Comprehensive Notes on Glycolysis and its Regulation

Enzymes Involved in Glycolysis

Quiz questions involve matching enzymes to their respective reaction steps in glycolysis. Key enzymes include: Hexokinase, Phosphoglycerate kinase, Pyruvate kinase, Glyceraldehyde 3-phosphate dehydrogenase, Aldolase, Phosphoglycerate mutase, Enolase, Phosphoglucose isomerase, Triosephosphate isomerase, and Phosphofructokinase. These enzymes catalyze specific reactions within the glycolytic pathway.

Control of Glycolysis

The glycolytic pathway is carefully regulated based on the energy needs of the cell.

  • In muscle cells, ATP and AMP levels signal energy availability.
  • High ATP levels indicate sufficient energy, inhibiting glycolysis.
  • Low energy is signaled by increased AMP levels, activating glycolysis to produce more ATP.

Why AMP Signals Low Energy

  • During exercise, ATP is hydrolyzed to ADP and inorganic phosphate (Pi).

  • Adenylate kinase catalyzes the transfer of phosphate from one ADP to another, producing ATP and AMP.

    ADP+ADP⇌ATP+AMPADP + ADP \rightleftharpoons ATP + AMP

  • This reaction generates additional ATP, and AMP signals a low 'energy charge' in the cell.

Energy Level Regulation

  • High Energy Levels: High ATP, low ADP, low AMP. Glycolysis is inhibited.
  • Low Energy Levels: Low ATP, high ADP, high AMP. Glycolysis is stimulated.

Regulation of Glycolysis in Muscle Cells

  • Glycolysis in muscle is regulated to meet the energy demands of contraction.

  • Phosphofructokinase (PFK) is stimulated by AMP.

  • Pyruvate kinase is stimulated by fructose 1,6-bisphosphate (feedforward loop).

  • When ATP is needed, adenylate kinase generates ATP from ADP:

    ADP+ADP⇌ATP+AMPADP + ADP \rightleftharpoons ATP + AMP

  • AMP signals the low-energy state.

Additional Regulatory Mechanisms in Muscle Cells

  • Hexokinase is allosterically inhibited by glucose 6-phosphate (negative feedback).
  • PFK and pyruvate kinase are inhibited by ATP.

Regulation of Glycolysis in the Liver

  • The liver is crucial for glucose and energy control.
  • It stores glucose as glycogen when energy levels are high and releases glucose when levels are low.
  • Phosphofructokinase (PFK) is regulated according to blood glucose levels.

Regulation of PFK by Fructose 2,6-Bisphosphate (F-2,6-BP)

  • High blood glucose increases fructose 2,6-bisphosphate (F-2,6-BP) levels.
  • F-2,6-BP is distinct from fructose 1,6-bisphosphate, which is a glycolytic intermediate.
  • Citrate inhibits phosphofructokinase, while fructose 2,6-bisphosphate is a powerful activator.

How F-2,6-BP affects Glycolysis

  • F-2,6-BP activates PFK, increasing the rate of glycolysis.
  • High blood-glucose levels lead to high concentrations of fructose 6-phosphate.
  • Abundant fructose 6-phosphate results in high concentrations of fructose 2,6-bisphosphate.
  • F-2,6-BP stimulates glycolysis by increasing PFK's affinity for fructose-6-phosphate.

Regeneration of NAD+

  • Glycolysis must regenerate NAD+NAD^{+} from the metabolism of pyruvate to maintain redox balance.
  • NAD+NAD^{+} is reduced to NADH during the oxidation of glyceraldehyde 3-phosphate.
  • The final step involves regenerating NAD+NAD^{+} through pyruvate metabolism via:
    • Anaerobic conditions.
    • Fermentation.
    • Aerobic conditions.

Alcoholic Fermentation

The net result of alcoholic fermentation maintains redox balance.

Lactic Acid Fermentation

The overall net result in converting glucose to lactate in lactic acid fermentation is described. Skeletal muscles can function anaerobically for short periods until fatigue sets in, caused by lactate buildup which drops the pH.

Lactate Fermentation and Exercise

During intense exercise, muscles run out of oxygen, relying on glycolysis and lactate fermentation for ATP.
Lactic acid builds up in the muscles.

Entry Points for Galactose and Fructose

Sugars are converted into glycolytic intermediates like Glucose.

Fructose 1-Phosphate Pathway

Most ingested fructose is metabolized in the liver via the fructose 1-phosphate pathway, which bypasses phosphofructokinase, a key regulator of glycolysis. Excessive fructose consumption may lead to type II diabetes.

Galactose-Glucose Conversion Pathway

Galactose 1-phosphate acquires a uridyl group from uridine diphosphate glucose (activated glucose) and converts into UDP-glucose.
UDP-galactose is recycled to UDP-glucose.

Galactosemia

Classic galactosemia results from deficient galactose 1-phosphate uridyl transferase activity.

Symptoms of Galactosemia

Symptoms include failure to thrive, jaundice, liver enlargement (leading to cirrhosis), and cataract formation as galactitol accumulates, causing osmotic imbalance in the lens.

Lactose Intolerance

Lactose intolerance (hypolactasia) occurs because most adults lack the enzyme to degrade lactose. A mutation that prevented lactase activity from diminishing in adults was beneficial due to milk availability from dairy farming.

Energy Conversion in Glycolysis

Under anaerobic conditions, glycolysis is the primary ATP source.

Quantification of Energy Conversion

1 mole of glucose yields 2 moles of ATP. With a DG for ATP hydrolysis around −50kJ mol−1-50 kJ \, mol^{-1}, about −100kJ mol−1-100 kJ \, mol^{-1} is conserved. However, burning 1 mole of glucose releases 2870kJ2870 kJ of energy, meaning glycolysis converts only ~3.5% of the available energy.

Efficiency of Glycolysis

Full oxidation of pyruvate in the mitochondria conserves much more energy as ATP (occurs only when oxygen is present).

Glycolysis and Cancer - The Warburg Effect

Many tumors exhibit high glucose uptake and glycolysis rates, fermenting pyruvate to lactate even in the presence of oxygen (aerobic glycolysis). This is known as the Warburg effect.

Diagnostic Utility of the Warburg Effect

The Warburg effect aids in tumor diagnosis and monitoring. 2-18F-2-D-deoxyglucose (FDG), a radioactive non-metabolizable glucose analog, localizes to tumors and is observed in positron emission tomography (PET) scans.

Summary of Glycolysis

Glycolysis converts glucose into 2 pyruvate, 2 ATP, and 2 NADH via 10 reactions, each catalyzed by a different enzyme. ATP is produced by substrate-level phosphorylation. Three irreversible steps are regulated by the cell's energy demand. Glycolysis is activated in muscles when energy levels are low and inhibited when high. In the liver, it’s activated when glucose levels are high and inhibited when low. Pyruvate is fermented to lactate (anaerobic) or oxidized in mitochondria (aerobic), producing more ATP.