lower potential energy in an H-O bondthan in an O=O bond?

H has a higher electronegativity than O.

O=O is not stable.

O=O contains a double bond.

The electrons are closer to the oxygen atom in the H-O bond.

None of the answers are correct.

How is the energy from the electron transport chain used mostdirectly?

to form the bonds of a water molecule

to pump hydrogen ions across the mitochondrial inner membraneinto the mitochondrial intermembrane space

to join ADP with a phosphate

to decompose glucose into pyruvate

to rotate ATP synthase

Which process provides the energy needed to pump protons acrossthe mitochondrial membrane during oxidative phosphorylation?

the transfer of electrons in redox reactions

the breakdown of ATP to ADP

the rotation of the ATP synthase rotor

oxidation of isocitrate

None of the answers are correct.

What causes the ATP synthase rotor to spin?

the passing of electrons through ATP synthase

the movement of protons across the mitochondrial membrane

the movement of electrons across the mitochondrial membrane

the synthesis of ATP from ADP and a phosphate group

None of the answers are correct.

What of the following generates the proton-motive force duringoxidative phosphorylation?

buildup of NADH molecules at protein complex I

movement of H⁺ ions across the mitochondrial membraneduring electron transport

rotation of the ATP synthase rotor

joining of H⁺ ions with oxygen to form water

None of the answers are correct.

Suppose a microorganism invades a cell, causing damage to ATPsynthase. How would cellular respiration be affected?

Electrons could not be passed down the electron transportchain.

ADP could not be joined with a phosphate to form ATP.

Hydrogen ions could not be pumped into the mitochondrialintermembrane space.

The cell could not release any energy from the glucosemolecule.

Glucose could not be broken down into pyruvate.

Which molecules transport electrons from the citric acid cycleto the electron transport chain?

NADH and FADH₂

ATP and ADP

GTP and GDP

oxaloacetate and citric acid

None of the answers are correct.

Which type of reactions control the energy release from glucoseduring the citric acid cycle?

acid-base reactions

redox reactions

random reactions

All answers are correct.

None of the answers are correct.

What is the role of NAD⁺ in cellular respiration?

to receive electrons from the citric acid cycle and deliver themto the electron transport chain

to catalyze the synthesis of ATP from ADP

to accumulate in the mitochondrial intermembrane space tofacilitate ATP synthesis

to serve as a substrate to bind to acetyl-CoA at the beginningof the citric acid cycle

None of the answers are correct.

10.

Which molecule enters the citric acid cycle?

glucose

pyruvate

ATP

acetyl-CoA

None of the answers are correct.

apricotgoose897

1. C6H12O6 + 6O2 Ã 6CO2 + 6H2O Glucose is being _____ to _____ and oxygen is being _____ to _____. a. oxidized; carbon dioxide; reduced; water b. oxidized; water; reduced; carbon dioxide Incorrect c. reduced; water; oxidized; carbon dioxide d. reduced; carbon dioxide; oxidized; water

2. Which of the following stages in the breakdown of the piece of toast you had for breakfast generates the most ATP? a. the citric acid cycle Incorrect b. glycolysis c. the digestion of starch to glucose d. oxidative phosphorylation

3. Which of the following is a catabolic pathway? a. gluconeogenesis b. production of starch Incorrect c. glycolysis d. production of glycogen

4. Per molecule, the oxidation of a fatty acid results in _____ energy storage than the oxidation of glucose. a. the same b. less Incorrect c. more d. None of the above; fatty acids cannot be oxidized.

5. Of the eight reactions in the citric acid cycle, how many are oxidation-reduction reactions? a. one b. two c. three d. four e. five

6. The process of glycolysis transfers the energy in sugar molecules into the stored energy of activated carriers such as ATP and NADH. Which statement is true about glycolysis? a. Only eukaryotic organisms do glycolysis. b. The ten reactions of glycolysis are identical in all eukaryotic cells, but are different in prokaryotic cells due to their diverse metabolic strategies. Incorrect c. The ten reactions of glycolysis are identical in almost all living organisms on the planet. d. There are many ways to oxidize glucose and transfer energy into activated carriers; the diversity of organisms on the planet is reflected in the diversity of their mechanisms of glycolysis.

7. Referring to the sentence completed in #13 above, the purpose of the latter reaction (in active muscle cells) is to _____. a. accumulate lactic acid in the cell, which provides additional energy for the production of ATP Incorrect b. slow down glycolysis until oxygen becomes available again c. accelerate the Krebs cycle, which will restore the oxygen supply d. regenerate NAD+ so that glycolysis can continue in the absence of oxygen

8. What are the products of the citric acid cycle? a. carbon dioxide, NADH, FADH2 and GTP b. carbon dioxide, NAD+, FAD and GTP c. NADH, FADH2, and acetyl CoA Incorrect d. carbon dioxide and water

9. Which statement is true about enzymes? a. Enzymes catalyze chemical reactions because they make inherently endergonic reactions exergonic. b. Each enzyme in a cell can catalyze many different chemical reactions involving chemically different substrates. Incorrect c. Enzymes can only catalyze reactions in one direction. d. After catalyzing a reaction, an enzyme is permanently altered and can no longer bind substrate. e. None of the above is true. 1

10. âG measures the change in free energy in a system as it converts reactant (Y) into product (X). When [X] = [Y], âG is equal to _____. a. RT b. ÎGÂ° + RT c. ÎGÂ° d. ln [X]/[Y]

11. The âGÂ° for the hydrolysis of ATP is -7.3 kcal/mol. What conditions in the cell would lead to a âG for ATP hydrolysis of -11 kcal/mol? a. An increase in the ATP/ADP ratio b. An increase in anabolic activity in the cell c. Transport of ATP out of the compartment where the reaction is occurring Incorrect d. A decrease in the ATP/ADP ratio

12. In the first reaction of glycolysis, a phosphate transferred from ATP to a glucose molecule to produce glucose-6-phosphate and ADP. Under conditions existing in the cell, the free energy change for this reaction is -8.0 kcal/mole. Which of the following statements is true? Recall that the âGÂ° for the hydrolysis of ATP is -7.3 kcal/mol. a. Glucose concentrations in the cell are always low, making this reaction more favorable than would be expected. b. This reaction is less favorable than would be expected, given the âGÂ° of ATP hydrolysis. c. Glucose-6-P concentrations in the cell are very low relative to glucose concentrations, making the reaction more favorable than would be expected. d. The âGÂ° for the addition of a phosphate to glucose must be -0.7 kcal/mole.

13. Protein E can bind to two different proteins, S and I. The binding reactions are described by the following equations and values: E + S --> ES Keq for ES = 10 E + I> EI Keq for EI = 2 Given the equilibrium constant values, which one of the following statements is true? a. When S is present in excess, no I molecules will bind to E. b. E binds I more tightly than S. c. The binding energy of the ES interaction is greater than that of the EI interaction. d. Changing an amino acid on the binding surface of I from a basic amino acid to an acidic one will probably make the free energy of association with E more negative.

BCH 3120 Lecture Notes - Lecture 3: Citric Acid Cycle, Oxidative Phosphorylation, Oxaloacetic Acid

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