Amino Acids Metabolism : Disposal of Nitrogen

No No Storage Amino Acids of Amino Acids in the body No No Storage Amino Acids of Amino Acids in the body amino acidsSo, amino acids must be obtained from 1.Diet 2.De novo synthesis 2.De novo synthesis (of non-essential aa) 3.Degradation 3.Degradation of protein (normal turnover) amino acidsSo, amino acids must be obtained from 1.Diet 2.De novo synthesis 2.De novo synthesis (of non-essential aa) 3.Degradation 3.Degradation of protein (normal turnover)

Amino Acids Pool Diet De novo synthesi s Degradation ProteinsynthesisProteinsynthesis Other Nitogen Other Nitogen- containing comp. Other Nitogen Other Nitogen- containing comp. Glucose & Fas..etc

Simultaneous synthesis & degradation of protein molecules Simultaneous synthesis & degradation of protein molecules Protein Turnover

Protein turnover synthesizeddegraded Most proteins in the body are constantly being synthesized & then degraded, permitting the removal of abnormal or unneeded proteins

Protein Degradation By Two Major Enzyme Systems 1- Ubiquitin-proteasome mechanism Energy-dependent Mainly for endogenous proteins (proteins synthesized within the cell) 2- Lysosomes 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 Catabolism

amino acids notUnlike glucose and fatty acids, amino acids are not stored by the body Amino acidsAmino acids in excess of biosynthetic needs are degraded. amino acidsDegradation of amino acids involves: First Stage Ammonia Removal of α-amino group Ammonia (NH 3 ) Second Stage Remaining carbon skeleton Energy metabolism Amino Acids Catabolism - Overview

1st phase of catabolism of amino acids: Removal of the α-amino groups 1st phase of catabolism of amino acids: Removal of the α-amino groups With production of Free Ammonia In Liver Small amount excreted in urine Urea

AmmoniaAmmonia is produced by all tissues from the catabolism of amino acids Ammonia ureaAmmonia is mainly disposed is via formation of urea in liver ammoina hyperammonemia CNS toxicityBlood level of ammoina must be kept very low, otherwise, hyperammonemia & CNS toxicity will occur ammoniaTo solve this problem, ammonia is transported from peripheral tissues to liver via formation of: Glutamine (most tissues) Alanine (muscle ) Amino Acids Catabolism - Overview

2 nd phase of A. A. catabolism Carbon skeletons of the α-ketoacids are converted to common intermediates of energy producing, metabolic pathways 2 nd phase of A. A. catabolism Carbon skeletons of the α-ketoacids are converted to common intermediates of energy producing, metabolic pathways 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 Metabolism Removal of Nitrogen from Amino Acids Removing the -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 Pathways Amino group (nitrogen) is removed from an amino acid by either Transamination 1- Transamination : by transaminases Oxidative Deamination 2- Oxidative Deamination: by glutamate dehydrogenase

-ketoglutarate -ketoglutarate accepts the  amino group from amino acids to glutamate  become glutamate by: Transaminases (aminotransferases) Transaminases (aminotransferases) Glutamate Glutamate : Glutamate dehydrogenase Glutamate dehydrogenase Ammonia Ammonia Transamination 1- Transamination ALL Amino Acids (except ) ALL Amino Acids (except lysine & threonine) Energy, glucose, FAs or KB Transaminase

Glutamate from transamination steps Glutamate (from transamination steps) by enzyme Glutamate Dehydrogenase Ammonia Ammonia -ketoglutarate Urea UreaCycle Urea used for transamination of further amino acids Oxidative deamination by Glutamate Dehydrogenase 2- Oxidative deamination by Glutamate Dehydrogenase

Diagnostic Value of Plasma Aminotransferases 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 ALT more specific ALT is more specific for liver disease than AST AST more sensitive AST is more 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 Ammonia Ammoniaall tissues Ammonia is produced by all tissues during metabolism of a variety of compounds Ammonia urea Ammonia is disposed of primarily by formation of urea in the liver level of ammonia in bloodmust be kept very low The level of ammonia in blood must be kept very low hyperammonemia Slightly elevated concentrations (hyperammonemia) are toxic to CNS So, There must be a mechanism by which Ammonia is moved from peripheral tissues to the liver for disposal as urea While at the same time Ammonia must be maintained at low levels in blood

Urea 1- Urea in the liver in the liver most importantis quantitatively the most important disposal route for ammonia liverUrea is formed in the liver from ammonia (urea cycle) Ureakidneys urineUrea travels in the blood from the liver to the kidneys where it is filtered to appear in urine Disposal of Ammonia

Glutamine 2- Glutamine in most peripheral tissues especially brain, sk.ms. & liver in most peripheral tissues especially brain, sk.ms. & liver ammonia glutamine synthaseIn most peripheral tissues, glutamate binds with ammonia by action of glutamine synthase brainin the brain, it is the major mechanism of removal of ammonia from the brain nontoxic storage & transport form of ammoniaThis structure provides a nontoxic storage & transport form of ammonia Glutamine is transported to blood to other organs esp. liver & kidneys glutaminaseIn the liver & Kidney, glutamine is converted to ammonia & glutamate by the enzyme glutaminase. Disposal of Ammonia cont.

Alanine 3- Alanine in skeletal muscles in skeletal muscles Ammoniaalanine Ammonia + Pyruvate form alanine in skeletal muscles Alanine is transported in blood to liver ammonia In liver, alanine is converted to pyruvate & ammonia glucose Pyruvate can be converted to glucose (by gluconeogenesis) Glucose (GLUCOSE - ALANINE PATHWAY)Glucose can enter the blood to be used by skeletal muscles (GLUCOSE - ALANINE PATHWAY) Disposal of Ammonia cont.

Alanine in Skeletal Muscles Glutamine in Most Tissues Esp. brain & Kidneys Urea in Liver

Urea Cycle Urea is produced in the liverUrea is produced in the liver From the liver, it is transported in the blood to the kidneys for excretion in urineFrom the liver, it is transported in the blood to the kidneys for excretion in urine Urea is composed of: Two nitrogen atoms free ammoniaFirst nitrogen atom is from free ammonia aspartateSecond nitrogen atom is from aspartate Carbon & oxygen atoms are from CO2

Reactions of the Urea Cycle First two reactions occur in the mitochondriaFirst two reactions occur in the mitochondria Remaining reactions occur in the cytosolRemaining reactions occur in the cytosol Ammonia + Aspartate + CO ATP UREA + Fumarate + 2 ADP + AMP + 2 Pi + PPi + 3 H 2 0 Synthesis of urea is irreversible 4 high-energy phosphates are consumed for synthesis of one molecule of 4 high-energy phosphates are consumed for synthesis of one molecule of urea urea

Overview of Urea Cycle

Fate of Urea Urea (synthesized in the liver) Blood Kidney Kidney intestine Urine cleaved by bacterial urease Ammonia Ammonia CO2 In stool Reabsorbed in blood

Hyperammonemia Hyperammonemia = Increase of ammonia level of blood Blood AmmoniaBlood Ammonia Normal level of blood ammonia is 5-50 mmol/LNormal level of blood ammonia is 5-50 mmol/L HyperammonemiaHyperammonemia A medical emergency as ammonia has a direct neurotoxic effect on CNS Ammonia intoxication :Ammonia 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: Tremors, slurring of speech, somnolence, vomiting, cerebral edema & blurring of vision At high concentrations, ammonia can cause coma & death

Types of Hyperammonemia Acquired Hyperammonemia 1- Acquired Hyperammonemia Liver diseases 1- Liver diseases : are common causes in adults Acute causes: 1- Acute causes: viral hepatitis, ischemia, hepatotoxins Chronic causes: 2- 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 Gatrointestinal Bleeding 2- Gatrointestinal Bleeding By action of bacteria of GIT on blood urea with production of big amounts of ammonia that is absorbed to blood.

Hereditary Hyperammonemia 1- Hereditary Hyperammonemia five enzymes Genetic deficiencies can occur for each of the five enzymes of the urea cycle (overall prevalence 1:300,000 live births) Ornithine 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 autosomal recessive In each case, failure to synthesize urea leads to hyperammonemia during the first weeks following birth mental retardationAll inherited disorders of the urea cycle enzymes result in mental retardation Types of Hyperammonemia Types of Hyperammonemia cont.

Limiting protein in dietLimiting protein in diet Administration of compounds that bind covalently to amino acidsAdministration of compounds that bind covalently to amino 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 Hyperammonemia

Hyperammonemia in Renal Failure Rena Failure Rena Failure urea blood urea levels are elevated urea Transfer of urea to intestine is increased Ammoniabacterial urease Much amounts of Ammonia is formed by bacterial urease Absorbed to blood Hyperammonemia Hyperammonemia 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 production responsible for ammonia production