Chemotrophic Energy Metabolism: Glycolysis and Fermentation

Introduction to Chemotrophic Energy Metabolism

Chemotrophic energy metabolism encompasses the cellular processes by which organisms extract energy from chemical compounds, primarily through the breakdown of organic molecules. This energy is essential for cellular activities such as movement, transport, and biosynthesis.

• Chemotrophs obtain energy by ingesting or engulfing food, which is then catabolized to release energy.

• All cellular chemical reactions constitute metabolism, organized into metabolic pathways.

Metabolic Pathways

Anabolic and Catabolic Pathways

Metabolic pathways are divided into two main types: anabolic and catabolic.

• Anabolic pathways synthesize cellular components (e.g., starch, glycogen), increase order, decrease entropy, and are endergonic (require energy).

• Catabolic pathways break down cellular constituents (e.g., glucose hydrolysis), decrease order, increase entropy, and are exergonic (release energy).

• Catabolism can be aerobic (with oxygen) or anaerobic (without oxygen).

ATP: The Primary Energy Molecule in Cells

Structure and Function of ATP

Adenosine triphosphate (ATP) is the main energy currency in cells, powering movement, transport, and enzymatic reactions.

• ATP consists of adenine, ribose, and three phosphate groups linked by phosphoanhydride bonds.

• ATP can be hydrolyzed to ADP and inorganic phosphate (Pi), releasing energy: Standard free energy change: kcal/mol.

• Other high-energy molecules include GTP, creatine phosphate, and reduced coenzymes (e.g., NADH).

Why ATP Hydrolysis Is Highly Exergonic

• Charge repulsion between negatively charged phosphate groups.

• Resonance stabilization of hydrolysis products (ADP and Pi).

• Increased entropy and solubility of products after hydrolysis.

ATP as an Intermediate Energy Carrier

• ATP can donate or accept phosphate groups due to its intermediate free energy of hydrolysis.

• ATP/ADP system is central for energy conservation and transfer in cells.

Chemotrophic Energy Metabolism and Biological Oxidation

Oxidation and Reduction in Cells

Energy-yielding oxidative reactions are central to chemotrophic metabolism. Biological oxidations typically involve the removal of both electrons and protons (dehydrogenation).

• Oxidation: Removal of electrons (often as hydrogen atoms).

• Reduction: Addition of electrons (often as hydrogen atoms).

• Enzymes called dehydrogenases catalyze these reactions.

• Coenzymes such as NAD+ serve as electron acceptors, becoming reduced to NADH.

Glucose Catabolism: Glycolysis and Fermentation

Overview of Glycolysis

Glycolysis is a universal pathway that converts glucose to pyruvate, generating ATP and NADH. It occurs in the cytosol and is common to both aerobic and anaerobic organisms.

• Net yield: 2 ATP and 2 NADH per glucose molecule.

• Divided into three phases:

  • Phase I: Preparation and cleavage (Gly-1 to Gly-5)

  • Phase II: Oxidation and ATP generation (Gly-6 and Gly-7)

  • Phase III: Pyruvate formation and ATP generation (Gly-8 to Gly-10)

Fate of Pyruvate: Aerobic vs. Anaerobic Conditions

• In the presence of oxygen: Pyruvate is oxidized to acetyl-CoA and enters the citric acid cycle (aerobic respiration).

• In the absence of oxygen: Pyruvate undergoes fermentation to regenerate NAD+.

Types of Fermentation

• Lactate fermentation: Pyruvate is reduced to lactate (e.g., in muscles, some bacteria).

• Alcoholic fermentation: Pyruvate is converted to ethanol and CO2 (e.g., in yeast, plant cells).

Regulation of Glycolysis and Gluconeogenesis

Allosteric Regulation

• Key enzymes are regulated by allosteric effectors (activators or inhibitors).

• AMP activates glycolysis, inhibits gluconeogenesis; acetyl-CoA does the opposite.

• Fructose-2,6-bisphosphate is a major regulator, activating glycolysis and inhibiting gluconeogenesis.

Alternative Substrates and Gluconeogenesis

Alternative Substrates for Glycolysis

• Other sugars (e.g., fructose, mannose) and glycerol can enter glycolysis after conversion to intermediates.

• Polysaccharides (starch, glycogen) are broken down to glucose-1-phosphate, which enters glycolysis.

Gluconeogenesis

• Gluconeogenesis synthesizes glucose from non-carbohydrate precursors (e.g., pyruvate, lactate).

• Not a simple reversal of glycolysis; key steps are bypassed by alternative reactions.

Clinical Relevance: Cancer Cell Metabolism

Aerobic Glycolysis in Cancer Cells

• Cancer cells often ferment glucose to lactate even in the presence of oxygen (Warburg effect).

• This supports rapid growth by providing biosynthetic precursors.

• Positron emission tomography (PET) can detect increased glucose uptake in tumors using radiolabeled glucose analogs.

Summary Table: Comparison of Glycolysis and Fermentation

Additional info: The above notes integrate and expand upon the provided lecture content, including definitions, examples, and regulatory mechanisms relevant to glycolysis and fermentation. Images included directly illustrate the chemical changes in oxidation and reduction reactions central to cell metabolism.