CAC/Krebs Cycle BCH400 UNR

Substrate mnemonic

Citrate is Krebs' Special Substrate for Making Oxaloacetate

Substrate mnemonic — Step 1

Citrate

Substrate mnemonic — Step 2

Isocitrate

Substrate mnemonic — Step 3

α-Ketoglutarate

Substrate mnemonic — Step 4

Succinyl-CoA

Substrate mnemonic — Step 5

Succinate

Substrate mnemonic — Step 6

Fumarate

Substrate mnemonic — Step 7

Malate

Substrate mnemonic — Step 8

Oxaloacetate

Enzyme mnemonic

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Enzyme mnemonic — Step 1

Citrate synthase

Enzyme mnemonic — Step 2

Aconitase

Enzyme mnemonic — Step 3

Isocitrate dehydrogenase

Enzyme mnemonic — Step 4

α-Ketoglutarate dehydrogenase complex

Enzyme mnemonic — Step 5

Succinyl-CoA synthetase

Enzyme mnemonic — Step 6

Succinate dehydrogenase

Enzyme mnemonic — Step 7

Fumarase

Enzyme mnemonic — Step 8

Malate dehydrogenase

At which steps is NAD⁺ reduced to NADH?

Steps 3, 4, and 8

At which step is FAD reduced to FADH₂?

Step 6

At which step is ATP produced?

Step 5

At which steps is ATP consumed directly by the Krebs cycle?

None

Reaction occurs in Step 1?

Condensation

Reaction occurs in Step 2?

Isomerization via dehydration and hydration

Reaction occurs in Step 3?

Oxidative decarboxylation

Reaction occurs in Step 4?

Oxidative decarboxylation

Reaction occurs in Step 5?

Substrate-level phosphorylation

Reaction occurs in Step 6?

Oxidation; FAD is reduced to FADH₂

Reaction occurs in Step 7?

Hydration; water is added to fumarate

Reaction occurs in Step 8?

Oxidation; NAD⁺ is reduced to NADH

How many NADH are produced per turn of the Krebs cycle?

3 NADH

How many FADH₂ are produced per turn of the Krebs cycle?

1 FADH₂

How much ATP or GTP is produced per turn?

1 ATP or GTP

How many CO₂ are released per turn?

2 CO₂

How many NADH are produced per glucose from the Krebs cycle?

6 NADH

How many FADH₂ are produced per glucose from the Krebs cycle?

2 FADH₂

How much ATP or GTP is produced per glucose from the Krebs cycle?

2 ATP or GTP

What is the purpose of the Krebs cycle?

Oxidize acetyl-CoA and capture energy in NADH, FADH₂, and ATP/GTP

What is the carbon count of citrate and isocitrate?

6 carbons each

What is the carbon count of α-ketoglutarate?

5 carbons

What is the carbon count of succinyl-CoA, succinate, fumarate, malate, and oxaloacetate?

4 carbons each

What is the mnemonic for energy-producing steps?

No Need to Ask For Number 1 — NADH at 3 and 4, ATP at 5, FADH₂ at 6, NADH at 8

What enters the Krebs cycle?

Acetyl-CoA (2 carbons)

What molecule combines with acetyl-CoA to begin the Krebs cycle?

Oxaloacetate (4 carbons)

What type of reaction releases CO₂?

Decarboxylation

What is oxidative decarboxylation?

Removal of CO₂ coupled to oxidation and reduction of NAD⁺ to NADH

Which steps are oxidative decarboxylations?

Steps 3 and 4

What happens during Step 3?

Isocitrate is oxidized and decarboxylated to α-ketoglutarate

What happens during Step 4?

α-Ketoglutarate is oxidized and decarboxylated to succinyl-CoA

What happens during Step 5?

Succinyl-CoA forms succinate and produces ATP/GTP

What happens during Step 6?

Succinate is oxidized to fumarate and FAD is reduced to FADH₂

What happens during Step 7?

Fumarate is hydrated to form malate

What happens during Step 8?

Malate is oxidized to oxaloacetate and NAD⁺ is reduced to NADH

What is substrate-level phosphorylation?

Direct production of ATP/GTP by transferring a phosphate group to ADP/GDP

What is the energy-producing reaction in Step 5 called?

Substrate-level phosphorylation

What happens to NAD⁺ when it is reduced?

It gains electrons and becomes NADH

How many CO₂ are released per glucose by the Krebs cycle?

4 CO₂

Why is the Krebs cycle considered amphibolic?

It participates in both catabolic and anabolic processes

What are the main regulatory enzymes of the Krebs cycle?

Citrate synthase, isocitrate dehydrogenase, and α-ketoglutarate dehydrogenase

What generally happens to Krebs cycle activity when NAD⁺ is available?

Oxidative reactions can proceed more readily

Where does the Krebs cycle occur in eukaryotic cells?

Primarily in the mitochondrial matrix; succinate dehydrogenase is embedded in the inner mitochondrial membrane

What is the role of NADH and FADH₂ after the Krebs cycle?

Deliver high-energy electrons to the electron transport chain

step 1

step 2

step 3

step 4

step 5

step 6

step 7

step 8

At which steps is CO₂ released?

Steps 3 and 4

What do the dehydrogenases produce?

NADH and CO₂