Overview of Metabolism

Overview of Metabolism

  • Definition of Metabolism: The sum total of all the chemical reactions that take place in a living cell.

    • Catabolism:

    • Description: The process of breaking down complex molecules.

    • Energy Type: Exergonic reactions; produce energy.

    • Anabolism:

    • Description: The process of combining simple molecules into complex ones.

    • Energy Type: Endergonic reactions; consume energy.

Metabolic Pathways

  • Overview of Metabolic Pathways:

    • There are thousands of chemical reactions occurring within a living cell.

    • Reactions are organized into pathways where the product of one reaction becomes the substrate (reactant) for the next.

  • Enzymes in Metabolism:

    • Every chemical reaction in metabolism is catalyzed by an enzyme.

    • Enzymes function as valves to regulate the flow of materials through the metabolic pathways.

  • Driving Nonspontaneous Reactions:

    • Nonspontaneous (thermodynamically unfavorable) reactions can be driven by coupling them with spontaneous (thermodynamically favorable) reactions.

  • TCA Cycle:

    • All metabolic pathways eventually converge into the TCA cycle.

    • Also known as: Tricarboxylic acid cycle, Krebs cycle, or Citric acid cycle.

  • ATP (Adenosine Triphosphate):

    • ATP is described as the universal currency of free energy in biological systems.

    • Acts as a common link between catabolism and anabolism.

    • Used to transfer chemical energy from catabolism to anabolism.

Three Mechanisms of ATP Generation

  1. Substrate-level phosphorylation:

    • Process: Transfers a high-energy phosphate group from an intermediate directly to ADP (Adenosine diphosphate).

  2. Oxidative phosphorylation:

    • Process: Involves the removal of electrons and their transfer through an electron transport chain to oxygen.

  3. Photophosphorylation:

    • Occurrence: Only occurs in chlorophyll-containing plant cells.

Protein and Amino Acid Metabolism

  • Protein and Amino Acid Catabolism:

    • Proteins obtained from diet or degraded cells are broken down into amino acids.

    • Prior to entering the TCA cycle, the amino group of amino acids must be removed, a process referred to as deamination.

    • Oxidation of amino acids leads to ATP production.

  • Protein and Amino Acid Anabolism:

    • Protein synthesis from amino acids occurs through translation in ribosomes.

    • Essential Amino Acids in humans: Valine (Val), Leucine (Leu), Isoleucine (Ile), Methionine (Met), Tryptophan (Trp), Phenylalanine (Phe), Threonine (Thr), Lysine (Lys), Histidine (His).

    • Essential amino acids must be obtained through diet as they cannot be synthesized by the body.

    • Nonessential Amino Acids: Can be synthesized by body cells through a process called transamination.

Nucleic Acid and Nucleotide Metabolism

  • Nucleic Acid and Nucleotide Catabolism:

    • Degradation of DNA and RNA is facilitated by various enzymes.

    • Nucleotides are broken down into nucleosides by enzymatic action.

    • Further degradation of nucleosides into bases (purines and pyrimidines) is carried out by enzymes.

    • Purines are degraded into uric acid; pyrimidines degrade into carbon dioxide (CO2), ammonia (NH3), and β-amino acids.

  • Nucleic Acid and Nucleotide Anabolism:

    • Encompasses DNA replication, repair, and recombination.

    • Involves RNA synthesis and processing.

    • Includes two pathways for nucleotide synthesis: De Novo nucleotide synthesis and the Salvage pathway for nucleotide synthesis.

Carbohydrate Metabolism

  • Carbohydrate Catabolism:

    • Enzymatic breakdown of starch and glycogen releases glucose.

    • Major pathways involved in glucose catabolism are:

    • Glycolysis

    • TCA Cycle

    • Pentose Phosphate Pathway.

  • Carbohydrate Anabolism:

    • Gluconeogenesis: The synthesis of glucose from non-carbohydrate sources.

    • Photosynthesis: The process through which glucose is synthesized in plants.

    • Glycogenesis: The storage of glucose in the form of polysaccharides.

Lipid Metabolism

  • Lipid Catabolism:

    • Known as lipolysis, involves the breakdown of triglycerides into glycerol and fatty acids.

    • Necessary for the oxidation of fatty acids in muscle, liver, and adipose tissues.

  • Lipid Anabolism:

    • Known as lipogenesis, is the process by which liver cells and adipose tissue synthesize lipids from glucose or amino acids.

    • This occurs when there is a caloric surplus, meaning more calories are consumed than required for ATP production.