Acetyl-CoA + Oxaloacetate โ Citrate: Committing to the Cycle!
Acetyl-CoA (Cโ)
CHโ
C=O
S-CoA
O
2-Carbon fuel High-energy thioester
๐
Citrate Synthase
Oxaloacetate (Cโ)
C=O
CHโ
CHโ
C=O
O
O
O
4-Carbon acceptor Regenerated later
โก Energy Investment: Thioester bond hydrolysis
The high-energy thioester bond in Acetyl-CoA powers this condensation
๐ฎ INSERTING YOUR TOKEN!
Acetyl-CoA is your "game token" - you insert it into the machine (citrate synthase) where it merges with the existing 4C platform.
This forms a 6C citrate molecule and commits you to playing through the entire cycle!
๐ฏ Key Points:
โ C-C bond formation - only step that builds a bond
โ Irreversible - commitment step to the cycle
โ Oxaloacetate acts as a catalyst (regenerated)
โ Energy from thioester bond makes reaction favorable
๐ Challenge Questions - Stage 1
Q1: Why is this step irreversible?
Q2: What happens to the CoA group?
Q3: Why is oxaloacetate called a catalyst?
๐ Stage 1 Complete! Score: 0/30
Stage 2: The Isomerization Shuffle ๐
Citrate โ Isocitrate: Getting Ready for Oxidation!
Citrate (Cโ)
COOH
CHโ
COH
CHโ
COOH
CHโ
O
O
O
O
6-Carbon Tertiary alcohol (unstable)
๐ฒ
Aconitase
Isocitrate (Cโ)
COOH
CHโ
CH
CHโ
COOH
CHโ
O
O
O
O
OH
6-Carbon Secondary alcohol (oxidizable!)
โก Energy Status: Near equilibrium
Two-step process via cis-aconitate intermediate
๐ง ADJUSTING THE SPARK PLUG!
Citrate's hydroxyl group is in the wrong position (tertiary). Aconitase "moves" it via dehydration then rehydration,
converting it to isocitrate's secondary alcohol - now perfectly positioned for oxidation!
๐ฏ Key Points:
โ Isomerization: Same formula, different structure
โ Moves OH group from tertiary โ secondary carbon
โ Involves dehydration then rehydration
โ Activates molecule for the first oxidation step
๐ Challenge Questions - Stage 2
Q1: Why is this rearrangement necessary?
Q2: What is the intermediate?
Q3: This reaction involves...
๐ Stage 2 Complete! Score: 0/60
Stage 3: First Oxidation Strike โก
Isocitrate โ ฮฑ-Ketoglutarate: First NADH & COโ!
Isocitrate (Cโ)
COOH
CHโ
CH(OH)
CHโ
COOH
CHโ
O
O
O
O
OH
6-Carbon Ready to oxidize
โก
Isocitrate Dehydrogenase
ฮฑ-Ketoglutarate (Cโ )
COOH
C=O
CHโ
COOH
CHโ
O
O
O
O
5-Carbon ฮฑ-keto acid
+1
NADH + Hโบ
โ
COโ Released
๐ FIRST ENERGY HARVEST!
โ Oxidation: Isocitrate loses 2 electrons
โ Decarboxylation: One carbon leaves as COโ
โ NADโบ reduced: To NADH + Hโบ (energy carrier)
โ Product: ฮฑ-Ketoglutarate (5 carbons)
๐ฏ FIRST PRIZE CLAW!
You're playing a claw machine (isocitrate dehydrogenase). You grab the electron prize (NADH) and drop the waste carbon (COโ) into the chute.
You walk away with one less carbon but an energy-rich NADH battery!
๐ Challenge Questions - Stage 3
Q1: What is the carbon count change?
Q2: What is the energy product?
Q3: What type of reaction is this?
๐ Stage 3 Complete! NADH: 0 | COโ: 1
Stage 4: Second Oxidation Blast ๐ฅ
ฮฑ-Ketoglutarate โ Succinyl-CoA: Second NADH & COโ!
ฮฑ-Ketoglutarate (Cโ )
COOH
C=O
CHโ
COOH
CHโ
O
O
O
O
5-Carbon ฮฑ-keto acid Next to be oxidized
๐ฅ
ฮฑ-Ketoglutarate Dehydrogenase Complex
Succinyl-CoA (Cโ)
COOH
CHโ
CHโ
COOH
S-CoA
O
O
O
O
4-Carbon High-energy thioester
+1
NADH + Hโบ
โ
COโ Released
+
CoA-SH Added
๐ SECOND ENERGY CAPTURE!
โ Oxidative decarboxylation (like Stage 3)
โ Uses same enzyme complex as pyruvate dehydrogenase
โ Produces high-energy thioester (succinyl-CoA)
โ 2nd NADH and 2nd COโ per acetyl-CoA
๐๏ธ SECOND TRASH COMPACTOR!
Another carbon gets kicked out as COโ waste, and you grab another NADH energy prize!
The enzyme complex is identical to the one that converts pyruvate โ acetyl-CoA.
๐ Challenge Questions - Stage 4
Q1: Total COโ released so far?
Q2: What makes succinyl-CoA special?
Q3: This enzyme is similar to...
๐ Stage 4 Complete! NADH: 0 | COโ: 2
Stage 5: Energy Cash-Out ๐ฐ
Succinyl-CoA โ Succinate: The TCA Cycle's Only Direct ATP!
Succinyl-CoA (Cโ)
COOH
CHโ
CHโ
COOH
S-CoA
O
O
O
O
High-energy thioester Ready to release energy
๐ธ
Succinyl-CoA Synthetase
Succinate (Cโ)
COOH
CHโ
CHโ
COOH
O
O
O
O
Standard acid Lower energy
+1
GTP โ GDP + Pi
โ
CoA-SH Released
๐ SUBSTRATE-LEVEL PHOSPHORYLATION!
โ Thioester bond energy transferred to GTP
โ GTP can phosphorylate ADP โ ATP (nucleoside diphosphate kinase)
โ Only direct energy currency produced in TCA cycle
โ Equivalent to 1 ATP per acetyl-CoA
๐ฆ CASHING A CHECK!
Your succinyl-CoA is a cashier's check (high-energy thioester).
The bank (succinyl-CoA synthetase) cashes it into GTP cash.
You can immediately convert GTP to ATP at the currency exchange!
๐ Challenge Questions - Stage 5
Q1: What is unique about this step?
Q2: GTP is equivalent to...
Q3: The CoA is...
๐ Stage 5 Complete! GTP: 1 | NADH: 0
Stage 6: FADHโ Battery Charge ๐
Succinate โ Fumarate: Membrane-Bound Oxidation!
Succinate (Cโ)
COOH
CHโ
CHโ
COOH
O
O
O
O
Saturated dicarboxylic acid No double bonds
โก
Succinate Dehydrogenase (Complex II)
Fumarate (Cโ)
COOH
=CH
=CH
COOH
O
O
O
O
Trans-unsaturated Double bond formed
+1
FADHโ
โ
FAD is reduced
๐ THIRD ENERGY CAPTURE!
โ Dehydrogenation: Removes 2 H atoms (2eโป + 2Hโบ)
โ Creates trans double bond (stereospecific)
โ FAD reduced โ FADHโ (electron carrier)
โ Enzyme is Complex II of electron transport chain
๐ RECHARGING THE BATTERY!
Succinate dehydrogenase is like a wireless charging pad. As succinate passes over it,
2 hydrogen atoms (electrons) jump onto FAD, recharging it to FADHโ.
This "battery" will later power the electron transport chain!
๐ Challenge Questions - Stage 6
Q1: What is the key change in succinate?
Q2: Where is this enzyme located?
Q3: FADHโ holds...
๐ Stage 6 Complete! FADHโ: 1 | NADH: 0
Stage 7: Water Addition Splash ๐ง
Fumarate โ L-Malate: Stereospecific Hydration!
Fumarate (Cโ)
COOH
=CH
=CH
COOH
O
O
O
O
Trans double bond No OH groups
๐ง
Fumarase
L-Malate (Cโ)
COOH
CH(OH)
CHโ
COOH
O
O
O
O
OH
Hydroxy-dicarboxylic acid OH added to C2
โก Energy Status: Near equilibrium
Adds HโO across double bond in trans configuration
๐ฆ FILLING THE TANK!
Fumarase is like a water station at a car wash. As fumarate drives through,
it gets a precise water spray added to its double bond, becoming malate -
perfectly positioned for the final oxidation step!
๐ฏ Key Points:
โ Hydration: Adds HโO across double bond
โ Stereospecific: Only L-malate formed (not D-)
โ No energy carriers produced
โ Prepares molecule for final oxidation
๐ Challenge Questions - Stage 7
Q1: What is added to fumarate?
Q2: This step is...
Q3: Energy carriers produced?
๐ Stage 7 Complete! Malate ready for final step!
Stage 8: Final Oxidation & Regeneration ๐
Malate โ Oxaloacetate: Third NADH & Cycle Complete!
L-Malate (Cโ)
COOH
CH(OH)
CHโ
COOH
O
O
O
O
OH
Hydroxy-dicarboxylic acid Ready for final oxidation
๐
Malate Dehydrogenase
Oxaloacetate (Cโ)
C=O
CHโ
CHโ
C=O
O
O
O
O
4-Carbon keto-dicarboxylic acid Cycle can restart!
+1
NADH + Hโบ
โป
Oxaloacetate regenerated!
๐ CYCLE COMPLETE!
โ 3rd NADH produced per acetyl-CoA
โ Oxaloacetate regenerated (catalyst restored)
โ Cycle ready for next acetyl-CoA
โ Net reaction complete!
โญ COMING FULL CIRCLE!
You've come full circle! Malate dehydrogenase is the exit ramp that drops you back at the starting line (oxaloacetate),
but not before grabbing one last NADH prize. The platform is ready for the next acetyl-CoA player!
๐ Challenge Questions - Stage 8
Q1: Why is this step highly favorable?
Q2: Total NADH per acetyl-CoA?
Q3: The cycle is now...
๐ Stage 8 Complete! NADH: 0 | FADHโ: 0
Stage 9: Energy Accounting ๐
Counting the Total Energy Harvested per Acetyl-CoA!
After completing the cycle, you count your winnings: 3 high-value NADH bonds (7.5 ATP),
1 FADHโ bond (1.5 ATP), and 1 cash GTP (1 ATP). Total โ 10 ATP per acetyl-CoA!
Not bad for one spin through the cycle!
๐ Challenge Questions - Stage 9
Q1: Why does NADH produce more ATP than FADHโ?
Q2: What happened to the 2 carbons from acetyl-CoA?
Q3: The cycle is efficient because...
๐ Energy Audit Complete! Total โ 10 ATP per cycle!
Stage 10: Master Control Panel ๐๏ธ
Regulation & Integration: The Power Plant Command Center!
Regulatory Enzymes
Citrate Synthase
Isocitrate DH
ฮฑ-Ketoglutarate DH
Three control points Rate-limiting steps
๐๏ธ
Regulatory Network
Key Regulators
Inhibitors: ATP, NADH, succinyl-CoA
Activators: ADP, Caยฒโบ
Feedback inhibition
Allosteric control Responds to cell energy status
โก Regulation Principle: Energy Status
High [ATP]/[NADH] = "Energy rich" โ Slow down cycle
High [ADP]/[Caยฒโบ] = "Energy needed" โ Speed up cycle
๐ฏ Key Regulatory Features:
โ Citrate Synthase: Inhibited by ATP, NADH, succinyl-CoA (feedback)
โ Isocitrate DH: Activated by ADP/Caยฒโบ, inhibited by ATP/NADH
โ ฮฑ-Ketoglutarate DH: Inhibited by ATP, NADH, succinyl-CoA
โ Amphibolic: Both catabolic (energy) and anabolic (biosynthesis)
The TCA cycle is like a smart factory that adjusts production based on inventory levels.
When ATP "inventory" is full, production slows down. When ADP "orders" pile up, production accelerates.
The factory also produces "spare parts" (intermediates) for other factories (biosynthetic pathways)!
โ Stage 9: Energy accounting: ~10 ATP per acetyl-CoA
โ Stage 10: Regulation: Allosteric control by ATP/NADH/ADP/Caยฒโบ
๐ Amphibolic Powerhouse: The TCA cycle is both energy-generating (catabolic) and provides biosynthetic precursors (anabolic) for amino acids, fatty acids, and more!