IMPORTANT METABOLIC PATHWAYS (SHORT NOTES)

DR.C.GANESAN M.D.,

PROFESSOR OF MEDICINE

Definition

Metabolic pathways are a series of enzyme-catalyzed biochemical reactions occurring within cells that convert substrates into products necessary for energy production, growth, maintenance, and cellular function.

Major Metabolic Pathways

1. Glycolysis

·         Occurs in the cytoplasm.

·         Converts glucose into pyruvate.

·         Produces ATP and NADH.

·         Functions under both aerobic and anaerobic conditions.

·         Major pathway for glucose utilization.

2. Glycogenesis

·         Formation of glycogen from glucose.

·         Occurs mainly in liver and skeletal muscle.

·         Stimulated by insulin.

·         Stores excess glucose as glycogen.

3. Glycogenolysis

·         Breakdown of glycogen into glucose.

·         Stimulated by glucagon and epinephrine.

·         Maintains blood glucose during fasting.

4. Gluconeogenesis

·         Formation of glucose from non-carbohydrate sources.

·         Occurs mainly in liver and kidney.

·         Precursors include lactate, glycerol, and amino acids.

·         Essential during prolonged fasting.

5. Pentose Phosphate Pathway (HMP Shunt)

·         Alternative pathway of glucose metabolism.

·         Produces NADPH and ribose-5-phosphate.

·         Important for fatty acid synthesis and antioxidant defense.

6. Citric Acid Cycle (Krebs Cycle/TCA Cycle)

·         Occurs in mitochondria.

·         Oxidizes acetyl-CoA to carbon dioxide.

·         Produces NADH, FADH₂, and GTP.

·         Central pathway of cellular metabolism.

7. Electron Transport Chain (ETC)

·         Located in the inner mitochondrial membrane.

·         Transfers electrons from NADH and FADH₂.

·         Generates ATP through oxidative phosphorylation.

·         Major source of cellular energy.

8. Oxidative Phosphorylation

·         Final stage of aerobic respiration.

·         Couples electron transport with ATP synthesis.

·         Produces the majority of ATP in cells.

Lipid Metabolism Pathways

9. Lipolysis

·         Breakdown of triglycerides into fatty acids and glycerol.

·         Occurs in adipose tissue.

·         Stimulated during fasting and exercise.

10. β-Oxidation of Fatty Acids

·         Occurs in mitochondria.

·         Converts fatty acids into acetyl-CoA.

·         Produces NADH and FADH₂.

·         Important source of energy during fasting.

11. Fatty Acid Synthesis

·         Formation of fatty acids from acetyl-CoA.

·         Occurs in cytoplasm.

·         Stimulated by insulin and excess carbohydrates.

12. Ketogenesis

·         Formation of ketone bodies in the liver.

·         Occurs during prolonged fasting, starvation, and diabetes.

·         Provides alternative fuel for brain and muscles.

13. Ketolysis

·         Utilization of ketone bodies by peripheral tissues.

·         Produces energy during carbohydrate deficiency.

14. Cholesterol Synthesis

·         Occurs mainly in liver.

·         HMG-CoA reductase is the rate-limiting enzyme.

·         Essential for steroid hormones and cell membranes.

Protein and Amino Acid Metabolism

15. Transamination

·         Transfer of amino groups between amino acids and keto acids.

·         Requires pyridoxal phosphate (Vitamin B6).

16. Oxidative Deamination

·         Removal of amino groups from amino acids.

·         Produces ammonia for urea synthesis.

17. Urea Cycle

·         Converts toxic ammonia into urea.

·         Occurs in liver.

·         Major pathway for nitrogen disposal.

18. Protein Synthesis

·         Formation of proteins from amino acids.

·         Occurs on ribosomes.

·         Essential for growth and repair.

Nucleotide Metabolism

19. Purine Synthesis

·         Produces adenine and guanine nucleotides.

·         Necessary for DNA and RNA formation.

20. Pyrimidine Synthesis

·         Produces cytosine, thymine, and uracil.

·         Essential for nucleic acid synthesis.

21. Purine Degradation

·         Ends with uric acid formation.

·         Excess uric acid may cause gout.

Specialized Metabolic Pathways

22. Heme Synthesis

·         Formation of heme molecule.

·         Occurs in liver and bone marrow.

·         Essential for hemoglobin synthesis.

23. Heme Degradation

·         Produces bilirubin.

·         Occurs in reticuloendothelial system.

24. Cori Cycle

·         Lactate produced in muscles is converted to glucose in the liver.

·         Maintains energy supply during exercise.

25. Glucose-Alanine Cycle

·         Transfers amino groups from muscle to liver.

·         Helps maintain glucose levels during fasting.

26. One-Carbon Metabolism

·         Involves folate and vitamin B12.

·         Essential for DNA synthesis and methylation reactions.

27. Detoxification Pathways

·         Occur mainly in liver.

·         Include Phase I (oxidation, reduction) and Phase II (conjugation) reactions.

·         Eliminate drugs and toxins.

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