Anabolisme et catabolisme

• Anabolism and catabolism are the two main divisions of metabolism, the sum of all chemical reactions that occur in an organism.

Anabolism

• Anabolism refers to the set of metabolic pathways that construct molecules from smaller units.
• These processes require energy, known as an endergonic process.
• Anabolism is essential for building and maintaining the body's tissues.
• Anabolic processes include the synthesis of complex molecules such as proteins, nucleic acids, polysaccharides, and lipids from smaller precursors.
• Anabolism is driven by enzymes, which catalyze the various steps of these biosynthetic pathways.
• Anabolic pathways often utilize reducing power, typically in the form of NADPH, to drive the formation of new chemical bonds.
• Examples of anabolism include:
  • Photosynthesis: Plants convert carbon dioxide and water into glucose using energy from sunlight.
  • Protein synthesis: Amino acids are linked together to form proteins based on genetic information encoded in DNA/RNA.
  • DNA replication: Nucleotides are assembled to create new DNA molecules.

Catabolism

• Catabolism refers to the set of metabolic pathways that break down complex molecules into simpler ones.
• These processes release energy, known as an exergonic process.
• Catabolism provides the energy and building blocks needed for anabolic processes.
• Catabolic pathways involve the degradation of molecules such as carbohydrates, lipids, and proteins, often through oxidation.
• Catabolism also involves enzymes to facilitate the breakdown of complex molecules.
• Catabolic pathways often generate reducing power, typically in the form of NADH and FADH2, which can be used to produce ATP via oxidative phosphorylation.
• Examples of catabolism include:
  • Glycolysis: Glucose is broken down into pyruvate, generating ATP and NADH.
  • Beta-oxidation: Fatty acids are broken down into acetyl-CoA, NADH, and FADH2.
  • Protein catabolism: Proteins are broken down into amino acids.

Energy Coupling

• Anabolism and catabolism are coupled through the transfer of energy.
• Energy released by catabolic reactions is used to drive anabolic reactions.
• ATP serves as the primary energy currency in the cell, linking catabolism and anabolism.
• Catabolic pathways generate ATP, while anabolic pathways consume ATP.
• Reducing power (NADH, FADH2, NADPH) also plays a crucial role in linking these processes.

Regulation

• Anabolism and catabolism are tightly regulated to maintain homeostasis and respond to changing environmental conditions.
• Regulation occurs through various mechanisms, including:
  • Enzyme regulation: Allosteric control, covalent modification (phosphorylation), and enzyme synthesis/degradation.
  • Hormonal control: Hormones like insulin, glucagon, and adrenaline influence metabolic pathways.
  • Substrate availability: The concentrations of substrates and products affect the rates of metabolic reactions.
  • Genetic regulation: Gene expression of enzymes involved in metabolism is regulated.

Metabolic Pathways

• Both anabolism and catabolism involve multiple interconnected metabolic pathways.
• Metabolic pathways are sequences of chemical reactions, where the product of one reaction becomes the substrate for the next.
• Pathways can be linear, branched, or cyclic.
• Intermediates in metabolic pathways are called metabolites.
• Metabolic pathways are often compartmentalized within cells (e.g., glycolysis in the cytoplasm, oxidative phosphorylation in the mitochondria).

Amphibolic Pathways

• Some metabolic pathways can function in both anabolism and catabolism, depending on the cellular conditions.
• These pathways are referred to as amphibolic.
• A prime example is the citric acid cycle (Krebs cycle), which oxidizes acetyl-CoA (catabolic) and provides precursors for biosynthesis (anabolic).
• The citric acid cycle generates energy (ATP, NADH, FADH2) during the oxidation of acetyl-CoA.
• It also provides intermediates for the synthesis of amino acids, nucleotides, and other essential molecules.