Formation of ammonia and Urea cycle by Dr. Ashok Kumar J

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


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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


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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


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


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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


Transaminases are named after the specific amino group donor


Pyruvate Glutamate

Alanine amino transferase (ALT)

Aspartate

Oxaloacetate


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


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


Oxidative deamination by L-Glutamate dehydrogenase

GlutamateαKetoglutarate

NH3NAD+/ NADP+

NADH + H+/ NADPH + H+

L-Glutamate dehydrogenase

+


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


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

+


Transdeamination


Pyruvate Glutamate

Transaminase

αKetoglutarate

NH3

L- Glutamate dehydrogenase

NADH+H+

NAD+

Transamination

Oxidative deamination


NH3

Sources of ammonia

Catabolism of Purines and Pyramidines

Catabolism of various nitrogen containing

compounds

Ammonia absorbed from the Gut

Glutaminase action on glutamine


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


Glutamate GlutamineGlutamine synthetase

NH4+ H2O

ATP ADP

Transport of ammonia


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


From muscle ammonia is transported as Alanine

Urea Cycle


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


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


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


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


• 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



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Health & Medicine

Formation of ammonia and Urea cycle by Dr. Ashok Kumar J