Amino Acids MetabolismAmino Acids Metabolism::

Disposal of NitrogenDisposal of Nitrogen

NoNo Storageof Amino AcidsAmino Acids

in the body

• So, amino acids amino acids must be obtained from

1.1.Diet Diet

2.2.DeDe novo synthesis novo synthesis (of non-essential aa)

3.3.DegradationDegradation of protein (normal turnover)

• So, amino acids amino acids must be obtained from

1.1.Diet Diet

2.2.DeDe novo synthesis novo synthesis (of non-essential aa)

3.3.DegradationDegradation of protein (normal turnover)

Amino Acids Amino Acids PoolPool

Degradation

Degradation

Protein

Protein

synthesis

synthesis

Protein

Protein

synthesis

synthesis Other NitogenOther Nitogen-containing compcontaining comp..Other NitogenOther Nitogen-

containing compcontaining comp..

SimultaneousSimultaneous

synthesis & degradationsynthesis & degradation

of protein moleculesof protein molecules

SimultaneousSimultaneous

synthesis & degradationsynthesis & degradation

of protein moleculesof protein molecules

Protein TurnoverProtein Turnover

Protein turnoverProtein turnover

Most proteins in the body are constantly being synthesizedsynthesized & then degradeddegraded,

permitting the removal of abnormal or unneeded proteins

Protein DegradationProtein Degradation

By Two Major Enzyme SystemsBy Two Major Enzyme Systems

1-1- Ubiquitin-proteasome mechanismUbiquitin-proteasome mechanism

• Energy-dependent• Mainly for endogenous proteins (proteins synthesized within the cell)

2- 2- LysosomesLysosomes

• Non-energy-dependent• Primarily for extracellular proteins as: - plasma proteins that are taken into cells by endocytosis - cell surface membrane proteins: for receptor-mediated endocytosis

Amino Acids CatabolismAmino Acids Catabolism

• Unlike glucose and fatty acids, amino acids amino acids are notnot stored by the body

• Amino acids Amino acids in excess of biosynthetic needs are degraded.

• Degradation of amino acids amino acids involves:

First Stage First Stage

Removal of α-amino group AmmoniaAmmonia (NH3)

Second Stage Second Stage

Remaining carbon skeleton Energy metabolism

Amino Acids Catabolism - OverviewAmino Acids Catabolism - Overview

1st phase of catabolism of amino acids:1st phase of catabolism of amino acids:

RRemoval of the α-amino groupsemoval of the α-amino groups

1st phase of catabolism of amino acids:1st phase of catabolism of amino acids:

RRemoval of the α-amino groupsemoval of the α-amino groups

With production

of

Free Free AmmoniaAmmonia

In Liver

Small amount excreted Small amount excreted in urinein urine

UreaUrea

• AmmoniaAmmonia is produced by all tissues from the catabolism of amino acids

• AmmoniaAmmonia is mainly disposed is via formation of ureaurea in liver

• Blood level of ammoina ammoina must be kept very low, otherwise, hyperammonemia hyperammonemia & CNS toxicity CNS toxicity will occur

• To solve this problem, ammoniaammonia is transported from peripheral tissues to liver via formation of:

Glutamine (most tissues)

Alanine (muscle)

Amino Acids Catabolism - OverviewAmino Acids Catabolism - Overview

22ndnd phase of A. A. catabolism phase of A. A. catabolism CCarbon skeletons of the α-ketoacids arbon skeletons of the α-ketoacids are converted to are converted to

common intermediates of energy producing, metabolic common intermediates of energy producing, metabolic pathwayspathways

22ndnd phase of A. A. catabolism phase of A. A. catabolism CCarbon skeletons of the α-ketoacids arbon skeletons of the α-ketoacids are converted to are converted to

common intermediates of energy producing, metabolic common intermediates of energy producing, metabolic pathwayspathways

• ATP, CO2 & H2O (by Citric acid cycle) • Glucose (by gluconeogenesis)• Fatty Acids (from acetyl CoA)• Ketone Bodies (from acetyl CoA)

• ATP, CO2 & H2O (by Citric acid cycle) • Glucose (by gluconeogenesis)• Fatty Acids (from acetyl CoA)• Ketone Bodies (from acetyl CoA)

Amino Acids MetabolismAmino Acids MetabolismRemoval of Nitrogen from Removal of Nitrogen from Amino AcidsAmino Acids

Removing the Removing the -amino group-amino group • Essential for producing energy from any amino acidEssential for producing energy from any amino acid

• An obligatory step for the catabolism of all amino acidsAn obligatory step for the catabolism of all amino acids

Deamination PathwaysDeamination Pathways

Amino group (nitrogen) is removed from an amino acid Amino group (nitrogen) is removed from an amino acid by eitherby either

1- TransaminationTransamination : by transaminases

2- Oxidative DeaminationOxidative Deamination: by glutamate dehydrogenase

-ketoglutarate-ketoglutarate accepts the

amino group from amino acids to become glutamate glutamate by :

Transaminases Transaminases (aminotransferases)(aminotransferases)

GlutamateGlutamate: Glutamate dehydrogenaseGlutamate dehydrogenase

AmmoniaAmmonia

1- TransaminationTransamination

ALL Amino Acids ALL Amino Acids ((exceptexcept lysine & threonine))

Energy, glucose, FAs or KBEnergy, glucose, FAs or KB

TransaminaseTransaminase

GlutamateGlutamate (from transamination stepsfrom transamination steps)

by enzyme Glutamate Dehydrogenaseby enzyme Glutamate Dehydrogenase

AmmoniaAmmonia -ketoglutarate

Urea Urea CycleCycle

Urea used for transamination

of further amino acids

2- Oxidative deamination Oxidative deamination by Glutamate Dehydrogenaseby Glutamate Dehydrogenase

Diagnostic Value of Plasma Diagnostic Value of Plasma AminotransferasesAminotransferases

• Aminotransferases are normally intracellular enzymes

• Plasma contains low levels of aminotransferases representing release of cellular contents during normal cell

turnover

• Elevated plasma levels of aminotransferases indicate damage to cells rich in these enzymes (as physical trauma or disease to tissue)

• Plasma AST & ALT are of particular diagnostic value

1- liver disease: Plasma ALT & AST are elevated in nearly all liver diseases but, particularly high in conditions that cause cell necrosis as: viral hepatitis toxic injury prolonged circulatory collapse

ALTALT is more specificmore specific for liver disease than AST ASTAST is more sensitivemore sensitive (as liver contains a large amount of AST)

2- Nonhepatic disease: as: Myocardial infarction Skeletal muscle disorders These disorders can be distinguished clinically from liver disease

Diagnostic Value of Plasma Aminotransferases

Metabolism of AmmoniaMetabolism of Ammonia

• AmmoniaAmmonia is produced by all tissuesall tissues during metabolism of a variety of compounds• AmmoniaAmmonia is disposed of primarily by formation of ureaurea in the liver• The level of ammonia in bloodlevel of ammonia in blood must be kept must be kept very lowvery low• Slightly elevated concentrations (hyperammonemiahyperammonemia) are toxic to CNS

So,There must be a mechanism by whichThere must be a mechanism by which

Ammonia is moved from peripheral tissues to the liver Ammonia is moved from peripheral tissues to the liver for disposal as ureafor disposal as urea

While at the same timeWhile at the same timeAmmonia must be maintained at low levels in bloodAmmonia must be maintained at low levels in blood

1- Urea Urea in the liverin the liver

• is quantitatively the most important most important disposal route for ammonia

• Urea is formed in the liver liver from ammonia (urea cycle)

• UreaUrea travels in the blood from the liver to the kidneyskidneys where it is filtered to appear in urineurine

Disposal of AmmoniaDisposal of Ammonia

2- GGlutamine lutamine in in most peripheral tissues most peripheral tissues especiallyespecially brain, sk.ms. & liver brain, sk.ms. & liver

• In most peripheral tissues, glutamate binds with ammoniaammonia by action of glutamine synthase glutamine synthase

• in the brainbrain, it is the major mechanism of removal of ammonia from the brain

• This structure provides a nontoxic storage & transport form of ammonia nontoxic storage & transport form of ammonia • Glutamine is transported to blood to other organs esp. liver & kidneys• In the liver & Kidney, glutamine is converted to ammonia & glutamate

by the enzyme glutaminaseglutaminase.

Disposal of Ammonia Disposal of Ammonia contcont . .

3- Alanine Alanine in skeletal musclesin skeletal muscles

• AmmoniaAmmonia + Pyruvate form alanine alanine in skeletal muscles• Alanine is transported in blood to liver• In liver, alanine is converted to pyruvate & ammoniaammonia • Pyruvate can be converted to glucoseglucose (by gluconeogenesis)• GlucoseGlucose can enter the blood to be used by skeletal muscles

(GLUCOSE - ALANINE PATHWAY)(GLUCOSE - ALANINE PATHWAY)

Disposal of Ammonia Disposal of Ammonia contcont . .

Disposal of Ammonia Disposal of Ammonia contcont . .

Alanine Alanine in Skeletal Musclesin Skeletal Muscles

GlutamineGlutaminein Most Tissuesin Most Tissues

Esp. brain & KidneysEsp. brain & Kidneys

UreaUreain Liverin Liver

Urea CycleUrea Cycle

• Urea is produced in the Urea is produced in the liverliver• From the liver, it is transported in the blood to the From the liver, it is transported in the blood to the kidneyskidneys for excretion in for excretion in

urine urine

Urea is composed of:Urea is composed of:

Two nitrogen atomsTwo nitrogen atoms• First nitrogen atom is from free ammoniafree ammonia• Second nitrogen atom is from aspartateaspartate

Carbon & oxygen atoms are from CO2Carbon & oxygen atoms are from CO2

Reactions of the Urea CycleReactions of the Urea Cycle

• First two reactionsFirst two reactions occur in the occur in the mitochondriamitochondria• RemainingRemaining reactions occur in the reactions occur in the cytosolcytosol

Ammonia + Aspartate + CO2 + 3 ATP

UREA + Fumarate + 2 ADP + AMP + 2 Pi + PPi + 3 H20

• Synthesis of urea is Synthesis of urea is irreversibleirreversible• 4 high-energy phosphates4 high-energy phosphates are consumed for synthesis of are consumed for synthesis of one one molecule of molecule of urea urea

Overview of Urea CycleOverview of Urea Cycle

Fate of UreaFate of Urea

Urea Urea (synthesized in the liver) (synthesized in the liver)

BloodBlood

KidneyKidney intestine

Urine cleaved by bacterial urease

AmmoniaAmmonia CO2

In stool Reabsorbed in blood

HyperammonemiaHyperammonemia= Increase of ammonia level of blood

• Blood Ammonia Blood Ammonia • Normal level of blood ammonia is 5-50 mmol/L Normal level of blood ammonia is 5-50 mmol/L

• Hyperammonemia Hyperammonemia A medical emergency as ammonia has a direct neurotoxic effect on CNSA medical emergency as ammonia has a direct neurotoxic effect on CNS

• Ammonia intoxicationAmmonia intoxication: : • It is defined as toxicity of the brain due to increase in ammonia level in the systemic blood.

• This increased ammonia will be directed to α ketoglutarate to form glutamic acid then glutamine

leading to interference with citric acid cycle so decrease ATP production in the brain cells. Clinical manifestations:Clinical manifestations: Tremors, slurring of speech, somnolence, vomiting, cerebral edema & blurring of vision At high concentrations, ammonia can cause coma & death

Types of HyperammonemiaTypes of Hyperammonemia

1- Acquired HyperammonemiaAcquired Hyperammonemia

1- Liver diseasesLiver diseases: are common causes in adults 1- Acute causes: Acute causes: viral hepatitis, ischemia, hepatotoxins

2- Chronic causes: Chronic causes: liver cirrhosis due to alcoholism, hepatitis, biliary obstruction…etc may result in the formation of collateral circulation around the liver So, portal blood is shunted directly into systemic circulation & detoxication of ammonia to urea is markedly impaired

2- Gatrointestinal BleedingGatrointestinal Bleeding By action of bacteria of GIT on blood urea with production of big amounts of

ammonia that is absorbed to blood.

1- Hereditary HyperammonemiaHereditary Hyperammonemia Genetic deficiencies can occur for each of the five enzymesfive enzymes of the urea cycle

(overall prevalence 1:300,000 live births)

Ornithine transcarbamoylase deficiencyOrnithine transcarbamoylase deficiency• X-linked • Most common deficiency among all 5 enzymes• Males are predominantly affected • Females carriers are clinically affected

All other urea cycle disorders are All other urea cycle disorders are autosomal recessiveautosomal recessive

• In each case, failure to synthesize urea leads to hyperammonemia during the first weeks following birth

• All inherited disorders of the urea cycle enzymes result in mental retardationmental retardation

Types of Hyperammonemia Types of Hyperammonemia cont.

• Limiting protein in dietLimiting protein in diet

• Administration of compounds that bind covalently to amino Administration of compounds that bind covalently to amino acids acids

To produce nitrogen-containing molecules that are excreted in the urine for example: Phenylbutyrate Phenylbutyrate given orally converted to phenylacetate that condenses with glutamine to form phenylacetylglutamine which is excreted in urine

Treatment of HyperammonemiaTreatment of Hyperammonemia

Hyperammonemia in Renal FailureHyperammonemia in Renal Failure Rena FailureRena Failure

blood ureaurea levels are elevated

Transfer of ureaurea to intestine is increased

Much amounts of AmmoniaAmmonia is formed by bacterial ureasebacterial urease

Absorbed to blood

HyperammonemiaHyperammonemia

• To reduce hyperammonemia:To reduce hyperammonemia: Oral neomycin reduces the amount of intestinal bacteria Oral neomycin reduces the amount of intestinal bacteria responsible for ammonia productionresponsible for ammonia production