Urea Cycle &hyperammonimias New

Urea Cycle and Hyperammonemia Done by Abdulaziz massoud alfaydi

Urea Ø The liver is the principle site of ammonia detoxification. Ø The pathway for the detoxification of ammonia in the liver is urea cycle, which carried out along a pathway utilizes enzymes found in both cytosol and mitochondria Ø High levels of ammonia in the blood are toxic, the normal rang is (10–20 μg/dL) Ø Nitrogen is transported between organs in organic forms such as alanine,, glutamine, glutamate and urea. Ø Urea is a soluble compound in water & less toxicthan ammonia Ø Then Urea finds its way to the kidneys to be excreted in the urine. Ø

Sources of Ammonia 

Many tissue particularly liver

Form ammonia from amino acids by Transdeamination  

NH3

Diet Body protein catabolism

Amino acid

Urea(57%)

Plasma proteins(7%) Body proteins(14%) Circulation (22%)

Sources and fat of Ammonia... Glutamine

Deamintion Glutamate Gln / Asn

NH3

Pyrimidine, Purine

Pool

Putrification Amines, Amino Sugars

Asparagine

Urea

Sources of Ammonia... Ammonia from amines: Amines in diet and monoamines as hormones or neurotransmitters give rise to ammonia by the action of amine oxidase. 



From purines and pyrimidines: During catabolism of purines and pyrimidinesammonia is removed which is formed from the amino group attached to the purines and pyrimidine rings. 



From bacterial action in intestine: Ammonia is produced by bacterial degradation of urea in lumen of intestine and from there it is absorbed by way of portal vein. 

Transportation of ammonia Ø There are two forms (glutamine and alanine) of ammonia transporter. Ø Glutamine is a temporary nontoxic storage it is transported to the liver Ø kidney remove (NH3) by glutaminaseand excreted its amide-N as ammonium salt (NH4+) in the urine

CONH2

COOH

CH2

CH2

CH2 CHNH2

glutaminase H2O

CH2 CHNH2

COOH

COOH

glutamine

glutamate

+ NH3

Biosynthesis of Urea Ø The Urea Cycle occurs mainly in liver

,urea

excreted by kidney

.

Aspartate

ØThe 2 nitrogen atoms of urea enter the Urea Cycle as NH3. and as the amino acid (aspartate) ØThe source of its carbon is carbon dioxide (Co2)from respiratory .

NH2

Co2

Urea cycle Krebs-Henseleit Or ornithine

The enzymes catalyzing the urea cycle reactions: 1)Carbamoyl phosphate synthetase 2)Ornithine transcarbamoylase 3)Argininosuccinate synthetase 4)Arginino succinase 5)Arginase.

Fumarate eturns to TCA cycle)

O

UREA

H2N-C- NH2

2ATP + HCO3- + NH4+

Arginine







Argininosuccinate Ornithine AMP+PPi

Carbamoyl phosphate  Ornithine transcarbamoylase

 -OOC-CH-NH3  CH2COO-

+

2ADP + Pi

Ornithine

Citrulline -Aspartate

Carbamoyl phosphate synthetase

Citrulline Pi ATP

CYTOPLASM

 argininosuccinate synthetase

MITOCHONDRIA

 argininosuccinase

arginase

3ATP + HCO3- + NH4+ + aspartate  2ADP + AMP + 2Pi + PPi + fumarate + urea

Step1: Formation of Carbamoyl Phosphate  1 mol of ammoniaand respiratory carbon

NH 3+ CO 2

dioxide condense in presence of 2 ATP ATP to form carbamoyl phosphate.  This reaction catalyzed by Carbamoyl

Phosphate Synthase (Types ǀ) .  Carbamoyl Phosphate Synthase is the

HCO3−

ADP O HO NH 3 Pi

committed stepof the Urea Cycle, and H N 2 irreversible it is allosterically regulated. ATP

C

OPO32−

carbonyl phosphate O C

O

−

carbamate

ADP O H 2N

C

OPO32−

carbamoyl phosphate

The activator N-acetylglutamate.



Ø Carbamoyl Phosphate Synthase has an

N-acetylglutamate O

absolute requirement N-acetylglutamate is H3C C N an allosteric activator whose binding H induces conformation change that enhances the affinity of the enzyme for ATP. Ø This derivative of glutamate is synthesized from acetyl-CoA & glutamate when cellular [glutamate] is high, signaling an excess of glutamate free amino acids due to protein breakdown H or dietary intake. + H3N

H C

COO−

CH2 CH2 COO−

C

(Glu) COO−

CH2 CH2 −

COO

Step 2: Formation of citrulline:-

ØCarbamoyl phosphate is transferred to Ornithine by Ornithine Transcarbamoylase a mitochondrial enzyme to form citrulline. Ø Citrullineleaves the mitochondria in exchange with ornithine which enters from cytosol through a mitochondrial inner membrane transport system. For each cycle, citrulline must leave the mitochondria, and ornithine must enter the mitochondrial matrix. Ø An ornithine/citrulline transporter in the inner mitochondrial membrane facilitates transmembrane fluxes of citrulline & ornithine.

Cytosol mitochondrial matrix carbamoyl phosphate Pi ornithine ornithine

citrulline citrulline

Step 3: Formation of Argininosuccinate :Ø One molecule of aspartate is added to citrulline forming argininosuccinate Argininosuccinate Ø Argininosuccinate synthetase catalyses the reaction with the

AMP+PPi

hydrolysis of ATP to AMP +PPi

argininosuccinate synthetase Citrulline

pyrophosphatasefurther splits P-P to 2 P, thus 2 high energy phosphate bonds get expended in this reaction

-Aspartate

-OOC-CH-NH3  CH2COO-

+

ATP

Step 4: Formation of Arginine ØArgininosuccinate is hydrolyzed by

Fumarate (returns to TCA cycle)

argininosuccinase enzyme to form arginineand fumarate.

Arginine

argininosuccinase

ØFumarate forms L-malate in TCA cycle

Argininosuccinate

The Urea Cycle &TCA Cycle are interconnected

Ø In TCA cycle Addition of H2O to fumarate from urea cycle forms L-malate and subsequently NAD+ dependent oxidation of malate forms oxaloacetate Ø which undergoes transaminationwith glutamate to regenerate Aspartate.

Ø These reactions are catalyzed by cytosolic fumarase and malate Dehyderogenase.

Step 5 : Formation of urea:-

O

ØArginase hydrolyses arginine to urea

H2N-C- NH2

and ornithine which is regenerated. UREA

Ø

ØOrnithine re-enters mitochondria for Arginine the operation of another urea cycle. arginase

Ornithine

Urea Regulation  Enzyme levels change with the protein content of diet During

starvation, activity of urea cycle enzymes are elevatedto meet the increased rate of protein catabolism.  High levels of glutamate leads to increased N-acetyl

glutamate and thereby enhanced activity of carbamoyl phosphate synthase-1, thus augmenting therate of urea synthesis  Arginine is an activator of N-acetyl glutamate synthase.

How ammonia is toxic to CNS 1. Failure of liver function  High [NH3] would drive Glutamine Synthase: glutamate + ATP + NH  glutamine + ADP + P 3 i 



This would decrease glutamate which precursor for synthesis

of the neurotransmitter GABA. 2. decreased of glutamate & high ammonia level would drive Glutamate Dehydrogenase reaction to reverse: glutamate+NAD(P)+ α -ketoglutarate + NAD(P)H + NH + 4 

The resulting depletion of α -ketoglutarate, an essential

Krebs Cycle intermediate , impair energy metabolism in the brain.

Types of Hyperammonemia : Acquired : qLiver cell failure : Liver cell cannot convert ammonia to urea (Cirrhosis of liver due to alcoholism, hepatitis or billiary obstruction). qRenal failure. qPortal blood bypasses liver and shunted directly into systemic circulation, due to formation of collateral circulation around liver, leading to elevated levels of circulating ammonia.

Hereditary Hyperammonemia 

Result of Genetic deficiency of any of the Urea Cycle

enzymes leads to hyperammonemia. 

Treatment of deficiency of Urea Cycle enzymes

(depends on which enzyme is deficient): w limiting protein intake to the amount barely adequate to supply amino acids for growth, while adding to the diet the α -keto acid analogs of essential amino acids. w Liver transplantation has also been used, since liver is the organ that carries out Urea Cycle.

Disorders Of Urea Cycle There are six disorders of urea cycle:





There are deficiencies of the each of the enzymes involved in urea cycle.



The symptoms are due to the high levels of ammonia in each disorder.

Symptoms of ammonia intoxication characterized by



ü ü ü ü ü ü

1.

Tremors, Slurring of speech, Blurring of vision, Vomiting, Irritability, And hepatic coma and death

1)N-Acetylglutamate synthetase Deficiency: Hyperammonemia and generalized



hyperaminoacidemia is noticed in a newborn whose liver contained no detectable ability tosynthesize Nacetyl glutamate

Therapy administration of carbamoyl glutamate



which is an activator of carbamoyl phosphate synthetase.

2)Hyperammonemia Type I: 

Deficiency of carbamoyl phosphate synthetase in Liver.

Treatment supplemented with benzoate





, phenylacetate and arginine(activator).

3) Hyperammonemia Type II:



x-linked inheritance. Males are affected. deficiency of



ornithine transcarbamoylase Orotic acid also increases because carbamoyl phosphate that



can not be used to form citrulline diffuses into the cytosol where it condenses with aspartate becoming orotic acid and orotic aciduriaoccurs.

4) Hypercitrullinemia : Due to Deficiency of argininosuccinate synthetase ,



citrulline is unable to condense with aspartate to form argininosuccinate ,and elevated levels of citrulline in blood and urine are seen. 

5) Argininosuccinic aciduria : Impaired ability to split argininosuccinate to form arginine due to the deficiency of argininosuccinase. argininosuccinate will be accumulated

6(Hyperargininemia Arginase deficiency is a rare disease that causes



many abnormality in the development and function of the central nervous system. Arginine accumulates and is excreted.



Hyperornithinemia Hyperammonemia syndrome:



Autosomal recessive disorder.



Elevated Ornithine and Ammonia levels in blood



due to the impaired transport of ornithine into



mitochondria via ornithine-citrulline antiporter. Urea cycle gets impaired and ammonia levels increase