Glycolysis

Introduction to Cell Biology & Biochemistry

Metabolism Overview

• Metabolism: The complex network of chemical reactions occurring within cells to modify ingested chemicals into necessary compounds to sustain life.

• Energy metabolism: The mechanisms the body uses to obtain and expend energy.

Types of Metabolism

• Anabolism:

  • Definition: The process of building larger compounds from smaller molecules.

  • Characteristics: Requires energy input for the synthesis of complex molecules.

• Catabolism:

  • Definition: The breakdown of larger molecules into smaller units.

  • Characteristics: Energy is released during this transformation.

Autotrophs vs. Heterotrophs

• Autotroph:

  • Definition: Organisms that synthesize their own food using inorganic sources, either through photosynthesis or chemosynthesis.

• Heterotroph:

  • Definition: Organisms that cannot synthesize their own food and must consume organic compounds produced by other organisms.

Key Pathways in Metabolism

• Major metabolic pathways discussed:

  • Glycolysis

  • Krebs cycle

  • Oxidative phosphorylation

  • Fermentation

Metabolic Pathways and Enzymes

• All reactions in a metabolic pathway are catalyzed by enzymes, which:

  • Serve as biological catalysts that facilitate reactions.

  • Regulate and determine which pathways are active under specific conditions.

• Pathways significant for:

  • Energy production (e.g., glycolysis and the Krebs cycle)

  • Biosynthesis of essential molecules, such as DNA bases and amino acids.

Enzymatic Limitations

• Not all organisms can perform every chemical conversion due to the absence of certain enzymes.

• Example: Humans cannot synthesize lysine and must obtain it through diet.

• Despite differences, core metabolic processes like the Krebs cycle are conserved across diverse life forms from bacteria to humans.

Cellular Respiration

Stages of Cellular Respiration

1. Glycolysis:

   • Converts glucose ($C<em>6H</em>{12}O<em>6$) into pyruvate ($CH</em>3COCOO^-$) through 10 enzyme-mediated steps.

   • Process occurs in the cytosol and does not require oxygen.

   • Free energy from glucose breakdown generates ATP and NADH.

2. Acetyl-CoA production:

   • Pyruvate is transformed into Acetyl-CoA in the presence of oxygen.

3. Krebs cycle:

   • Acetyl-CoA enters the Krebs cycle (Citric Acid Cycle), where it is further oxidized, generating CO2, NADH, and FADH2.

4. Electron transport and oxidative phosphorylation:

   • High-energy carriers (NADH and FADH2) transfer electrons through a series of complexes resulting in ATP production and water formation as byproducts.

Definitions and Key Concepts

• Gibbs Energy ($ΔG$):

  • Definition: Indicates the spontaneity of a chemical reaction. Calculated using the equation:
    $ΔG = ΔH - TΔS$

  • Where:

    • $ΔH$ = change in enthalpy (heat exchange)

    • $T$ = temperature in Kelvin

    • $ΔS$ = change in entropy (degree of disorder)

• Exergonic Reactions:

  • Characteristics:

  • ΔG < 0

  • Spontaneous, releases energy.

• Endergonic Reactions:

  • Characteristics:

  • ΔG > 0

  • Non-spontaneous, requires energy input (e.g., ATP).

Coupling Reactions

• Example Reactions:

  • ATP hydrolysis:
    $ATP + H<em>2O ightleftharpoons ADP + P</em>i$ with $ΔG = -30 ext{ kJ/mol}$

  • Formation of sucrose from glucose and fructose:
    $Glucose + Fructose + ATP <br>ightleftharpoons Sucrose + ADP + P_i$ with $ΔG = -3 ext{ kJ/mol}$

  • Reaction of glucose with fructose:
    $Glucose + Fructose <br>ightleftharpoons Sucrose$ with $ΔG = +27 ext{ kJ/mol}$

Glycolysis Mechanism

Overview

• Glycolysis is divided into two phases:

  • Energy Investment Phase: Consumes ATP in the initial steps (Steps 1 and 3)

  • Energy Payoff Phase: Produces ATP later in the pathway (Steps 7 and 10)

Steps of Glycolysis

1. Hexokinase: Glucose is phosphorylated to form Glucose-6-phosphate, consuming 1 ATP.

2. Phosphoglucose isomerase: Converts Glucose-6-phosphate into Fructose-6-phosphate.

3. Phosphofructokinase:

   • Second phosphorylation forms Fructose-1,6-bisphosphate via input of another ATP.

4. Aldolase: Cleaves Fructose-1,6-bisphosphate into Glyceraldehyde-3-phosphate and Dihydroxyacetone phosphate.

5. Triose phosphate isomerase: Interconverts glyceraldehyde-3-phosphate and dihydroxyacetone phosphate.

6. Glyceraldehyde-3-phosphate dehydrogenase: Converts glyceraldehyde-3-phosphate into 1,3-bisphosphoglycerate, generating NADH.

7. Phosphoglycerate kinase: Convert 1,3-bisphosphoglycerate into 3-phosphoglycerate, producing ATP via substrate-level phosphorylation.

8. Phosphoglycerate mutase: Converts 3-phosphoglycerate to 2-phosphoglycerate.

9. Enolase: Converts 2-phosphoglycerate into phosphoenolpyruvate.

10. Pyruvate kinase: Converts phosphoenolpyruvate into pyruvate, producing ATP.

Fate of Pyruvate

• Two main pathways for pyruvate post-glycolysis:

  • Aerobic Respiration:

  • Converted into Acetyl-CoA for entry into Krebs cycle, producing CO2 and NADH.

  • Anaerobic Conditions:

  • Converted into lactate (in muscles) or ethanol (in yeast), allowing fermentation under low oxygen.

Key Differences Between Pathways

• Glycolysis:

  • Anaerobic process, takes place in cytosol, yield of 2 ATP per glucose.

• Gluconeogenesis:

  • Energy-consuming, synthesis of glucose from non-carbohydrate precursors.

Visual Aids

• Figure of Carbon Cycling: Illustrates the interaction between autotrophic (photosynthetic) and heterotrophic domains, indicating the substantial turnover of carbon (approximately $4 × 10^{11}$ metric tons annually).

• Thermodynamics: Key principles governing reaction spontaneity and energy transfer.

Useful Glossary

• Metabolism: The entirety of chemical processes vital for sustaining life.

• Energy metabolism: The dynamics of energy acquisition and consumption.

• Exergonic: Reaction characterized by release of energy, ΔG < 0.

• Endergonic: Reaction requiring energy input, ΔG > 0.

• Autotroph: Self-sustaining organisms, harnessing inorganic substances for food.

• Heterotroph: Consumers dependent on organic materials produced by others.

• All content reflects comprehensive principles key to cell biology and biochemistry, offering insights into fundamental life processes corresponding to metabolic functions.