Major Metabolic Pathways

Major metabolic pathways are essential for understanding how our bodies convert food into energy. These processes, like glycolysis and the citric acid cycle, play a crucial role in nutrition, biochemistry, and cellular function, impacting overall health and energy balance.

1. Glycolysis

   • Converts glucose into pyruvate, producing a net gain of 2 ATP and 2 NADH.
   • Occurs in the cytoplasm and does not require oxygen (anaerobic process).
   • Key regulatory enzymes include hexokinase, phosphofructokinase, and pyruvate kinase.

2. Citric Acid Cycle (Krebs Cycle)

   • Takes place in the mitochondria and processes acetyl-CoA to produce NADH, FADH2, and GTP.
   • Completes the oxidation of carbohydrates, fats, and proteins.
   • Regulated by the availability of substrates and the energy needs of the cell.

3. Electron Transport Chain and Oxidative Phosphorylation

   • Located in the inner mitochondrial membrane, it uses electrons from NADH and FADH2 to create a proton gradient.
   • ATP is synthesized via ATP synthase as protons flow back into the mitochondrial matrix.
   • Oxygen serves as the final electron acceptor, forming water.

4. Gluconeogenesis

   • The synthesis of glucose from non-carbohydrate precursors, primarily in the liver.
   • Key enzymes include pyruvate carboxylase and phosphoenolpyruvate carboxykinase.
   • Important for maintaining blood glucose levels during fasting or intense exercise.

5. Glycogenesis and Glycogenolysis

   • Glycogenesis is the process of converting glucose to glycogen for storage, primarily in the liver and muscle.
   • Glycogenolysis is the breakdown of glycogen back into glucose when energy is needed.
   • Regulated by hormones such as insulin (promotes glycogenesis) and glucagon (promotes glycogenolysis).

6. Pentose Phosphate Pathway

   • A metabolic pathway parallel to glycolysis that generates NADPH and ribose-5-phosphate.
   • NADPH is crucial for biosynthetic reactions and antioxidant defense.
   • Ribose-5-phosphate is essential for nucleotide synthesis.

7. Fatty Acid Synthesis

   • Occurs in the cytoplasm and involves the conversion of acetyl-CoA into fatty acids.
   • Key enzyme is fatty acid synthase, which catalyzes the elongation of fatty acid chains.
   • Requires NADPH and is regulated by insulin and energy status.

8. Fatty Acid Oxidation (Beta-oxidation)

   • Takes place in the mitochondria, breaking down fatty acids into acetyl-CoA units.
   • Produces NADH and FADH2, which enter the electron transport chain for ATP production.
   • Regulated by the availability of fatty acids and energy needs of the cell.

9. Ketogenesis

   • The production of ketone bodies from excess acetyl-CoA, primarily in the liver.
   • Occurs during prolonged fasting or low-carbohydrate diets when glucose is scarce.
   • Ketone bodies serve as an alternative energy source for tissues, especially the brain.

10. Urea Cycle

    • A series of biochemical reactions in the liver that convert ammonia to urea for excretion.
    • Helps detoxify ammonia, a byproduct of amino acid metabolism.
    • Key enzymes include carbamoyl phosphate synthetase and arginase.

11. Amino Acid Metabolism

    • Involves the synthesis and degradation of amino acids, which are the building blocks of proteins.
    • Transamination and deamination are key processes for amino acid interconversion.
    • Amino acids can be used for energy, converted to glucose, or used in biosynthesis.

12. Cholesterol Biosynthesis

    • A multi-step process primarily occurring in the liver, converting acetyl-CoA into cholesterol.
    • Key regulatory enzyme is HMG-CoA reductase, which is targeted by statin drugs.
    • Cholesterol is essential for cell membrane structure and the synthesis of steroid hormones.

13. Purine and Pyrimidine Metabolism

    • Involves the synthesis and degradation of purines (adenine and guanine) and pyrimidines (cytosine, thymine, and uracil).
    • Nucleotides are synthesized de novo or salvaged from breakdown products.
    • Important for DNA and RNA synthesis, as well as energy metabolism (ATP).

14. Photosynthesis (Calvin Cycle and Light Reactions)

    • Light reactions capture solar energy to produce ATP and NADPH, occurring in the thylakoid membranes.
    • The Calvin Cycle uses ATP and NADPH to convert carbon dioxide into glucose in the stroma.
    • Essential for converting solar energy into chemical energy, supporting life on Earth.