For medical, Paramedicals and allied health Science students

1. Catabolism of
Amino Acids
(Removal of amino group)
By
Dr. Santhosh Kumar N
Associate Professor of Biochemistry

2. Synthesis of non
essential amino acids
Dietary protein
Breakdown of
tissue proteins
Amino acids pool
4 –8 mg/dl
Sources of amino acids Fate of amino acids
Formation of Structural protein
(eg: tissue proteins)
Biosynthesis of peptide hormones,
haemoglobin, myoglobin & enzymes
Synthesis of biological imp peptides
(eg: Glutathione)
Biosynthesis of NPN substances (eg:
Urea, uric acid, creatine, creatinine)
↑se in the fed state &
↓se in the post absorptive state

3. • All the catabolic pathway of amino acids
(removal of amino group) involves
Transamination, Deamination reactions
to form α-keto acid and Ammonia.
• Further carbon skeleton either enters
carbohydrate (glucogenic) or lipid
metabolisms (ketogenic)
• Ammonia is transported from the muscle
to liver & converted into urea than
excreted.

4. Transamination
Reactions
Transfer of amino group from α-amino acid to α-”C” atom of α-keto acid (α-
KG) to form a new α-amino acid & a new α-keto acid

5. .
Amino-transferase / transaminase
Transfer of amino group from α-amino acid to α-”C” atom of α-KG to form a
new α-amino acid & a new α-keto acid

6. Serum Alanine amino transferase (ALT)
PLP
L- Alanine + α- Ketoglutarate Pyruvate + L-Glutamate
Serum Aspartate amino transferase (AST)
PLP
L- Aspartate + α- Ketoglutarate Oxaloacetate + L-Glutamate

7. Significance of transamination reactions
• Provides the amino groups from different a.a’s into common product L-glutamate (L-Glu)
• L-Glu only a.a whose α-amino group can be directly removed by oxidative deamination.
• L-Glu used as an amino group donor in the synthesis of non essential amino acids
• All amino acids can be transaminated except lysine, threonine, proline and OH- proline.

8. • ALT(5 to 45 IU/L): Specific for liver diseases & also increases in acute hepatitis, hepatic
Jaundice
• AST (5 to 35 IU/L): Specific for the cardiac lesions (serum ↑se after 3-8hrs onset of chest
pain) & also acute liver diseases
AST/ALT ratio (De Ritis ratio) :
 Normal AST/ALT ratio is 1
• More is seen in Alcoholic hepatitis, hepatitis with cirrhosis, liver metastasis, MI.
• Less is seen in acute hepato cellular injury, toxic exposure, extra hepatic obstruction
(cholestasis)
• Low values of transaminases are observed in Vit-B6 deficiency

9. Deamination Reactions
Removal of amino group from α-amino acid in the
form of ammonia & α-keto acid (liver & kidney).
Oxidative
deamination
Non oxidative
deamination

10. Oxidative deamination
Catalyzed by enzymes Glutamate dehydrogenase
&L & D-amino acid oxidase.
Removal of ammonia from the amino
acids with the link of oxidation process

11. Glutamate dehydrogenase(GLDH)
(In hepatocytes)
GTP , ATP & NADH
-
GLDH can use either NAD+ or NADP+ as the acceptor of reducing equivalents
GLDH increased in cases of liver disease (hepato cellular damage )
+
GDP , ADP

12. L & D-amino acid oxidases
(kidneys & liver)

13. Non Oxidative deamination
Dehydratase enzyme deaminates OH-group
containing amino acids (PLP as coenzyme)
Removal of ammonia from the amino
acids without link of oxidation process
Serine + H2O
Serine
dehydratase
Pyruvate
+ NH4
+
Threonine α- ketoglutarate
+ NH4
+
Threonine
dehydratase

14. METABOLISM
OF
AMMONIA

15. From amino acids via
Transamination &
deamination
Degradation of
biogenic amines
Ammonia
Formation Metabolic fate of ammonia
Converted to urea (urea cycle)
Synthesis of non essential amino
acids
Formation of Purines &
Pyrimidines
Maintains the acid base balance
via NH4
+ ions
From the amino group of
purines & Pyrimidines
By the action of intestinal
bacteria (urease) on urea Synthesis of Glutamine
Formation of amino sugars

16. Transport of Ammonia
• Ammonia is transported from muscle to
liver in two transport forms
– Glutamine & Alanine
• but not as free ammonia.
• I. Glutamine is a major transport &
temporary storage form of ammonia
Ammonia + Glutamate
Glutamine
Glutamate
ATP
ADP+ Pi
Glutamate synthetase
(Muscle)
Glutaminase
(Liver)
H2O
NH4
+

17. II. Alanine
• Imp NH3 transporter from muscle to liver by
glucose – alanine cycle (in starvation).
• Glutamate can transfer its α-amino group to
pyruvate by the action of ALT to form
alanine.
• Liver promptly removes the ammonia from
the portal blood.
(GLDH)

18. Clinical Aspect Of Ammonia
The concentration of ammonia in blood : 40 to 70 μgm/dl
Elevation of NH3 in blood is found to be toxic to the body - Ammonia toxicity/
Hyperammonemia
Acquired
hyperammonemia
Inherited
hyper Ammonemia
result of liver cirrhosis
Leads to reducing the
synthesis of urea
results from genetic
defects in the urea
cycle enzymes

19. characterized by: A peculiar flapping tremor.
Slurring of speech.
Blurring of vision &.
in severe cases coma & death due to ↑sed NH3 conc. in blood & brain
Treatment
• restriction of dietary Proteins
• Increased Arginine in diet, bypasses Arginosuccinase defect
• Drugs like benzoate and phenyl acetate,
• Hemodialysis

20. ↑↑↑sed ammonia
leads to increased conc. of glutamine
act as
an osmotically active solute in brain astrocytes
Triggers
uptake of water into the astrocytes to maintain osmotic
balance
leads
swelling of the cells – coma
Terminal stages of ammonia intoxication characterized
by cerebral edema & increased cranial pressure

21. DISPOSAL OF AMMONIA
UREA CYCLE
(“Krebs Henseleit Cycle” )

22. Ammonia Aspartate
CO2
Urea
Site of synthesis: Liver
Location of enzymes: partly
mitochondrial & partly cytosolic.
Energetics: requires 4 moles of ATP per
each turn of cycle.

24. Eukaryotes have two forms of CPS:
• Mitochondrial CPS-I uses ammonia as its nitrogen donor and participates in urea
biosynthesis.
• Cytosolic CPS - II uses glutamine as its nitrogen donor and is involved in pyrimidine
biosynthesis.

25. Regulation of the Urea cycle
Regulated by substrate availability
Higher the rate of ammonia form higher the urea
Stimulation of urea cycle enzymes
occur in response to high protein diet (or) prolonged fasting when
gluconeogenesis from a.a’s high

26. Allosteric regulation
Acetyl CoA + Glutamate
N- Acetyl glutamate (NAG)
N- Acetyl glutamate synthase
high protein diet,
Arg & starvation
CO2 + NH4
+
Carbamoyl Phosphate
Synthetase-I
Carbamoyl phosphate
+
+

27. Significance Of Urea Cycle
• Converts toxic ammonia into non toxic urea
• Forms semi essential amino acid –Arginine & non essenial amino acid -proline
• It disposes off two waste products, ammonia & bicarbonate
• Ornithine is a precursor for the formation of polyamines like putrescine, spermidine &
spermine
• It involved in metabolic integration of nitrogen metabolism
• Fumarate synthesized by urea cycle, links the transamination reactions, through urea
cycle –citric acid cycle

29. Disposal of Urea
Broken down to CO2 and NH3 by the bacterial enzyme Urease (in intestine)
Urea produced in the liver freely diffuses and is transported in blood to kidneys
and excreted.

30. Deficiency of any of the urea cycle enzymes would result in hyper ammonemia.
INHIRITED DISORDERS OF UREA CYCLE

31. Inherited disorders Clinical features
Hyper ammonemia type-I  Carbamyl phosphate synthetase -1 (CRP-1) enzyme defect
 Ammonia toxicity is occurs.
Ornithinemia (or)
Hyper ammonemia type-II
 Ornithine transcarbamylase enzyme defect
 ↑sed levels of glutamine, NH3 & ornithine seen in blood.
Citrullinemia
 Arginino succinate synthetase enzyme defect
 Mental retardation
 ↑sed levels of NH3& citrulline are seen in blood.
Argininosuccinic
acidurias
 It is inherited disorder in fatal (before 2yrs of age)
 Arginino succinase enzyme defect.
 Mental retardation.
 ↑sed levels of Arginino-succinate are seen in blood & urine.
Hyper argininemia  Arginase enzyme defect.
 Hyper ammonemia is occurs.
 ↑sed excretion of lysine, cystine,ornithine & arginine in
urine.

32. Urea cycle disorders
due to
transporter defect

33. Mutation of the ORNT-1 gene that encodes the
mitochandrial membrane ornithine
permase
Failure of transport cytosolic ornitine into
mitochondria leads to hyperammonemia
&hyperornithineamia
Absence of ornithine, mitochondrial CP,
carbomoylates lysine to homocitrulline
leads to homocitrulinuria
HHH syndrome
(hyper ornithinemia, hyper
ammonemia, homocitrullinuria)
syndrome)

34. Accumulation of CP in the mitochondria,
enters the cytoplasm
synthesis of Orotic acid
(intermediate in the pyrimidine synthesis)
Accumulates leads to orotic aciduria
Orotic aciduria in hyperammonemia
type-II
(X-linked chromosomal deficiency)
CPS-II

35. Clinical significance of Urea
Normal ranges
 Blood Urea is 15 to 40 mg/dl
 Blood urea nitrogen (BUN) is 7 to 20 mg/dl
Physiological variation:
 Increases urea conc. more in males & Increases with age & high protein diet
 Decreases in pregnancy (due to hemodilution)

36. Pathological conditions
hyper Uremia (Increased urea levels in blood)
Pre renal
(Increased protein
breakdown)
Renal
(Increased in kidney
disease)
Post renal
(Obstruction to flow of urine,
GFR decreased)
- Dehydration
- Prolonged fever
- Diabetic coma
- Severe burns
- Acute & chronic
glomerulonephritis
-Nephrotic syndrome
-Renal failure
-Pyelonephritis
- Stones in the urinary tract
- Enlargement of prostate
- Tumors of the bladder
Hypo uremia
Decreased urea levels in blood seen in Malnutrition , Severe liver diseases (liver hepatitis)
& Decreased protein intake

37. Blood urea nitrogen (BUN) is 7 to 20 mg/dl
Higher BUN level low BUN level in
Renal disease,
Dehydration,
Tissue damage (severe burns) & also
high protein diet.
Severe liver disease or damage,
Malnutrition and
Second or third trimester of pregnancy
Blood BUN levels is mainly dependent on its rate of glomerular filtration and
tubular reabsorption ,it plays a vital role in diagnosing kidney function
Clinical significance of BUN

38. THANKYOU