Metabolism of Glucose and Energy Production

Overview of Glycolysis

• Definition: Glycolysis is the metabolic pathway that converts glucose into pyruvate, producing energy in the form of ATP.

• Reactions: The glycolytic pathway consists of 10 sequential enzymatic reactions.

Storage of Glucose

• Monosaccharide vs Polysaccharide:

  • Glucose primarily enters glycolysis as a monosaccharide but is stored in cells as polysaccharides.

  • Storage Forms:

  • Starch in plant cells.

  • Glycogen in animal cells.

• Importance of Polysaccharides:

  • Storing glucose as monosaccharides increases solute concentration, potentially causing osmotic imbalance and influx of water into cells.

  • Polysaccharides are generally insoluble in water, preventing osmotic overload.

Glycogen Storage and Breakdown

• Location of Storage:

  • Glycogen is predominantly stored in liver cells (hepatocytes) and, to a lesser extent, in skeletal muscle.

• Glycogenolysis:

  • Process of breaking down glycogen into glucose monomers. Enzyme involved: Glycogen Phosphorylase.

  • Mechanism: Phosphorylase uses inorganic phosphate to break the glycosidic bond, producing glucose 1-phosphate through a reaction called phosphorylysis (as opposed to hydrolysis).

• Conversion to Glucose 6-Phosphate:

  • Catalyzing Enzyme: Phosphoglucomutase.

  • Reaction Overview: Transfers phosphate from carbon 1 to carbon 6, forming glucose 6-phosphate, which can enter glycolysis.

Synthesis of Glucose

• Gluconeogenesis:

  • Definition: The synthesis of glucose from non-carbohydrate precursors, primarily occurring in the liver (and sometimes kidneys).

  • Precursor Examples: Three-carbon or four-carbon molecules such as pyruvate.

• The Cori Cycle:

  • Relationship between skeletal muscle and liver during intense exercise.

  • Process:

  • Skeletal muscle converts glucose to pyruvate (glycolysis), then into lactate under anaerobic conditions (lactic acid fermentation).

  • Lactate is released into the bloodstream, taken up by hepatocytes, and converted back into pyruvate, which can then be converted into glucose through gluconeogenesis.

Pathway Differences Between Glycolysis and Gluconeogenesis

• Irreversible Reactions in Glycolysis:

  • Key Enzymatic Reactions:

  • Hexokinase: Glucose to Glucose 6-phosphate.

  • Phosphofructokinase-1 (PFK-1): Fructose 6-phosphate to Fructose 1,6-bisphosphate.

  • Pyruvate Kinase: Phosphoenolpyruvate to Pyruvate.

• Gluconeogenesis Requirements:

  • To convert pyruvate back to glucose, gluconeogenesis uses different enzymes for the three irreversible steps of glycolysis.

Steps to Synthesize Glucose from Pyruvate

1. Pyruvate to Phosphoenolpyruvate (PEP):

   • Requires two steps involving:

     • Pyruvate Carboxylase: Adds CO2 to pyruvate, forming oxaloacetate using ATP energy.

     • Phosphoenolpyruvate Carboxykinase (PEPCK): Converts oxaloacetate to phosphoenolpyruvate using GTP.

2. Subsequent Reactions:

   • Incorporate glycolytic enzymes for the remaining steps, reversing glycolysis with exceptions of the irreversible reactions.

   • Total energy cost: six high-energy phosphate bonds (4 ATP and 2 GTP) to form one glucose molecule.

Regulation of Glycolysis and Gluconeogenesis

• Simultaneity:

  • Glycolysis and gluconeogenesis cannot occur simultaneously in order to avoid counterproductive energy expenditure.

• Allosteric Regulation of Key Enzymes:

  • Phosphofructokinase-1 (PFK-1) and Fructose 1,6-bisphosphatase are subject to regulation based on cellular energy needs.

  • Example of Regulation:

  • High ATP levels stimulate PFK-1 while inhibiting Fructose 1,6-bisphosphatase.

  • High AMP indicates low energy, stimulating glycolysis to generate ATP.

• Fructose 2,6-bisphosphate:

  • Key regulator of both pathways:

  • Stimulates PFK-1, promoting glycolysis.

  • Inhibits Fructose 1,6-bisphosphatase, inhibiting gluconeogenesis.

  • Formed by phosphorylation of Fructose 6-phosphate by Phosphofructokinase-2 (PFK-2).

Phosphofructokinase-2 (PFK-2)

• Dual Activity Enzyme:

  • Catalyzes the conversion of Fructose 6-phosphate to Fructose 2,6-bisphosphate and vice versa.

  • Unphosphorylated State: Activates glycolysis by forming fructose 2,6-bisphosphate.

  • Phosphorylated State: Promotes gluconeogenesis by converting fructose 2,6-bisphosphate back to fructose 6-phosphate.

• **Regulation Mechanism:

  • Reponses to hormonal signals like epinephrine can shift the balance between glycolysis and gluconeogenesis through phosphorylation states of PFK-2, leading to increased glucose production in times of need (e.g., during stress responses).