Done by Abdulaziz massoud alfaydi

Urea Cycle and Hyperammonemia

UreaThe liver is the principle site of ammonia detoxification.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

High levels of ammonia in the blood are toxic, the normal rang is (10–20 μg/dL)

Nitrogen is transported between organs in organic forms such as alanine,, glutamine, glutamate and urea.

Urea is a soluble compound in water & less toxic than ammonia

Then Urea finds its way to the kidneys to be excreted in the urine.

Many tissue particularly liver

Form ammonia from amino

acids by Transdeamination

Sources of Ammonia

Amino acid

Diet

Body protein catabolism

NH3 Urea(57%)

Plasma proteins(7%)

Body proteins(14%)

Circulation (22%)

Sources and fat of Ammonia...

NH3

Pool

Glutamine

Asparagine

Urea

Glutamate

Deamintion

Amines,Amino Sugars

Pyrimidine, Purine

Gln / Asn

Putrification

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 pyrimidines

ammonia 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 ammoniaThere are two forms (glutamine and

alanine) of ammonia transporter.

Glutamine is a temporary nontoxic

storage it is transported to the liver

kidney remove (NH3) by glutaminase

and excreted its amide-N as ammonium

salt (NH4+) in the urine

CONH2

CH2

CH2

CHNH2

COOHglutamine

COOH

CH2

CH2

CHNH2

COOHglutamate

H2O+ NH3

glutaminase

The Urea Cycle occurs mainly in

liver ,urea excreted by kidney .

The 2 nitrogen atoms of urea enter

the Urea Cycle as NH3.and as the

amino acid (aspartate)

The source of its carbon is carbon

dioxide (Co2) from respiratory .

Biosynthesis of Urea

Co2

Aspartate

NH2

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.

3ATP + HCO3- + NH4

+ + aspartate 2ADP + AMP + 2Pi + PPi + fumarate + urea

CYTOPLASM MITOCHONDRIA

Ornithine

Citrulline

argininosuccinate synthetase argininosuccinase arginase

AMP+PPi

-Aspartate

Argininosuccinate

ATP

Arginine

Fumarate(returns

to TCA cycle)

Pi

Ornithine

Citrulline

Ornithine transcarbamoylase

Carbamoyl phosphate

2ATP + HCO3- + NH4

+

2ADP + Pi

Carbamoyl phosphate synthetase

O

H2N-C- NH2

UREA

Ornithine

-OOC-CH-NH3+

CH2COO-

1 mol of ammonia and respiratory

carbon dioxide condense in presence of

2 ATP to form carbamoyl phosphate.

This reaction catalyzed by Carbamoyl

Phosphate Synthase (Types ǀ) .

Carbamoyl Phosphate Synthase is the

committed step of the Urea Cycle, and

irreversible it is allosterically regulated.

NH3+ CO2

H2N C OPO32

O

H2N C O

O

HO C

O

OPO32

HCO3

ATP

NH3

ADP ATP

Pi

ADP

carbonyl phosphate

carbamate

carbamoyl phosphate

Step1: Formation of Carbamoyl Phosphate

The activator N-acetylglutamate.

Carbamoyl Phosphate Synthase has an

absolute requirement N-acetylglutamate is

an allosteric activator whose binding

induces conformation change that

enhances the affinity of the enzyme for ATP.

This derivative of glutamate is synthesized

from acetyl-CoA & glutamate when cellular

[glutamate] is high, signaling an excess of

free amino acids due to protein breakdown

or dietary intake.

NH

C COO

CH2

CH2

COO

H

CH3C

O

N-acetylglutamate

H3N+ C COO

CH2

CH2

COO

H

glutamate (Glu)

Step 2: Formation of citrulline-:

Carbamoyl phosphate is transferred to Ornithine by Ornithine Transcarbamoylase a mitochondrial enzyme to form citrulline.

Citrulline leaves 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.

An ornithine/citrulline transporter in the inner mitochondrial membrane facilitates transmembrane fluxes of citrulline & ornithine.

Cytosol

mitochondrial matrix

carbamoyl phosphate Pi

ornithine citrulline

ornithine citrulline

Step 3:Formation of Argininosuccinate-:

One molecule of aspartate is added

to citrulline forming

argininosuccinate

Argininosuccinate synthetase

catalyses the reaction with the

hydrolysis of ATP to AMP +PPi

pyrophosphatase further splits P-P

to 2 P, thus 2 high energy phosphate

bonds get expended in this reaction

Citrulline

AMP+PPi

-Aspartate

Argininosuccinate

ATP-OOC-CH-NH3

+

CH2COO-

argininosuccinate synthetase

Step 4: Formation of Arginine

Argininosuccinate is

hydrolyzed by

argininosuccinase enzyme to

form arginine and fumarate.

Fumarate forms L-malate in

TCA cycle

Argininosuccinate

Arginine

Fumarate

(returns to TCA cycle)

argininosuccinase

The Urea Cycle &TCA Cycle are interconnected

In TCA cycle Addition of H2O to fumarate from urea cycle forms L-malate and subsequently NAD+ dependent oxidation of malate forms oxaloacetate

which undergoes transamination with glutamate to regenerate Aspartate.

These reactions are catalyzed by cytosolic fumarase and malate Dehyderogenase.

Step 5 : Formation of urea-:

Arginase hydrolyses arginine to

urea and ornithine which is

regenerated.

Ornithine re-enters mitochondria for

the operation of another urea cycle.

Arginine

O

H2N-C- NH2

UREA

Ornithine

arginase

Urea Regulation

Enzyme levels change with the protein content of diet

During starvation, activity of urea cycle enzymes are

elevated to 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 the rate 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 + NH3 glutamine + ADP + Pi

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)+ a-ketoglutarate + NAD(P)H + NH4+

The resulting depletion of a-ketoglutarate, an essential

Krebs Cycle intermediate , impair energy metabolism in the

brain.

Types of Hyperammonemia :Acquired :

Liver cell failure : Liver cell cannot convert

ammonia to urea (Cirrhosis of liver due to

alcoholism, hepatitis or billiary obstruction).

Renal failure.

Portal 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):

limiting protein intake to the amount barely adequate to

supply amino acids for growth, while adding to the diet the a-

keto acid analogs of essential amino acids.

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

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 to synthesize

N-acetyl 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

aciduria occurs.

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

Thank you