TCA cycle reactions and regulation

TCA Cycle - The Cell's Powerhouse

• Location: Mitochondrial matrix.
• Function: Oxidizes Acetyl-CoA, generating 3 NADH, 1 FADH₂, 1 GTP, and $2CO_2$ per cycle.
• Key regulated enzymes:
  • Citrate Synthase (inhibited by ATP, citrate)
  • Isocitrate Dehydrogenase (Rate-limiting): activated by ADP; inhibited by ATP, NADH
  • α-Ketoglutarate Dehydrogenase: activated by Ca²⁺; inhibited by succinyl-CoA, NADH

⭐ Amphibolic pathway: provides precursors for other pathways like gluconeogenesis (malate) and heme synthesis (succinyl-CoA).

📌 Mnemonic: Citrate Is Kreb's Starting Substrate For Making Oxaloacetate.

TCA Cycle Steps - An Enzymatic Merry‑Go‑Round

• Location: Mitochondrial matrix.
• Process: A sequence of 8 enzymatic reactions that oxidizes acetyl-CoA.
• Initiation: Acetyl-CoA (2C) + Oxaloacetate (4C) → Citrate (6C).
• 📌 Mnemonic (Substrates): Citrate Is Krebs' Starting Substrate For Making Oxaloacetate.

⭐ The TCA cycle itself produces very little ATP directly (one GTP/ATP per cycle). Its primary role is to generate a large number of high-energy electrons in the form of NADH and FADH₂, which then fuel the major ATP production via the electron transport chain.

TCA Regulation - Speed Bumps & Green Lights

Regulated by cellular energy status. High energy (↑ATP, ↑NADH) slows the cycle; low energy (↑ADP, ↑NAD⁺) speeds it up.

• Citrate Synthase (Irreversible)
  • Inhibitors (Speed Bumps): ATP, NADH, Citrate, Succinyl-CoA.
• Isocitrate Dehydrogenase (Rate-Limiting Step)
  • Activators (Green Lights): ADP, Ca²⁺.
  • Inhibitors: ATP, NADH.
• α-Ketoglutarate Dehydrogenase (Irreversible)
  • Activators: Ca²⁺.
  • Inhibitors: NADH, Succinyl-CoA.

⭐ In active muscle, ↑Ca²⁺ not only triggers contraction but also stimulates Isocitrate Dehydrogenase and α-Ketoglutarate Dehydrogenase, boosting ATP production to meet demand.

Yield & Links - TCA's Grand Central Station

• Net Yield (per Acetyl-CoA):
  • 3 NADH
  • 1 FADH₂
  • 1 GTP (ATP equivalent)
  • 2 CO₂
• Total Energy: Approx. 10 ATP per turn via oxidative phosphorylation.

• Metabolic Crossroads: TCA intermediates are hubs for biosynthesis.
  • Citrate → Fatty Acid Synthesis
  • α-Ketoglutarate → Amino Acid Synthesis (e.g., Glutamate)
  • Succinyl-CoA → Heme Synthesis
  • Malate → Gluconeogenesis

⭐ The TCA cycle is amphibolic (both catabolic and anabolic). Anaplerotic ("filling up") reactions, like pyruvate carboxylase converting pyruvate to oxaloacetate, are vital to replenish intermediates siphoned off for biosynthesis.

• The TCA cycle occurs in the mitochondrial matrix, generating 3 NADH, 1 FADH₂, and 1 GTP per Acetyl-CoA.
• Key irreversible enzymes are Citrate Synthase, Isocitrate Dehydrogenase, and α-Ketoglutarate Dehydrogenase.
• Isocitrate dehydrogenase is the primary rate-limiting enzyme.
• The cycle is activated by indicators of low energy (ADP, Ca²⁺) and inhibited by indicators of high energy (ATP, NADH).
• It's an amphibolic pathway, supplying precursors for gluconeogenesis and heme synthesis.