Related Subjects:
|Fat Metabolism
|Glucose Metabolism
|Protein metabolism
|Glycolysis Krebs Electron Transport Chain
Overview of Protein Metabolism
Protein metabolism involves the breakdown and utilization of proteins and amino acids for energy, growth, and repair of tissues. This process is essential for maintaining cellular function and overall health.
Key Concepts
- Types of Proteins:
- Structural Proteins: Provide support and shape to cells and tissues (e.g., collagen, keratin).
- Enzymatic Proteins: Catalyze biochemical reactions (e.g., digestive enzymes, metabolic enzymes).
- Transport Proteins: Carry substances across cell membranes or in the bloodstream (e.g., haemoglobin, albumin).
- Digestion and Absorption:
- Proteases: Enzymes that break down proteins into peptides and amino acids in the stomach and small intestine.
- Absorption: Amino acids are absorbed through the intestinal lining into the bloodstream.
- Amino Acid Pool:
- A collective term for all free amino acids available in the body for protein synthesis and other functions.
- Protein Synthesis:
- Involves the transcription of DNA to mRNA and translation of mRNA to form polypeptides.
- Occurs in the ribosomes of cells.
- Protein Catabolism:
- Deamination: Removal of the amino group from amino acids, producing ammonia and a keto acid.
- Urea Cycle: Converts toxic ammonia into urea for excretion in urine.
- Glucogenic and Ketogenic Amino Acids: Glucogenic amino acids can be converted into glucose; ketogenic amino acids can be converted into ketone bodies.
- Regulation of Protein Metabolism:
- Hormones: Insulin promotes protein synthesis; cortisol promotes protein breakdown.
- Growth Factors: Stimulate protein synthesis and cell growth (e.g., IGF-1).
Detailed Pathways
- Digestion and Absorption:
- Proteins -> (Pepsin, Trypsin, Chymotrypsin) -> Peptides -> (Peptidases) -> Amino Acids.
- Amino acids are transported into enterocytes and then into the bloodstream.
- Deamination and Urea Cycle:
- Deamination:
- Amino acid -> Keto acid + Ammonia (NH3).
- Urea Cycle:
- Ammonia -> Carbamoyl phosphate.
- Carbamoyl phosphate + Ornithine -> Citrulline.
- Citrulline + Aspartate -> Argininosuccinate.
- Argininosuccinate -> Arginine + Fumarate.
- Arginine -> Urea + Ornithine (cycle repeats).
- Protein Synthesis:
- Transcription: DNA -> mRNA in the nucleus.
- Translation: mRNA -> Polypeptide chain in the ribosome.
- Post-translational modifications to form functional proteins.
- Amino Acid Metabolism:
- Glucogenic Amino Acids: Can be converted into glucose through gluconeogenesis.
- Ketogenic Amino Acids: Can be converted into ketone bodies through ketogenesis.
- Some amino acids are both glucogenic and ketogenic.
Clinical Relevance
- Liver Disease:
- Impaired urea cycle function can lead to hyperammonemia.
- Reduced albumin synthesis can cause edema and ascites.
- Kidney Disease:
- Impaired excretion of urea can lead to uraemia.
- Malnutrition:
- Protein-energy malnutrition (PEM) can lead to muscle wasting and weakened immunity.
- Kwashiorkor and marasmus are severe forms of PEM.
- Inborn Errors of Metabolism:
- Genetic disorders affecting amino acid metabolism (e.g., phenylketonuria, maple syrup urine disease).
Summary
Understanding protein metabolism is crucial for recognizing its impact on health and disease. Key pathways include digestion and absorption of proteins, deamination and the urea cycle for ammonia detoxification, and protein synthesis for cellular functions. Hormonal regulation plays a significant role in these processes, influencing metabolic outcomes in conditions such as liver and kidney disease, malnutrition, and genetic metabolic disorders.