Bioenergetics 2

CSUN Overview

  • California State University, Northridge

  • Course Topic: Bioenergetics

  • Quiz Date: Feb 11

Glycogen Metabolism

  • Key Concept: Glycogen metabolism is an integral part of energy production and regulation in the body.

Glycolysis: Two Phases

Phase 1: Energy Investment Phase

  • ATP Investment: 2 ATP required for initiating glycolysis.

  • Process: Glucose is phosphorylated to glucose-6-phosphate (G6P) by hexokinase.

  • Intermediate Produces: G6P is a negatively charged molecule which is trapped inside the cell.

Phase 2: Energy Generation Phase

  • Total ATP Production: 4 ATP produced, net gain of 2 ATP after investment.

  • NADH Production: 2 molecules produced.

  • End Products: 2 pyruvate or 2 lactate.

Glycolysis Process Overview

Key Steps In Glycolysis:

  1. Phosphorylation by ATP (Hexokinase): Glucose → Glucose-6-phosphate

  2. Rearrangement: Glucose-6-phosphate → Fructose-6-phosphate → Fructose-1,6-bisphosphate.

  3. Splitting to G3P: This process leads to the formation of two 3-carbon molecules (G3P).

  4. Further Reaction Steps: Includes oxidation and substrate-level phosphorylation to yield ATP and pyruvate.

Glucose Transport Proteins

  • GLUT4 Role in Muscle Cells:

    • Facilitates glucose uptake; activated by insulin during high glucose conditions or exercise.

    • Translocates from intracellular compartments to the cell surface when insulin binds to its receptor.

Importance of Phosphorylation

  • Hexokinase Function: Traps glucose in the cell due to the phosphorylated state of glucose-6-phosphate.

  • Irreversible reaction that prevents glucose from exiting the cell.

Phosphofructokinase (PFK)

  • Controlling Enzyme: Regulates the committed step of glycolysis.

  • Allosteric Regulation:

    • ATP: Inhibitory

    • AMP: Activatory

    • Citrate: Inhibitory when energy is abundant.

Reaction Velocities in Glycolysis

  • Low ATP Levels: Increased PFK activity, enhancing glycolysis.

  • High ATP Levels: Decreased PFK activity, inhibiting glycolysis.

Fate of Pyruvate

  • Aerobic Conditions: Pyruvate enters the mitochondria to support further ATP production through oxidative phosphorylation.

  • Anaerobic Conditions: Converted to lactate in the absence of oxygen, regenerating NAD+ to maintain glycolytic activity.

Glycogen Catabolism

  • Enzyme Functions:

    • Glycogen Phosphorylase: Breaks down glycogen to glucose-1-phosphate.

    • Phosphoglucomutase: Catalyzes the conversion from glucose-1-phosphate to glucose-6-phosphate.

Glucose-6-Phosphate Destinations

  • Possible Pathways:

    • Enter glycolysis or be dephosphorylated by glucose-6-phosphatase in the liver for glucose release to the blood.

Regulation of Glycogen Metabolism

  • Glycogen Phosphorylase Regulation:

    • Allosteric regulation by AMP, ADP (activators) and ATP, glucose-6-phosphate (inhibitors).

  • Glycogen Synthase Regulation: Activated by glucose-6-P; plays a crucial role during high blood glucose levels.

  • Calcium Regulation: In skeletal muscle, Ca2+ enhances glycogen breakdown during contraction.

Key Enzymatic Reactions in Glycolysis

  • Hexokinase: Glucose → Glucose 6-P

  • Glycogen Phosphorylase: Glycogen → Glucose 1-P

  • Phosphofructokinase: Fructose 6-P → Fructose 1,6-bisP

  • Pyruvate Kinase: PEP → Pyruvate

  • Lactate Dehydrogenase: Pyruvate → Lactate

Conclusion

  • Understanding glycolysis and glycogen metabolism is essential for comprehending energy production in cellular processes.