Unit 11: Citric Acid Cycle simple

How much ATP per glucose?

32 ATP/glucose

How much ATP from pyruvate?

12.5 ATP/pyruvate

How much ATP did glycolysis provide total?

7 ATP

How much ATP per Acetyl CoA?

10 ATP/acetyl CoA through CAC

How much ATP per NADH?

2.5 ATP/NADH by oxidative phosphorylation

How much ATP per FADH₂?

1.5 ATP/FADH₂ by oxidative phosphorylation

What is the cost from pyruvate to acetyl CoA?

2.5 ATP (NAD+ → NADH)

How many of the 5 steps in PDH complex are catalyzed by dehydrogenase enzymes?

4 steps

What is the conversion of pyruvate to acetyl CoA catalyzed by?

Pyruvate dehydrogenase (PDH) complex

What enzymes are required for PDH complex?

3 enzymes- 2 dehydrogenase, 1 transferase

What are the stoichiometric factors for PDH complex?

CoA-SH & NAD+

What are the regenerated factors in PDH complex?

TPP, lipoate, & FAD

Why is the PDH complex an irreversible step?

• Loss of CO₂

• Highly exothermic

• Body cannot convert acetyl CoA to glucose

What is the role of thiamine in PDH complex?

• Acidic C & creates electron sink

• Allows for decarboxylation of substrate

• Regenerated in next step

What is the role of lipoic acid in PDH complex?

• Prosthetic group linked to Lysine

• Very flexible

• Undergoes ox/red

• Regenerated in subsequent step

• Can transfer acyl groups

What is the role of Coenzyme A (CoA-SH) in PDH complex?

• Carrier of acyl groups

• Creates thioester/acetyl CoA

What are the substrates for PDH complex?

Pyruvate, CoA-SH, & NAD+

What are the products of PDH complex?

Acetyl-CoA, NADH, CO₂

What does the PDH complex mechanism match in the CAC?

Step 4 with alpha-ketoglutarate dehydrogenase

What does enzymatic control of PDH complex involve?

• Product feedback ratios: ATP/ADP, Acetyl CoA/CoA-SH, NADH/NAD+

• Reversible phosphorylation of Serine residue in E₁

• Increased [pyruvate], the substrate causes stimulation of the enzyme

• Hormones (vasopressin, epinephrine) & alpha-adrenergic agonists (norepinephrine) stimulate PDH complex by increasing [Ca++]

• Increase [Ca++] causes stimulation of enzyme

  • Signal for excitation-contraction coupling

• Insulin (favors storage of fuel sources) stimulates (acetyl CoA can → fatty acids)

How many steps does the CAC have?

8 steps

How many steps are irreversible in the first 4 steps of CAC?

3 steps, so cycle only moves in 1 direction

What happens in step 1 of CAC?

Acetyl CoA (2 carbons) is added to oxaloacetate (4 carbons)

What happens in steps 3 & 4 of CAC?

CO₂ is “lost”

What is the net input of carbon for gluconeogenesis?

There is none

What are the steps 5-8 for CAC?

Readily reversible

What are steps 6-8 extremely similar to?

Steps 1-3 in fatty acid oxidation

What starts the cycle for CAC?

1 step (no resources lost or gained)

How many steps of CAC involve dehydrogenase enzymes & produce NADH or FADH₂?

4 steps

How many steps directly produce GTP which can be converted to ATP?

1 step

How many steps are required to “set up” an oxidation (no resources lost or gained)?

2 steps

What is needed for 1st step with Acetyl CoA + Oxaloacetate → Citrate?

• Citrate synthase (no ATP required)

• Very large -deltaG: irreversible

• Starts cycle

What is needed for 2nd step with Citrate → Isocitrate?

• Aconitase

• Reversible dehydration/hydration that moves position of oxygen atom

• Allows oxygen to be available for oxidation in next step

• Set up step: no resources gained or lost

What is needed for 3rd step with Isocitrate → alpha-ketoglutarate?

• Isocitrate dehydrogenase

• CO₂ is lost in ox/red reaction: irreversible

• NADH gained

What is needed for 4th step with alpha-ketoglutarate → Succinyl CoA?

• alpha-ketoglutarate dehydrogenase

• CO₂ is lost in ox/red reaction: irreversible

• Mechanism is identical to PDH complex

• Gains NADH

• Requires TPP, lipoic acid, FAD, CoA-SH, NAD+

What are the purposes of steps 5-8 in CAC?

Regenerate oxaloacetate

What are steps 5-8 in CAC all & many?

• All reversible

• Many linked to other pathways

What is needed for 5th step with Succinyl CoA → Succinate?

• Succinyl CoA Synthetase (ATP involved)

• CoA-SH regenerated

• Requires Histidine for mechanism

• GTP gained (can be converted to ATP)

• Substrate level phosphorylation

What are steps 6 & 7 of CAC?

Highly stereospecific

What is needed for 6th step with Succinate → Fumarate?

• Succinate dehydrogenase

• FADH₂ gained

What does C-O → C=C have?

NADH

What does C-C → C=C have?

FADH₂

What is needed for 7th step with Fumarate → Malate?

• Fumarase

• Reversible hydration/dehydration

• Set up step: no resources gained or lost

What is needed for 8th step with Malate → Oxaloacetate?

• Malate dehydrogenase

• NADH gained

What steps in CAC generate 1 NADH each?

Steps 3, 4, & 8

• Oxidize C-O bond

What step in CAC generates 1 FADH₂?

Step 6

• Oxidize C-H bond

What step in CAC generates 1 ATP?

Step 5

What is the purpose of anaplerotic reactions?

Replenish CAC intermediates if they are low/diminshed

• Intermediates can be used (“drained out”) for other pathways

What enzyme is used for anaplerotic reactions?

Aminotransferase (transfers a nitrogen)

• Converts an amino acid to keto acid (CAC or glycolysis intermediate)

What are the 3 anaplerotic reactions?

• alpha-ketoglutarate → glutamate (Glu)

• Oxaloacetate → asparate (Asp)

• Pyruvate → Alanine (Ala)

What is the role of biotin?

• Carrier of CO₂ (HCO₃-)

  • ATP is required to add CO₂ (HCO₃-) to biotin

• Prosthetic group covalently bound to Lysine (like lipoic acid)

What type of enzyme is biotin?

Carboxylase (adds CO₂-)

What enzyme can convert pyruvate to oxaloacetate?

Carboxylase

What does the enzymatic control of CAC involve?

Steps 1, 3, & 4 (irreversible steps)

What is the CAC stimulated by?

• Increased concentrations of ADP (substrate)

  • Signals that fuel sources are getting depleted

• Calcium (signal of muscle excitation-coupling & energy need)

What is the CAC inhibited by?

• Succinyl CoA (product of step 4: after 3 irreversible steps)

• ATP & NADH (cell has sufficient fuel supplies)

• Citrate (step 1- product feedback inhibition)