Chapter 7.4: The Krebs Cycle

Question 1

What happens during Reactions 2 and 3: Isomerization of the Krebs Cycle?

Answer 1

 A water molecule is removed from one carbon, then water is added to a different carbon  As a result, an –H group and an –OH group change positions  The product is an isomer of citrate called isocitrate

Question 2

What happens during Reaction 7: The Third Oxidation of the Krebs Cycle?

Answer 2

o Reaction 7: The Third Oxidation  Succinate is oxidized to fumarate  FAD is the electron acceptor

Question 3

What happens during Reaction 5: The Second Oxidation of the Krebs Cycle?

Answer 3

• Reaction 5: The Second Oxidation
  
  • α-ketoglutarate is decarboxylated
  • The succinyl group left after the removal of CO₂ joins to coenzyme A, forming succinyl-CoA
    
    • Two electrons are extracted, and they reduce another molecule of NAD⁺ to NADH

Question 4

What happens during Reactions 8 and 9: Regeneration of Oxaloacetate of the Krebs Cycle?

Answer 4

• A water molecule is added to fumarate, forming malate

Malate is then oxidized forming a 4-carbon molecule of oxaloacetate and two electrons that reduce a molecule of NAD⁺ to NADH

Question 5

Where does the Krebs cycle take place?

Answer 5

mitochondrial matrix

Question 6

What are the products of the Krebs Cycle?

Answer 6

• The Krebs Cycle forms 3 CO₂ molecules, 4 ATP molecules, 10 NADH molecules, and 2 FADH₂ molecules

Question 7

What happens during Reaction 4: The First Oxidation of the Krebs Cycle?

Answer 7

• Isocitrate is oxidized, yielding a pair of electrons that reduce a molecule of NAD⁺ to NADH
• Then the oxidized intermediate is decarboxylated
  
  • The central carboxyl group splits off to form CO₂ yielding a 5-carbon molecule called α-ketoglutarate

Question 8

What happens during Reaction 6: Substrate-Level Phosphorylation of the Krebs Cycle?

Answer 8

 The bond between the 4-carbon succinyl group and CoA is cleaved, and the energy released drives the phosphorylation of guanosine disphosphate (GDP), forming guanosine triphosphate (GTP)  GTP can transfer a phosphate to ADP converting it into ATP  The 4-carbon molecule that remains is called succinate

Question 9

What happens during Reaction 1: Condensation of the Krebs Cycle?

Answer 9

 Citrate is formed from acetyl-CoA and oxaloacetate