Ammonia and amino acid metabolism

AMMONIA METABOLISMThe ways of ammonia formation

1. Oxidative deamination of amino acids

2. Deamination of physiologically active amines and nitrogenous bases.

3. Absorption of ammonia from intestine (degradation of proteins by intestinal microorganisms results in the ammonia formation).

4. Hydrolytic deamination of AMP in the brain (enzyme – adenosine deaminase)

Ammonia is a toxic substance to plants and animals (especially for brain)

Normal concentration: 25-40 mol/l (0.4-0.7 mg/l)

Ammonia must be removed from the organism

Terrestrial vertebrates synthesize urea (excreted by the kidneys) - ureotelic organisms

Urea formation takes place in the liver

Birds, reptiles synthesize uric acid

Peripheral Tissues Transport Nitrogen to the Liver

Two ways of nitrogen transport from peripheral tissues (muscle) to the liver:

1. Alanine cycle. Glutamate is formed by transamination reactions

Glutamate is not deaminated in

peripheral tissues

Nitrogen is then transferred to pyruvate to form alanine, which is released into the blood.

The liver takes up the alanine and converts it back into pyruvate by transamination.

The glutamate formed in the liver is deaminated and ammonia is utilized in urea cycle.

Reaction catalyzed by glutamate dehydrogenase.

The glutamate dehydrogenase of mammalian liver has the unusual capacity to use either NAD or NADP as cofactor. The mammalian enzyme is allosterically regulated by GTP and ADP.

Ammonia transport in the form of glutamine. Excess ammonia in tissues is added to glutamate to form glutamine, a process catalyzed by glutamine synthetase. After transport in the bloodstream, the glutamine enters the liver and NH4 is liberated in mitochondria by the enzyme glutaminase.

2. Nitrogen can be transported as glutamine.

Glutamine synthetase catalyzes the synthesis of glutamine from glutamate and NH4

+ in an ATP-dependent reaction:

Hypotheses toxicity of ammonia

A. The binding of ammonia in the synthesis of glutamate causes an outflow of α-ketoglutarate from the tricarboxylic acid cycle, with decreased formation of ATP energy and deteriorates the activity of cells.

B. Ammonium ions NH4 + caused alkalization of blood plasma. This increases the affinity of hemoglobin for oxygen (Bohr effect), the hemoglobin does not release oxygen to the capillaries, resulting the cells hypoxia occurs.

C. The accumulation of free NH4 + ion in the cytosol affects the membrane potential and intracellular enzymes work - it competes with ion pumps, Na + and K +.

D. The producing ammonia tramsform glutamic acid - glutamine - an osmotically active substance. This leads to water retention in the cells and the swelling that causes swelling of tissues. In the case of nervous tissue it can cause brain swelling, coma and death.

E. The use of α-ketoglutarate and glutamate to neutralize the ammonia causes a decrease in the synthesis of γ-aminobutyric acid (GABA) inhibitory neurotransmitter of the nervous system.

Urea cycle - a cyclic pathway of urea synthesis first postulated by H.Krebs

THE UREA CYCLE

The sources of nitrogen atoms in urea molecule:- aspartate;- NH4

+.

Carbon atom comes from CO2.

SYNTHESIS OF UREA

NH 3 + CO2 + 2 АТФ + H2O H2N C

O

O ~ P

OH

OH

O + 2 АДФ + H3PO4

Carbamoilphosphate synthase

carbamilphosphate

ATP ADP

H2N C

O

O ~ P

OH

OH

O +

NH 2

(CH 2)3

HC NH 2

COOH

NH

(CH 2)3

HC NH 2

COOH

C O

NH 2

+ H 3PO4

carbamilphosphateornitine

citrulline

Ornitine-carbomoil transferasa

The carbamoyl phosphate generated in the mitochondria now donates its carbamoyl group to ornithine, which is formed in the cytosol but enters the mitochondrion via a spe cific inner-membrane transport system. The product is citruiline:

The second amino group required for urea synthesis now arrives in the form of aspartate, which in turn acquired it from glutamate by the action of aspartate transaminase in the cytosol. The amino group of aspartate condenses reversibly with the carbamoyl carbon atom of citrulline in the presence of ATP to form argininosuccinate; this reaction is catalyzed by argminosuccinate synthetase:

SYNTHESIS OF UREA

NH

(CH2)3

HC NH2

COOH

C O

NH2

COOH

CH

CH2

H2N

COOH

+ + АТФNH

(CH2)3

HC NH2

COOH

C N

NH2

CH

COOH

CH2

COOH

NH

(CH2)3

HC NH2

COOH

C NH

NH

CH

COOH

CH2

COOH + АMФ + H4P2O7

citrulline aspartate

arginino-succinate

Arginino-succinate synthase

NH

(CH2)3

HC NH2

COOH

C NH

NH

CH

COOH

CH2

COOH

NH

(CH2)3

HC NH2

COOH

C NH

NH2

CH

COOH

CH

COOH

+

arginino-succinate arginine

fumarate

Arginino-succinate lyase

In the next reaction argininosuccinate undergoes a elimination reaction by the action of argininosuccinate lyase to form free arginine and fumarate:

SYNTHESIS OF UREA

NH

(CH2)3

HC NH2

COOH

C NH

NH2

+ H2O

NH2

(CH2)3

HC NH2

COOH

+ C O

NH2

NH2

arginine

ornitineurea

arginase

The Linkage between Urea Cycle, Citric Acid Cycle and Transamination of Oxaloacetate

Fumarate formed in urea cycle enters citric acid cycle and is converted to oxaloacetate.

Fates of oxaloacetate: (1) transamination to aspartate, (2) conversion into glucose,(3) condensation with acetyl CoA to form citrate,(4) conversion into pyruvate.

Diagnostic significance of the determination of urea in urine.

25-30 g/day of urea is excreted in normal conditions.

The increase of urea in urine occurs in high fever, malignant anemia, poisoning by phosphorus, intensive decomposition of protein in organism. The decrease of urea in urine occurs in liver diseases, kidney unsufficiency, acidosis.

Fates of carbon skeleton of amino acids

Transformation of AA carbone backboneTransformation of AA carbone backbone

Alanine

Tyrosine

Leucine

Acetoacetate

Homogentizinic acid

Urocainic acid

Lysine

Phenylalanine

Serine Glycine

Treonine

Glucose

Pyruvate AcetylCoA

Isoleucine

Cystine

Cysteine

Glutamate

Oxaloacetate

Ketoglutarate

SuccinylCoA

Fumarate

Serine

Phenylpyruvate

PhenyllactateBenzoic acid Phenylacetate

Phenylalanine

oxyphenylpyruvate

Mono-, DiiodtyrosineTyrosine

Triiodtyronine

Tyroxine

Maleiloacetate

Homogentisinic acid

AcetoacetateFumarate

Fumariloacetate

horepinephrine

Dopamine

DOPA

epinephrine

Melanine

Dioxyindol

AA

BB BB CC

DD

EEEE

A – PKUB – Tyrosinosis, scurvyC - Alkaptonuria

D – AlbinosisE – Cretinism, Graves' diseaseF – Parkinsons’ disease

FF

INBORN ERRORS OF AMINO ACIDS METABOLISM

Alcaptonuria - inherited disorder of the tyrosine metabolism caused by the absence of homogentisate oxidase.

homogentisic acid is accumulated and excreted in the urine turns a black color upon exposure to air

In children: urine in diaper may darken

In adults: darkening of the ear dark spots on the on the sclera and cornea arthritis

Phenylketonuria is caused by an absence or deficiency of phenylalanine hydroxylase or of its tetrahydrobiopterin cofactor.

Phenylalanine accumulates in all body fluids and converts to phenylpyruvate.

Defect in myelination of nervesThe brain weight is below normal.Mental and physical retardations.The life expectancy is drastically shortened. Diagnostic criteria:

phenylalanine level in the blood FeCl3 test DNA probes (prenatal)

Glutamate

Glutamine

CO2

ketoglutarate

carbamylglutamate

Aminobutaric acid

methylaspartate

NH3

TRANSAMINASE

AcetatePyruvate

ASPARTATE

Asparagine

CO2

Oxaloacetate

Alanine

Aspartylphosphate

NH3

TRANSAMINASE

LisineMetyonine

Fumarate

Maple syrup urine disease - the disorder of the oxidative decarboxylation of -ketoacids derived from valine, isoleucine, and leucine caused by the missing or defect of branched-chain dehydrogenase. The levels of branched-chain amino acids and corresponding -ketoacids are markedly elevated in both blood and urine.

The urine has the odor of maple syrup

The early symptoms: lethargy ketoacidosis unrecognized disease leads to seizures, coma, and death mental and physical retardation