Enzymes, Metabolism, and Cellular Energy Processes

Enzymes: Biological Catalysts

Cells possess chemical compounds that control internal reactions. Enzymes are proteins acting as catalysts, accelerating chemical reactions without harming the cell. Each enzyme acts on a specific substrate. Enzyme names often end with the suffix -ase (e.g., maltase, lactase).

Denaturing Proteins: High temperatures break molecular links in proteins.

Metabolic Pathways

Metabolism types include:

  • Photolithotrophs: Light energy, CO2 (plants, algae, cyanobacteria).
  • Photoorganotrophs: Light energy, organic C (some bacteria).
  • Chemolithotrophs: Chemical energy, CO2 (nitrifying bacteria).
  • Chemoorganotrophs: Chemical energy, organic C (animals, fungi, protozoa, bacteria).

Enzyme Classes:

Oxidoreductases: Catalyze redox reactions (e.g., dehydrogenases).

Transferases: Transfer functional groups (e.g., transaminases, kinases).

Hydrolases: Act in food digestion (e.g., glucosidases, lipases).

Isomerases: Rearrange atoms within a molecule (e.g., epimerases).

Lyases: Remove groups without hydrolysis (e.g., decarboxylases).

Ligases: Join molecules using ATP (e.g., synthetases, carboxylases).

Krebs Cycle:

The Krebs cycle is central to aerobic cellular respiration, oxidizing acetyl-CoA to release CO2. Electrons (e-) and protons (H+) are used in subsequent oxidations, forming reducing power (NADH, FADH2) and GTP. This process occurs in the mitochondria.

Electron Transport Chain:

The final stage of respiration, where electrons from NADH and FADH2 pass through mitochondrial complexes, releasing energy to form ATP. Oxygen is the final electron acceptor, forming water.

Fermentation:

Anaerobic reactions regenerating NAD+ from NADH. Lactic fermentation produces lactate (e.g., in yogurt production). Alcoholic fermentation produces ethanol (e.g., in yeast).

Glycolysis vs. Gluconeogenesis

Gluconeogenesis (glucose synthesis) is not the reverse of glycolysis (glucose breakdown). Key differences include:

  • Pyruvate to phosphoenolpyruvate conversion requires mitochondrial enzymes.
  • Fructose 1,6-bisphosphate to fructose-6-phosphate conversion occurs in the cytosol.
  • Glucose to glucose-6-phosphate conversion occurs in the endoplasmic reticulum.

Glycogen Synthesis (Glycogenesis)

Glycogen synthesis from glucose-6-phosphate involves glucose-1-phosphate and uridine diphosphate glucose (UDP-glucose). It occurs in muscles and liver.

Fatty Acid Biosynthesis

Fatty acid synthesis differs from breakdown: it occurs in the cytosol, uses acyl carrier protein (ACP), and NADPH as a reducing agent.