Dr Ashok Kumar J ; IMS; MSU
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Protein metabolism and
Urea CycleDr. Ashok Kumar J
Associate ProfessorInternational School of Medicine
Management and Science UniversityMalaysia
Dr Ashok Kumar J ; IMS; MSU
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OBJECTIVES: To learn …….
• Transamination reaction• Deamination reaction• Sources for ammonia• Transport of ammonia• Detoxification of ammonia (Urea cycle)• Ammonia toxicity• Hyperammonemia
Dr Ashok Kumar J ; IMS; MSU
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Catabolism of proteins
C
R
HNH2
COO-
α-amino group
α- carboxyl group
Side chain group
Individual amino acids differ from each other with respect to their side chain group
Amino Acids
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• Removal of the α amino group is an obligatory step in the catabolism of amino acids• The α amino group is removed as Ammonia
• Ammonia is converted to Urea (in Liver)
• The carbon skeleton of the amino acid is metabolized
- Energy production- Glucose synthesis- Fat synthesis
Dr Ashok Kumar J ; IMS; MSU
Cataboliosm of Amino acids
C
R
HNH2
COO-
NH3
UREA
in Liver
Glucogenic amino acids Glucogenic and ketogenic
Ketogenic amino acids
Alanine, Arginine, Asparagine, AspartateAsparagine, Cysteine, MethionineGlutamate, Glutamine, Glycine, HistidineProline, Serine, Threonine,Valine
TyrosineIsoleucinePhenylalanineTryptophan
LeucineLysine
5Dr Ashok Kumar J ; IMS; MSU
Dr Ashok Kumar J ; IMS; MSU
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Removal of Amino group of amino acid as ammonia
TransaminationOxidative deaminationAmino acid α- Ketoacid
(carbon skeleton)
Amonia
Transamination
Alanine αKetoglutarate
Pyruvate Glutamate
Transaminase (amino transferase)Pyridoxal phosphate (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 Ketoglutarate is a αketoacid most frequently used in transamination reactions
Transamination reaction does not form free ammonia
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Transaminases are named after the specific amino group donor
Alanine αKetoglutarate
Pyruvate Glutamate
Alanine amino transferase (ALT)
Aspartate
Oxaloacetate
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TCA Cycle
TCA Cycle
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 acids
9Dr Ashok Kumar J ; IMS; MSU
Amino acids which don't under go transamination – Lysine and threonine
DeaminationAmino group of amino
acid is removed as free ammonia
Oxidative deaminationAmino acids are oxidized (removal of Hydrogen)
and the α amino group of the amino acid is removed as free ammonia
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Oxidative deamination by L-Glutamate dehydrogenase
GlutamateαKetoglutarate
NH3NAD+/ NADP+
NADH + H+/ NADPH + H+
L-Glutamate dehydrogenase
+
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Allostearic regulators of L-Glutamate dehydrogenase
Allostearic inhibitors: ATP, GTP Allostearic activator: ADP
L-amino acid oxidase
L-Aminoacid
αKetoacid
NH3FMN FMNH2
O2H2O2
2 H2O + O2
Catalase
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L- aminoacid oxidase
•Requires FMN as the coenzyme•Enzyme activity is low in kidney and liver• It is of little importance
D-amino acid oxidaseUse FAD as coenzyme
Nonoxidative deamination
Removal of the α amino group of amino acid as free ammonia without oxidation of the amino acid
DehydrataseActs on hydroxyl amino acids to remove ammonia
Serine
Pyruvate
NH3H2O
Serine dehydratase
PLP
+
13Dr Ashok Kumar J ; IMS; MSU
Transdeamination
Alanine αKetoglutarate
Pyruvate Glutamate
Transaminase
αKetoglutarate
NH3
L- Glutamate dehydrogenase
NADH+H+
NAD+
Transamination
Oxidative deamination
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NH3
Sources of ammonia
Catabolism of Purines and Pyramidines
Catabolism of various nitrogen containing
compounds
Ammonia absorbed from the Gut
Glutaminase action on glutamine
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Blood level is about 80 to 100 μg/dlExcess of Ammonia is toxic to the CNS
Ammonia
UREA
Used for synthesis of glutamate and glutamine
Excreted in urineActs as buffer in kidney
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Glutamate GlutamineGlutamine synthetase
NH4+ H2O
ATP ADP
Transport of ammonia
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Ammonia is transported as glutamine from CNS to liverFormation of glutamine is first line of defence against ammonia toxicity
Glutamine is carried through blood to liver
Glutamine GlutamateGlutaminease
NH4+H2O
•Glutamine converted to ammonia and glutamate in the liver by glutaminase
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From muscle ammonia is transported as Alanine
Urea Cycle
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Urea• disposable form of ammonia• Accounts for 90% of nitrogen containing
component of the urine• One nitrogen of urea is supplied by free
ammonia• the other comes from the aspartate
• The carbon and oxygen comes from carbon dioxide
• First two reactions of urea cycle takes place in mitochondria and the remaining reactions in cytoplasm of the hepatocyte
CO2 NH4
2ATP
2ADPCarbamoyl Phosphate
Carbamoyl PhosphateSynthetase I
+
OrnithineOrnithineTranscarbamoylase
L-Citrulline
Argininosuccinate
ATPL-Aspartate AMP+PPi
Argininosuccinate Synthase
L-Arginine
Arginase
Urea
H2O
Fumarate
ArgininosuccinateLyase
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Aspartate
TCA Cycle
Mitochondria
L-Citrulline
Regulation of urea cycle
Carbamoyl phosphate synthetase I
N- Acetyl glutamate
+
Synthesis of N-acetyl glutamate is enhanced by high protein diet and amino acids especially arginine
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Heperammonemia
Genetic defects of the urea cycleBlood level of ammonia is increased
Ammonia has direct neurotoxic effect on the CNS • Ammonia intoxication Symptoms:
Tremors, Vomitting, slurring of speech, Blurring of vision coma accompanied by cerebral edema and increased cranial pressure If not treated – death
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Liver disease is the common causeCirrosis of liver caused by alcoholism, hepatitis or biliary obstruction - results in flow of portal blood directly into systemic circulation
In fulminant liver failure gut is sterilized to reduce further increase in blood ammonia
• Glutamate is used for glutamine formation• Depletion of Glutamate• Glutamate is required for the synthesis of Gamma amino
butyric acid (GABA)• Depletion of inhibitory neurotransmitter GABA
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• Ammonia reacts with α-ketoglutarate to form glutamate. • Depleted levels of α-ketoglutarate I• Impair function of the tricarboxylic acid (TCA) cycle in neurons• Depletion of energy in neurons.
Why ammonia is toxic to human body ?
Hereditary HyperammoniemiaCause: Genetic deficiency of each of the five enzymes of urea cycle
Hyperammonemia Type 1
Carbamoyl phosphate synthase I deficiencyHyperammonemia Type 2
deficiency of ornithine transcarbamoylase (OTC)Citrullinemia
Defect in argininosuccinate synthase activity
ArgininosuccinicaciduriaAbsence of argininosuccinase
HyperargininemiaArginase activity is deficient
24Dr Ashok Kumar J ; IMS; MSU
Dr Ashok Kumar J ; IMS; MSU
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• Thank you
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