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CATABOLISM OF PROTEINS
AND
AMINO ACIDS
1
• In animals,amino acids undergo oxidative
degradation in 3 different metabolic
circumstances:
• 1-During normal synthesis and degradation of
cellular proteins, some amino acids, that are not
needed for new protein synthesis, undergo
OXİDATİVE DEGRADATİON
• 2-When a diet is rich in protein, the surplus
amino acids are catabolized ( in the liver amino
acids can't be stored)
• 3-During starvation and uncontrolled DM, when
carbohydrates are unavailable or improperly
utilized, cellular proteins are used as fuel.
2
3
• Animals convert α-amino nitrogen to
varied end products as ammonia, uric acid
or urea.
• Humans are ureotelic and excrete non-
toxic, water-soluble urea.
4
BİOSYNTHESİS OF UREA
• Urea biosynthesis occurs in 4 states
• 1-Transamination
• 2-Oxidative deamination of glutamate.
• 3-Ammonia transport
• 4-Reactions of urea cycle.
5
6
• Transamination
• Transamination transfers α-amino nitrogen
to α-ketoglutarate, forming glutamate.
• Transamination interconverts pairs of α-
amino acids and α-ketoacids.
7
• Cofactor:Pyridoxal phosphate
• Pyridoxamine is the intermediate in the reaction.
• Alanine-pyruvate amino transferase (alanine
aminotransferase) and glutamate α-
ketoglutarate amino transferase (glutamate
aminotransferase) catalyze the transfer of amino
groups to pyruvate (forming alanine) or to α-
ketoglutare (forming glutamate)
8
9
10
• Glutamate releases its amino group as
ammonia in the liver.
• In hepatocytes, glutamate is transported
from cytosol into mitochondria, where it
undergoes OXİDATİVE DEAMİNATİON by
glutamate dehydrogenase.
11
12
• L-glutamate is the only amino acid that
undergoes oxidative deamination at an
appreciable rate in mammalian tissues.
• L-glutamate dehydrogenase (GDH)
occupies a central position in nitrogen
metabolism.(mitochondrial matrix)
13
• GDH reaction is a reversible reaction that can
produce glutamate from α-ketoglutarate or
convert glutamate to α-ketoglutarate and NH3
• Hepatic GDH can use either NAD+ or NADP+,
as the acceptor of reducing equivalents.
• Glutamate serves as a precursor of
ammonia.Mitochondrial glutamine synthetase
catlyses this energy requiring reaction (ATP),
consuming a molecule of ammonia.
14
15
• Glutamine can serve as a buffer for
ammonia utilization as a source of
ammonia and carrier of amino groups.
Glutamine, along with alanine, is a key
transporter of amino groups between
various tissues and liver and is present in
greater concentrations than most amino
acids in blood.
16
• Glutaminase hydrolyses glutamine to
glutamate and NH4+.
• This reaction is important in the kidney for
the management of proton transport and
pH control.
17
18
Aminotransferase + GDH action TRANSDEAMiNATiON
• GDH operates at an important intersection
of carbon and nitrogen metabolism.
• The mammalian GDH is allosterically
regulated by GTP (-modulator) and ADP
(+ modulator)
19
• Oxidative deamination of amino acids
• L-amino acid oxidases of liver and kidney
produces NH3 and α-keto acid directly,
using FMN as a cofactor.(through α-imino
acid)
• FMNH2 is converted to FMN, using O2 and
produces H2O2 which is decomposed by
catalase.
20
Non-oxidative deamination
• Hydroxyaminoacids (serine, threonine) are
non-oxidatively deaminated by
dehydratase to form keto acids (pyruvate,
and α-ketobutyrate) and NH3.
21
22
• The NH4+ ,from intestine and kidney, is
transported in the blood to liver.
• In the liver, the ammonia from all sources
is disposed of by urea synthesis.
23
• Symptoms:Tremor, slurred speech,
blurred vision, coma
• Ammonia Encephalopathy
• When ammonia concentration increases
in blood and other biological fluİds, it
diffuses across blood-brain barrier.
24
glutamine, which acts as an osmotically
active solute (osmolyte) in brain
astrocytes.
• Uptake of water into astroyctes to maintain
osmotic balance leads to swelling of cells
and coma.
25
• NH3 may be toxic to brain because it reacts with
α-ketoglutarate to form glutamate.
• Depleted levels of α-ketoglutarate impair TCA
cycle function.
• Excretion into urine of ammonia produced by
renal tubular cells facilitates cation conservation
and regulation of acid-base balance.
• NH3 production from intracellular renal glutamine
increases in metabolic acidosis, decreases in
metabolic alkalosis.
26
Urea Cycle
• Urea is the principal nitrogenous excretion
product in humans.
• The urea cycle was the first metabolic
cycle to be well defined.
• Its description preceded that of TCA
cycle.
27
28
• Fumarate ,which enters the mitochondria, may
be recycled through TCA cycle to oxaloacetate:
• Addition of H2O to fumarate forms L-malate and
subsequent NAD+-dependent oxidation of
malate in the mitochondrion forms oxaloacetate
(malate dehydrogenase)
• Each NADH molecule can generate up to 2.5
ATP during mitochondrial respiration, greatly
reducing the overall energetic cost of urea
synthesis.
29
30
31
• The urea cycle is split between the mitochondrial
matrix and the cytosol:
• The first 2 steps occur in the mitochondrion.
• Citrulline, which is formed in the mitochondrion,
moves into the cytosol by a specific passive
transport system.
• The cycle is completed in the cytosol.
• Ornithine, which is regenerated, is transported
back across the mitochondrial membrane.
32
• The steady state levels of N-acetyl
glutamate are determined by glutamate,
acetyl coA and arginine
• Urea cycle enzymes increase or decrease
in response to high or low-protein diet
• Starvation and high-protein diets elevate
enzyme levels to cope with increased
ammonia production that accompanies
enhanced protein degradation.
33
• .Urea synthesis and excretion are
decreased and NH4+ excreton is increased
during acidosis to excrete protons into the
urine.
• .Infants born with defects in any of the first
4 enzymes may appear normal at birth,
but rapidly become lethargic, lose body
temperature and may have difficulty in
breathing.
34

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Catabolism of proteins and amino acids

  • 2. • In animals,amino acids undergo oxidative degradation in 3 different metabolic circumstances: • 1-During normal synthesis and degradation of cellular proteins, some amino acids, that are not needed for new protein synthesis, undergo OXİDATİVE DEGRADATİON • 2-When a diet is rich in protein, the surplus amino acids are catabolized ( in the liver amino acids can't be stored) • 3-During starvation and uncontrolled DM, when carbohydrates are unavailable or improperly utilized, cellular proteins are used as fuel. 2
  • 3. 3
  • 4. • Animals convert α-amino nitrogen to varied end products as ammonia, uric acid or urea. • Humans are ureotelic and excrete non- toxic, water-soluble urea. 4
  • 5. BİOSYNTHESİS OF UREA • Urea biosynthesis occurs in 4 states • 1-Transamination • 2-Oxidative deamination of glutamate. • 3-Ammonia transport • 4-Reactions of urea cycle. 5
  • 6. 6
  • 7. • Transamination • Transamination transfers α-amino nitrogen to α-ketoglutarate, forming glutamate. • Transamination interconverts pairs of α- amino acids and α-ketoacids. 7
  • 8. • Cofactor:Pyridoxal phosphate • Pyridoxamine is the intermediate in the reaction. • Alanine-pyruvate amino transferase (alanine aminotransferase) and glutamate α- ketoglutarate amino transferase (glutamate aminotransferase) catalyze the transfer of amino groups to pyruvate (forming alanine) or to α- ketoglutare (forming glutamate) 8
  • 9. 9
  • 10. 10
  • 11. • Glutamate releases its amino group as ammonia in the liver. • In hepatocytes, glutamate is transported from cytosol into mitochondria, where it undergoes OXİDATİVE DEAMİNATİON by glutamate dehydrogenase. 11
  • 12. 12
  • 13. • L-glutamate is the only amino acid that undergoes oxidative deamination at an appreciable rate in mammalian tissues. • L-glutamate dehydrogenase (GDH) occupies a central position in nitrogen metabolism.(mitochondrial matrix) 13
  • 14. • GDH reaction is a reversible reaction that can produce glutamate from α-ketoglutarate or convert glutamate to α-ketoglutarate and NH3 • Hepatic GDH can use either NAD+ or NADP+, as the acceptor of reducing equivalents. • Glutamate serves as a precursor of ammonia.Mitochondrial glutamine synthetase catlyses this energy requiring reaction (ATP), consuming a molecule of ammonia. 14
  • 15. 15
  • 16. • Glutamine can serve as a buffer for ammonia utilization as a source of ammonia and carrier of amino groups. Glutamine, along with alanine, is a key transporter of amino groups between various tissues and liver and is present in greater concentrations than most amino acids in blood. 16
  • 17. • Glutaminase hydrolyses glutamine to glutamate and NH4+. • This reaction is important in the kidney for the management of proton transport and pH control. 17
  • 18. 18
  • 19. Aminotransferase + GDH action TRANSDEAMiNATiON • GDH operates at an important intersection of carbon and nitrogen metabolism. • The mammalian GDH is allosterically regulated by GTP (-modulator) and ADP (+ modulator) 19
  • 20. • Oxidative deamination of amino acids • L-amino acid oxidases of liver and kidney produces NH3 and α-keto acid directly, using FMN as a cofactor.(through α-imino acid) • FMNH2 is converted to FMN, using O2 and produces H2O2 which is decomposed by catalase. 20
  • 21. Non-oxidative deamination • Hydroxyaminoacids (serine, threonine) are non-oxidatively deaminated by dehydratase to form keto acids (pyruvate, and α-ketobutyrate) and NH3. 21
  • 22. 22
  • 23. • The NH4+ ,from intestine and kidney, is transported in the blood to liver. • In the liver, the ammonia from all sources is disposed of by urea synthesis. 23
  • 24. • Symptoms:Tremor, slurred speech, blurred vision, coma • Ammonia Encephalopathy • When ammonia concentration increases in blood and other biological fluİds, it diffuses across blood-brain barrier. 24
  • 25. glutamine, which acts as an osmotically active solute (osmolyte) in brain astrocytes. • Uptake of water into astroyctes to maintain osmotic balance leads to swelling of cells and coma. 25
  • 26. • NH3 may be toxic to brain because it reacts with α-ketoglutarate to form glutamate. • Depleted levels of α-ketoglutarate impair TCA cycle function. • Excretion into urine of ammonia produced by renal tubular cells facilitates cation conservation and regulation of acid-base balance. • NH3 production from intracellular renal glutamine increases in metabolic acidosis, decreases in metabolic alkalosis. 26
  • 27. Urea Cycle • Urea is the principal nitrogenous excretion product in humans. • The urea cycle was the first metabolic cycle to be well defined. • Its description preceded that of TCA cycle. 27
  • 28. 28
  • 29. • Fumarate ,which enters the mitochondria, may be recycled through TCA cycle to oxaloacetate: • Addition of H2O to fumarate forms L-malate and subsequent NAD+-dependent oxidation of malate in the mitochondrion forms oxaloacetate (malate dehydrogenase) • Each NADH molecule can generate up to 2.5 ATP during mitochondrial respiration, greatly reducing the overall energetic cost of urea synthesis. 29
  • 30. 30
  • 31. 31
  • 32. • The urea cycle is split between the mitochondrial matrix and the cytosol: • The first 2 steps occur in the mitochondrion. • Citrulline, which is formed in the mitochondrion, moves into the cytosol by a specific passive transport system. • The cycle is completed in the cytosol. • Ornithine, which is regenerated, is transported back across the mitochondrial membrane. 32
  • 33. • The steady state levels of N-acetyl glutamate are determined by glutamate, acetyl coA and arginine • Urea cycle enzymes increase or decrease in response to high or low-protein diet • Starvation and high-protein diets elevate enzyme levels to cope with increased ammonia production that accompanies enhanced protein degradation. 33
  • 34. • .Urea synthesis and excretion are decreased and NH4+ excreton is increased during acidosis to excrete protons into the urine. • .Infants born with defects in any of the first 4 enzymes may appear normal at birth, but rapidly become lethargic, lose body temperature and may have difficulty in breathing. 34