Ammonia metabolism

ammonia metabolism

2. Metabolism of Ammonia
•Ammonia is constantly liberated in the metabolism of
amino acids & other nitrogenous compounds.
•At physiological pH, ammonia exists as NH4
+ions.

3. •Amino acids –by transdeamination.
•Biogenic amines.
•Amino group of purines & Pyrimidine catabolism.
•By action of intestinal bacteria (urease) on urea.

5. Transamination
AlanineαKetoglutarate
PyruvateGlutamate
Transaminase (amino transferase)
Pyridoxalphosphate (PLP)
Transfer of amino group of an amino acid to ketoacid to form
new Ketoacid corresponding to the original amino acid, and
new amino acid corresponding to the original ketoacid
α Ketoglutarateis
a αketoacidmost
frequently used in
transamination
reactions
Transamination
reaction does
not form free
ammonia
Dr Ashok Kumar J ; IMS; MSU
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6. Transaminases are named after
the specific amino group donor
AlanineαKetoglutarate
PyruvateGlutamate
Alanine amino
transferase (ALT)
Aspartate
Oxaloacetate
Dr Ashok Kumar J ; IMS; MSU
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TCA Cycle
TCA Cycle

7. Functions of Transamination reaction
1.Amino acids are converted to their respective
ketoacids(carbon skeleton)
Further metabolized by TCA cycle to produce energy or
used to form glucose or Fat
2. Helps in the synthesis of nonessential amino acidsDr Ashok Kumar J ; IMS; MSU
7Amino acids which don't under go transamination –Lysine and threonine

8. Deamination
Amino group of amino
acid is removed as
free ammonia
Oxidative deamination
Amino acids are oxidized (removal of Hydrogen)
and the α amino group of the amino acid is
removed as free ammonia
Dr Ashok Kumar J ; IMS; MSU
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9. Oxidative deamination by
L-Glutamate dehydrogenase
Glutamate
αKetoglutarate
NH3
NAD+/ NADP+NADH + H+/
NADPH + H+
L-Glutamate dehydrogenase
+
Dr Ashok Kumar J ; IMS; MSU
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Allostearicregulators of L-Glutamate dehydrogenase
Allostearicinhibitors: ATP, GTP
Allostearicactivator: ADP

10. Transdeamination
AlanineαKetoglutarate
PyruvateGlutamate
Transaminase
αKetoglutarate
NH3
L-Glutamate dehydrogenase
NADH+H+
NAD+
Transamination
Oxidative deamination
Dr Ashok Kumar J ; IMS; MSU
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11. Transport & storage of NH3
•There is a regular & constant production of NH3 from
various tissues, its concentration in the circulation is
low (10-20mg/dl).
•Body has an efficient mechanism for NH3 transport &
its utilization for urea cycle.

12. •The transport of ammonia between various tissues & the
liver mostly occurs in the form of glutamate or alanine &
not as free NH3.
•Alanine is important for NH3 transport from muscle to liver
by glucose -alanine cycle.

13. Role of glutamate
•Glutamine is a storehouse of NH3.
•It is present in highest concentration (8 mg/dl in
adults) in blood among the amino acids.
•Glutamine serves as a storage & transport form of
NH3.
•Synthesis mostlyoccurs in liver, brain & muscle.

14. •Glutamineis freely diffusible in tissues, hence easily
transported.
•Glutamine synthase (mitochondrial enzyme) is responsible for
synthesis of glutamine from glutamate & ammonia, requires
ATP & Mg2+.
•Glutamine can be deaminatedby hydrolysis to release
ammonia by glutaminase.
•Glutaminasefound in kidney & intestinal cells.

15. Synthesis of glutamine & its conversion to glutamate
COO-
CH2
CH2
H –C -NH3
+
COO-
CO-NH2
CH2
CH2
H –C –NH3
+
COO-
NH4
+ H2OGlutamine
Synthetase
ATPADP + Pi
Mg2+
Glutamate Glutamine
Glutaminase
H2ONH4
+
LIVER
BRAIN
Deamination

16. •Ammonia is essential for the synthesis of non-essential
amino acids, purines, pyrimidines, amino sugars &
aspargine.
•Ammonium ions are very important to maintain acid-base
balance of the body.

17. Disposal of ammonia
•Ammoniotelic: The aquatic animals dispose off NH3
into the surrounding water.
•Uricotelic: Ammonia is converted mostly to uric acid.
E.g.Reptiles& Birds.
•Ureotelic: The mammals including man convert NH3
to urea.
•Urea is non-toxic, soluble & easily excreted.

19. •The urea cycle is the first metabolic pathway to be elucidated.
•The cycle is known as Krebs–Henseleiturea cycle.
•Ornithine is the first member of the reaction, it is also called as
Ornithine cycle.
•Urea is synthesized in liver & transported to kidneysfor excretion
in urine.

20. •The two nitrogen atoms of urea are derived from two different
sources, one from ammonia & the other directly from the a-amino
group of aspartic acid.
•Carbon atom is supplied by CO2
•Urea is the end product of protein metabolism (amino acid
metabolism).

21. •Urea accounts for 80-90% of the nitrogen containing substances
excreted in urine.
•Urea synthesis is a five-step cyclic process, with five distinct
enzymes.
•The first two enzymes are present in mitochondria while the rest
are localized in cytosol.

23. •The overall reaction may be summarized as:
•NH3 + CO2 + Aspartate →Urea + fumarate
•2ATPs are used in the 1streaction.
•Another ATP is converted to AMP + PPiin the 3rdstep, which is
equivalent to 2 ATPs.
•The urea cycle consumes 4 high energy phosphate bonds.
•Fumarateformed in the 4thstep may be converted to malate.

24. •Malate when oxidisedto oxaloacetate produces 1 NADH
equivalent to 2.5 ATP.
•So net energy expenditure is only 1.5 high energy phosphates.
•The urea cycle & TCA cycle are interlinked & it is called as "urea
bicycle".

25. Toxicity of ammonia
•Elevation of blood ammonia concentration is harmful to
the Brain.
•Ammonia accumulation results in slurring of speech &
blurring of the vision & cause tremors.
•It may lead to coma, finally death, if not corrected.

26. Hyperammonemia
•Increased levels of ammonia in blood.
•It may be genetic or acquired.
•Impairment in urea synthesis due to a defect in any one of
the five enzymes in urea cycle.
•Acquired hyperammonemiamay be due to hepatitis,
alcoholism, where urea synthesis becomes defective, hence
NH3 accumulates.

27. Ammonia toxicity
•Increased levels of ammonia crosses BBB, formation of glutamate.
•More utilization of α-ketoglutarate.
•Decreased levels of α-Ketoglutaratein Brain.
•α-KG is a key intermediate in TCA cycle.
•Decreased levels impairs TCA cycle.
•Decreased ATP production.
Glutamate
NADPH + H+NADP+
GDH
α-Ketoglutarate+ NH3

28. DisoedersDefectiveEnzyme Products accumulated
Hyperammonaemia-1 CarbamoylPhosphate
Syntethase-1 Ammonia
Hyperammonaemia-2 Ornithinetranscarbomylase
(oroticaciduria-most common)Ammonia
CitrullinemiaArginininosuccinateSyntheatseCitrulline
ArginosuccinicaciduriaArgininosuccinatelyaseArginosuccinate
ArgininemiaArginase Arginine

29. •Increased levels of ammonia results in
žSlurring of speech
žBlurring of the vision
žConvulsions
žNausea, Vomiting
žNeurological Deficits
žMental Retardation
žComa & Death.

30. Diagnosis
•Increased levels of ammonia in blood & urine.
•Increased glutamine –in CSF, excreted in urine.
•Decreased blood urea levels.
•Urea cycle intermediates accumulate in blood &
excreted in urine.

31. •Intravenous administration of sodium benzoate,
phenyllacetate.
•These condense with glycine & glutamate to form water
soluble products that can be easily excreted.
•By this, ammonia can be trapped & removed from the
body.
•In toxic hyperammonemia, hemodialysismay become
necessary.