Overview and regulation of glycolysis

Overview and regulation of glycolysis

Overview and regulation of glycolysis

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Glycolysis Overview - Sugar Breakdown Basics

Universal, anaerobic pathway in the cytosol breaking down one glucose molecule ($C_6H_{12}O_6$) into two pyruvate molecules. This foundational process is the primary source of ATP in some cells and the initial step for cellular respiration in others. It consists of ten enzyme-catalyzed reactions.

  • Net Reaction: $Glucose + 2 NAD⁺ + 2 ADP + 2 P_i → 2 Pyruvate + 2 NADH + 2 H⁺ + 2 ATP + 2 H₂O$
  • Phases:
    • Energy Investment (Preparatory): Consumes 2 ATP.
    • Energy Generation (Payoff): Produces 4 ATP and 2 NADH via substrate-level phosphorylation.
  • Net Energy Yield: 2 ATP and 2 NADH per glucose.

Glycolysis pathway with investment and payoff phases

⭐ In erythrocytes, which lack mitochondria, glycolysis is the sole source of ATP. A genetic deficiency in Pyruvate Kinase (PK), a key glycolytic enzyme, leads to hemolytic anemia as red blood cells cannot maintain membrane integrity.

Irreversible Steps - The Commitment Issues

  • Mnemonic (📌): How Glycolysis Pushes Forward Process: Kinases (Hexokinase, Glucokinase, PFK-1, Pyruvate Kinase).

  • Three steps with large negative ΔG, acting as unidirectional gates controlling glycolytic flux.

  • 1. Hexokinase / Glucokinase (Glucose → G6P)

    • Hexokinase: Most tissues. Inhibited by its product, G6P. High affinity (low $K_m$).
    • Glucokinase: Liver & pancreas. Induced by insulin. Not inhibited by G6P. Low affinity (high $K_m$).
  • 2. Phosphofructokinase-1 (PFK-1) (Fructose-6-P → F-1,6-BP)

    • Rate-limiting step.
    • Activators: ↑ AMP, ↑ Fructose-2,6-bisphosphate.
    • Inhibitors: ↓ ATP, ↓ Citrate.

Fructose-2,6-bisphosphate is the most potent allosteric activator of PFK-1. Its levels are controlled by the bifunctional enzyme PFK-2/FBPase-2, which is regulated by insulin and glucagon.

  • 3. Pyruvate Kinase (PEP → Pyruvate)
    • Activator: ↑ Fructose-1,6-bisphosphate (feed-forward regulation).
    • Inhibitors: ↓ ATP, ↓ Alanine. Glucagon (in liver) promotes phosphorylation and inhibition.

Regulation - The On/Off Switches

  • Irreversible enzymes are the key regulatory points: Hexokinase/Glucokinase, Phosphofructokinase-1 (PFK-1), Pyruvate Kinase.
  • Rate-limiting step: PFK-1.
  • Hexokinase: Inhibited by its product, Glucose-6-P.
  • Glucokinase (Liver/Pancreas): Higher Km, induced by insulin. Not inhibited by G6P.
  • PFK-1:
    • Activators: ↑AMP, ↑Fructose-2,6-bisphosphate.
    • Inhibitors: ↑ATP, ↑Citrate.
  • Pyruvate Kinase:
    • Activators: ↑Fructose-1,6-bisphosphate (feed-forward).
    • Inhibitors: ↑ATP, ↑Alanine.

Regulation of Glycolysis Pathway

High-Yield: Fructose-2,6-bisphosphate is the most potent allosteric activator of PFK-1. Its levels are controlled by a bifunctional enzyme: Insulin activates PFK-2 (↑F-2,6-BP), pushing glycolysis forward. Glucagon activates FBPase-2 (↓F-2,6-BP), inhibiting glycolysis.

Clinical Correlates - When Sugars Go Rogue

  • Pyruvate Kinase (PK) Deficiency: Autosomal recessive disorder causing chronic hemolytic anemia. ↓ ATP production leads to rigid RBC membranes and extravascular hemolysis.
    • Presents with jaundice, splenomegaly, and pigmented gallstones.
    • Peripheral smear shows echinocytes (burr cells).
  • Arsenic Poisoning: Arsenate ($AsO_4^{3-}$) competes with inorganic phosphate, bypassing the ATP-generating step at glyceraldehyde-3-phosphate dehydrogenase (GAPDH), leading to a net zero ATP yield from glycolysis.

⭐ In states of hypoxia or ischemia (e.g., MI, shock), anaerobic glycolysis dominates. The resulting buildup of NADH shunts pyruvate to lactate, causing lactic acidosis and a high anion gap.

High‑Yield Points - ⚡ Biggest Takeaways

  • Glycolysis is a cytosolic pathway that converts glucose into 2 pyruvate, yielding a net 2 ATP and 2 NADH.
  • It is an anaerobic process, not requiring oxygen.
  • The rate-limiting enzyme is Phosphofructokinase-1 (PFK-1).
  • PFK-1 is strongly activated by fructose-2,6-bisphosphate and AMP.
  • Key inhibitors of PFK-1 include ATP and citrate, signaling energy abundance.
  • The three irreversible steps are catalyzed by Hexokinase/Glucokinase, PFK-1, and Pyruvate Kinase.

Practice Questions: Overview and regulation of glycolysis

Test your understanding with these related questions

A 45-year-old man is brought to the emergency department by ambulance after vomiting blood. The patient reports that he only ate a small snack the morning before and had not eaten anything for over 24 hours. At the hospital, the patient is stabilized. He is admitted to a surgical floor and placed on NPO with a nasogastric tube set to intermittent suction. He has been previously diagnosed with liver cirrhosis. An esophagogastroduodenoscopy (EGD) has been planned for the next afternoon. At the time of endoscopy, some pathways were generating glucose to maintain serum glucose levels. Which of the following enzymes catalyzes the irreversible biochemical reaction of this process?

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Flashcards: Overview and regulation of glycolysis

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Pyruvate kinase defiency results in increased levels of _____, which causes decreased Hb affinity for O2

TAP TO REVEAL ANSWER

Pyruvate kinase defiency results in increased levels of _____, which causes decreased Hb affinity for O2

2,3-BPG

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