Metabolism:Catabolism & Biosynthesis
Unlock the secrets of cellular energy! Discover how catabolism breaks down molecules while biosynthesis builds them up, plus master amino acid processing fundamentals.
What Is Metabolism?
Metabolism encompasses all chemical reactions that occur within living organisms to maintain life. These biochemical processes enable cells to grow, reproduce, maintain structures, and respond to environmental changes.
Key Components of Metabolism:
- Catabolism: Breaks down complex molecules into simpler ones, releasing energy
- Anabolism (Biosynthesis): Builds complex molecules from simpler ones, consuming energy
Metabolism maintains cellular homeostasis through carefully regulated pathways that balance energy production and consumption. Understanding these processes reveals how organisms convert nutrients into usable energy and building blocks for growth.
Catabolism: Breaking Down Molecules
Catabolism represents the destructive phase of metabolism where complex organic molecules break down into simpler compounds. This process releases stored chemical energy, making it available for cellular work.
Major Catabolic Pathways:
Glycolysis
Glucose breaks down into pyruvate, producing ATP and NADH
Citric Acid Cycle
Pyruvate oxidizes completely, generating CO₂, ATP, NADH, and FADH₂
Beta-Oxidation
Fatty acids break down into acetyl-CoA units for energy production
Catabolism Process Flow
Complex Molecules
Carbohydrates, Fats, Proteins
Enzymatic Breakdown
Hydrolysis & Oxidation
Energy Release
ATP, NADH, FADH₂
Biosynthesis (Anabolism): Building Complex Molecules
Biosynthesis Process Flow
Simple Precursors
Amino acids, Sugars, Fatty acids
Energy Investment
ATP, NADPH consumption
Complex Products
Proteins, Nucleic acids, Lipids
Biosynthesis or anabolism constructs complex molecules from simpler building blocks. This energy-requiring process builds cellular components necessary for growth, repair, and reproduction.
Key Biosynthetic Pathways:
Protein Synthesis
Amino acids link together to form polypeptide chains and functional proteins
Gluconeogenesis
Non-carbohydrate precursors convert into glucose for energy storage
Fatty Acid Synthesis
Acetyl-CoA units combine to create long-chain fatty acids for membrane formation
Transamination: Amino Acid Conversion
What is Transamination?
Transamination transfers amino groups (-NH₂) from amino acids to keto acids, creating new amino acids without net nitrogen loss.
Key Enzymes
- • ALT (Alanine aminotransferase)
- • AST (Aspartate aminotransferase)
- • Various other aminotransferases
Biological Importance
Enables synthesis of non-essential amino acids and maintains amino acid pools for protein synthesis.
Transamination Reaction Example:
Alanine + α-Ketoglutarate ⇌ Pyruvate + Glutamate
Catalyzed by Alanine Aminotransferase (ALT)
Deamination: Removing Amine Groups
Deamination removes amino groups from amino acids, producing ammonia (NH₃) and corresponding keto acids. This process occurs when amino acids serve as energy sources or during protein catabolism. Disrupted deamination can contribute to metabolic disorders and conditions requiring medical intervention.
Clinical Relevance
Metabolic disorders like insulin resistance (resistencia a la insulina y su relación con enfermedades metabólicas) can affect amino acid metabolism and overall metabolic health.
Types of Deamination:
Oxidative Deamination
NAD⁺ or FAD accepts electrons during amino group removal
Non-oxidative Deamination
Direct removal without electron transfer, often involving water
Deamination Consequences
Ammonia Production
Toxic ammonia must be quickly converted or excreted
Keto Acid Formation
Can enter gluconeogenesis or citric acid cycle
Detoxification Need
Requires urea cycle activation in liver
Urea Cycle: Detoxifying Ammonia
The urea cycle converts toxic ammonia into less harmful urea, which can be safely excreted in urine. This critical detoxification pathway occurs primarily in liver hepatocytes.
Urea Cycle Steps
Carbamoyl Phosphate Formation
NH₃ + CO₂ + 2ATP → Carbamoyl phosphate
Citrulline Synthesis
Carbamoyl phosphate + Ornithine → Citrulline
Argininosuccinate Formation
Citrulline + Aspartate + ATP → Argininosuccinate
Arginine Formation
Argininosuccinate → Arginine + Fumarate
Urea Formation
Arginine → Urea + Ornithine
Clinical Significance
Hyperammonemia
Elevated blood ammonia levels due to urea cycle defects can cause neurological damage
Liver Function
Urea cycle efficiency reflects hepatic health and metabolic capacity
Energy Cost
Requires 4 ATP molecules per urea molecule produced
Essential vs Non-Essential Amino Acids
Essential Amino Acids
Cannot be synthesized by the human body and must be obtained through diet.
Histidine
Isoleucine
Leucine
Lysine
Methionine
Phenylalanine
Threonine
Tryptophan
Valine
Non-Essential Amino Acids
Can be synthesized by the human body from other amino acids and metabolic intermediates.
Alanine
Arginine*
Asparagine
Aspartate
Cysteine*
Glutamate
Glutamine*
Glycine*
Proline*
Serine
Tyrosine*
*Conditionally essential under certain conditions
Conditionally Essential Amino Acids
Some non-essential amino acids become essential during periods of illness, stress, or rapid growth when the body’s synthesis capacity cannot meet increased demands. These include arginine, cysteine, glutamine, glycine, proline, and tyrosine.
FAQs About Metabolism
What is the difference between catabolism and biosynthesis?
Catabolism breaks down complex molecules into simpler ones, releasing energy in the form of ATP. Biosynthesis (anabolism) builds complex molecules from simpler precursors, requiring energy input. These processes work together to maintain cellular energy balance and provide building blocks for growth and repair.
How does the urea cycle detoxify ammonia?
The urea cycle converts toxic ammonia into less harmful urea through a series of enzymatic reactions in the liver. This five-step process combines ammonia with carbon dioxide and aspartate, ultimately producing urea that can be safely excreted through urine, preventing ammonia toxicity.
Why are some amino acids considered essential?
Essential amino acids cannot be synthesized by the human body in sufficient quantities to meet physiological needs. They must be obtained through dietary protein sources. The body lacks the enzymatic pathways necessary to produce these amino acids from other metabolic intermediates.
What happens during transamination reactions?
Transamination transfers amino groups from amino acids to keto acids, creating new amino acids without net nitrogen loss. This process allows the body to synthesize non-essential amino acids and maintain amino acid pools necessary for protein synthesis and other metabolic functions.
How does metabolism regulate energy balance?
Metabolism maintains energy homeostasis through coordinated regulation of catabolic and anabolic pathways. When energy is needed, catabolic processes increase to break down stored nutrients. When energy is abundant, anabolic processes dominate to build energy reserves and cellular components.
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References & Further Reading
- 1. Berg, J. M., Tymoczko, J. L., & Stryer, L. (2019). Biochemistry (8th ed.). W. H. Freeman. NCBI Bookshelf
- 2. Nelson, D. L., & Cox, M. M. (2017). Lehninger Principles of Biochemistry (7th ed.). W. H. Freeman.
- 3. Voet, D., & Voet, J. G. (2016). Biochemistry (4th ed.). John Wiley & Sons.
- 4. National Institute of Health. (2023). “Amino Acid Metabolism.” PMC Article
- 5. American Chemical Society. (2024). “Metabolic Pathways in Biochemistry.” ACS Education
- 6. Mayo Clinic. (2024). “Metabolic Syndrome and Related Disorders.” Mayo Clinic
- 7. Educational Resources: Metabolismo TV and other educational platforms provide accessible content on metabolic health and optimization strategies.