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L18 - Liver-Nitrogen and Protein Metabolism
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Lecture by Despo Papachristodoulou. Notes already organised according to the lecture objectives!
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L18 – Liver: Nitrogen and Protein MetabolismExplain the concept of protein turnover, dynamic equilibrium and amino acid pools
Slide 5 – Protein turnover
• Body proteins continuously degraded to amino acids and resynthesised
• Average turnover in an adult = 300-400g per day
• Turnover variable
• Most proteins have half-lives of several days
• Structural proteins (e.g. collagen may have half-lives of years)
• Hormones and digestive enzymes degraded very rapidly, within half-lives of minutes
Slide 6 – Amino acid pool
• Free amino acids
• Very low concentrations inside cells or in the blood stream
• Mixing and exchange with other free amino acids distributed throughout the body
Explain the concept of nitrogen balance and the causes of positive and negative nitrogen
balance
Slides 9-12 – Nitrogen balance
• In normal healthy adults: total amount of nitrogen taken in the diet as protein should be
equal to the amount of nitrogen excreted from the body in the form of urea, uric acid,
creatinine and NH4+
- N(intake) = N(excretion)
• Subject then described as being nitrogen balanced
• Rate of body protein synthesis (and other N-containing compounds) is equal to the rate
of degradation
• Positive nitrogen balance:
- N(intake) > N(excretion)
- Protein synthesis exceeds the rate of breakdown
- During normal growth in children
- In convalescence after serious illness (recovery)
- After immobilisation after an accident
- During pregnancy
• Negative nitrogen balance:
- N(intake) < N(excretion)
- In starvation
- During serious illness, in late stages of some cancers
- In injury and trauma
- If not corrected and becomes prolonged, there will be irreversible loss of essential body
tissue, will ultimately lead to death
, Outline the pathways of protein degradation
Slide 13 – Pathways to protein degradation
• Most cellular proteins:
- Recognised as old or damaged
- Removed by the ubiquitin breakdown system
- Give a mixture of the 20 amino acids
• Foreign exogenous proteins:
- Old or damaged sub cellular organelles
- Taken into vesicles by endocytosis or autophagocytosis, vesicle fuses with lysosomes,
proteolytic enzymes degrade proteins into amino acids
- Starvation and hormones (e.g. cortisol) increase rates of protein breakdown in muscle
Explain how amino acids are classified as essential or nonessential and the significance
of this classification
Slide 3 – Is dietary protein essential?
• Some amino acids are essential
• Amino acids are required as building blocks for proteins
• Also for synthesis of neurotransmitters, creatine, carnitine, haem, purines and
pyrimidines
• Act as a source of blood glucose in fasting and starvation
- Brain and RBCs need glucose, RBCs only can use glucose because they have no
mitochondria
• We can only make 10 amino acids
Slide 7 – Protein requirements
• No storage form of protein in the body to replace proteins and other N containing
compounds
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