Metabolism of Amino Acids - PDF

Metabolism of Amino Acids - II 1 Outline  Common catabolic pathways of amino Acids  Transport of ammonia  Pathways of amino acid degradation (the carbon skeleton) and associated disorders  Sources and fates of blood ammonia  Urea cycle and its re...

Metabolism of Amino Acids - II 1 Outline  Common catabolic pathways of amino Acids  Transport of ammonia  Pathways of amino acid degradation (the carbon skeleton) and associated disorders  Sources and fates of blood ammonia  Urea cycle and its regulation  Hyperammonemia and ammonia toxicity  Possible treatments for defective urea cycle 2  Catabolism of Amino Acids  At normal condition, AAs derive10-15% of energy (human) 3 Common Catabolic pathways of Amino Acids  Transamination: H Reversible transfer of R1 C COO - + R2 C COO - aminogroup from + NH3 O transaminable aminoacids to -Keto acids PLP Transaminase  Mainly in liver, kidney, H - - brain and heart. R1 C COO + R2 C COO + O NH3  Amino acids are transaminable except lysine, threonine, proline  ALT and AST are predominantly and hydroxyproline present in the liver, but AST is also found in heart, muscle, and kidneys. 4 Common catabolic pathways …  Deamination: associated with or without dehydrogenation (oxidative or non-oxidative)  Oxidative (aerobic dehydrogenase):  Major sites: liver and kidney  Mitochondrial glutamate dehydrogenase  L-amino acid oxidases & D-Amino acid oxidases within peroxisomes  Non-oxidative: Mainly By Dehydratases (Various organs):  E.g. Hydroxyamino acids  Serine → pyruvate + NH4+  Threonine  -ketobutyrate + NH4+ 5  Common catabolic pathways …  Transdeamination  Sequential occurrence of transamination and deamination 6  Other Catabolic Pathways … (Cont’d)  Deamidation:  Glutamine & asparagine contain R group amides  Released as NH4+ by deamidation  Asparagine is deamidated by asparaginase,  Produce aspartate and NH4+  Glutaminase acts on glutamine, important in liver and kidney:  Forming glutamate & NH4+  Hydrolytic deamination:  important for deamination of nitrogenous bases 7 Transport of Ammonia  Skeletal muscle ammonia removal & transport by glucose-alanine cycle  Transport of ammonia from most other tissues to liver in the form of glutamine.  Other Source of Ammonia:  Nucleotide Catabolism → Release of free ammonia → Formation of glutamine by glutamine synthetase 8 Transport of Ammonia From Skeletal Muscle to Liver: Glucose-Alanine Cycle 9 Pathways of amino acid degradation – Carbon-Skeleton 10  Six amino acids degraded to pyruvate  Serine can be catabolized into Glycine or pyruvate  Tryptophan is degraded to Alanine and Acetoacetate  Threonine catabolism via two pathways via dehydrogenase (mitochondrial) and dehydratase (cytosolic) 11 Cytosolic Threonine catabolism (main pathway) 12  Glycine Synthesis and Degradation  Glycine cleavage enzyme system is major pathway  Important route for generation of one carbon unit 13  Degradation of aromatic amino acids  Cleavages of aromatic rings in biological systems are catalyzed by dioxygenases and monooxygenases. 1. Tryptophan degradation 14 Degradation of aromatic amino acids 2. Degradation of Phenyl Alanine and Tyrosine  Tyrosine to Homogentisate is catalyzed by 4-hydroxy- phenylpyruvate dioxygenase ◦ Requires O2 and Vit C ◦ Also generate dehydroascorbate, CO2 and H2O 15  Degradation of aromatic amino acids 2. Degradation of Phenyl Alanine and Tyrosine 16 Phenylketonuria (PKU) & Tyrosinemia (type II & III)  Tyrosinemias type II & III - if untreated, weakness, liver damage, mental retardation. 17  Phenyketonuria  Caused by deficiency of hepatic phenylalanine hydroxylase or of its tetrahydrobiopterin cofactor.  Neurological problems (mental retardation (major manifestation), seizures, tremors, microcephaly, etc) due to reduced production of catecholamines  Hypopigmentation (light skin, hair, blue eyes) due to reduced melanin production 18 Backup pathway for phenylalanine degradation in PKU patients  Urine contains high phenylalanine, phenylpyruvate and phenylacetate due to its increase in plasma  Accumulation of phenylacetate in tissues and body fluids results in ‘mousty odor’ of the urine. Accumulation of phenylalanine leads to: 1. Defective “serotonin” formation. 2. Impairment of melanin synthesis Treatment: By giving diet having very low levels of phenylalanine. 19  Alkaptonuria and Tyrosinaemia type 1  Alkaptonuria:  Causes accumulation of Homogentisate  Dark urine when stay in air  Tyrosinaemia type 1:  Usually present with severe liver disease 20 Alkaptonuria Arthritis is a long-term complication of alkaptonuria Dark spots on the on the sclera and cornea Darkening of the ear Accumulation of oxidized homogentisic acid pigment in connective tissue (ochronosis) 21 Albinism Phenylalanine Tyrosine Thyroxine  Genetically Melanin determined lack or DOPA tyrosinase deficit of enzyme tyrosinase Dopamine  Tyrosinase in melanocytes oxidize tyrosine to DOPA and DOPA- quinone Norepinephrine Epinephrine 22  Symptoms of albinism: Inhibition of production or lack of melanin in skin, hair, eyes Increased sensitivity to sunlight Increased risk of skin cancer development sun burns photophobia decrease of vision acuity 23 Five amino acids degraded to α-ketoglutarate 24 Amino acids degraded to succinyl CoA 25 Methionine derivative: S-adenosyl methionine (SAM)  A powerful methyl donor  Methylation is important in biosynthesis of hormones, proteins, neurotransmitters, phospholipids , L-carnitine, creatine, specific membrane function. 26  Homocystinuria  Deficiency of cystathionine synthase, cofactors or other related problems  High urinary levels of homocysteine, a substrate of the impaired enzyme.  High plasma homocysteine causes chronic progressive skeletal abnormalities, mental retardation and severe thrombotic tendencies.  Treatment by two forms ◦ by high doses of vitamin B6 ◦ limiting intake of methionine 27 Cysteine Catabolism  Nutritionally semi-essential present in the form of L-cystine in ECM.  Liver regulate free cysteine pool  Level regulated by increased/ decreased synthesis of glutathione by glu-cysteine ligation/breakdown  H2S is physiologically active gasotransmitter.  Regulate various function such as angiogenesis, synaptic transmission, transcription, etc 28 Degradation of the Branched-chain Amino Acids  Branched amino acids degraded in, muscle, adipose, kidney and brain to α-keto acids 29 Genetic Defects of Branched Chain Amino Acid Metabolism 30 Maple syrup urine disease  The disorder of the oxidative decarboxylation of - ketoacids derived from valine, isoleucine, and leucine due to deficiency of the branched-chain α-keto acid 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  Incidence of the disease: approximately 1 in 200,000  Included in most newborn screening programs like PKU. 31  Maple syrup urine disease  The early symptoms:  Vomiting and loss of appetite  fatigue  ketoacidosis  mental and physical retardation  unrecognized disease leads to seizures, coma, and death  Treatment plans: Dietary restriction 32  Catabolism of Lysine  Catabolize into acetyl-CoA (ketogenic) ◦ Pipecolic acid can be metabolized back to α-amino-adipic semialdehyde by reversible reaction manner.  Trymethyllysine residue can generate carnitine upon the protein breakdown 33 Fate of Nitrogen From AA Catabolism 34  Sources and Fates of blood ammonia 1. Sources of blood ammonia: Main source: transamination followed by deamination of amino acids. Glutaminase in intestine Minor sources Deamination of nitrogenous bases. Putrefaction of dietary proteins 2. Disposal of blood ammonia: Anabolic: Synthesis of urea, non-essential amino acids, purines, pyrimidines, porphyrins and sugaramines. Catabolic: Excreted in the form of NH4+ after conjugation with H+ in exchange with Na+ Glutaminase reaction in kidney Direct deamination of other amino acids by other deaminases in kidney. 35 Sources of Nitrogen and Urea Formation 36 UREA CYCLE: Formation of Carbamoyl Phosphate 37  UREA CYCLE: Formation of ammonia TCA cycle 38  UREA CYCLE: For Removal of ammonia Key regulatory enzyme: Carbamoyl Phosphate Synthetase I Allosteric Activator: N-acetyl-glutamate TCA cycle 39  Linkage of Citric Acid and Urea cycles/ Fumarate (in cytosol)  Citric acid cycle (in mitochondria) 40 Regulation of the urea cycle  By dietary condition and availability of substrates: “Feed forward” regulation of the enzyme carbamoyl phosphate synthase I (CPS I)  Arginine  N-acetylglutamate (NAG) synthesis allosteric activation of CPS I  Concentration of NAG is determined by Concentrations of substrates: acetyl CoA and glutamate Concentration of arginine, which activates N- acetylglutamate synthetase.  Increase in ornithine synthesis  in urea cycle intermediate 41 Blood Urea Level  Normal blood urea level: 8 - 25 mg/dL.  Urea is excreted in the urine by kidney & its concentration ranges between 20 - 40 gm/day.  Blood urea level decreases in advanced liver diseases  Blood urea level in advanced renal disease/ Renal failure: 200-300 mg/dL  Types of Uremia/ Condition of increased blood urea level:  Pre-renal uremia: Caused by decrease in the blood volume as in salt and water depletion, severe prolonged vomiting or diarrhea, etc  Renal uremia: Due to decrease in total urinary output  Post-renal uremia: Due to obstruction to urine flow 42 Metabolic Disorders of Urea Biosynthesis  Comparatively rare, but medically devastating,  Show similar clinical signs & symptoms ◦ can characterize any number of different molecular level defects in a given enzyme  Rational therapy of the disorders must be based on: ◦ understanding of the relevant enzyme catalyzed reactions in both normal & impaired individuals 43 Cont’d  Identification of accumulated intermediates & additional products prior to a metabolic block (defective step)  Can implicate the reaction that is impaired  Provides the basis for metabolic screening tests  Precise diagnosis the disorders requires quantitative assay of the activity of the enzyme suspected to be defective  Finally, the gene that encodes the mutant enzyme is cloned;  its DNA sequence compared to that of the wild-type gene to identify the specific mutation(s) that cause the disease 44 Cont’d  All defects in urea synthesis result in ammonia intoxication  The effects are most severe when the metabolic block occurs  Clinical symptoms are common to all urea cycle disorders include: ◦ Vomiting ◦ Intermittent ataxia (poor muscle control and clumsy movement), irritability, ◦ Lethargy (causes you to feel sleepy or fatigued and sluggish) ◦ Severe mental retardation 45 Hyperammonemia (Ammonia intoxication)  Normal blood NH3 level: 40 - 70 μg/100ml.  Hyperammonemia: >80 g/dL in blood leading to ammonia intoxication.  Types of hyperammonemia ◦ Acquired hyperammonemia:  Eg. Caused by liver cirrhosis, hepatitis, cancer ◦ Inherited hyperammonemia: Due to urea cycle disorders.  e.g. genetic deficiencies of Urea cycle enzymes.  Congenital hyperammonmia Type I: carbamoyl phosphate synthetase deficiency  Congenital hyperammonmia Type II: Ornithine transcarbamoylase deficiency 46  Ammonia toxicity: Examples  Enhances amination of α – ketoglutarate to glutamate in brain  ↓ cellular respiration/ ATP   Glutamine formation  ↓ cellular pool of glutamic acid  ↓ γ – amino butyric acid (GABA)  High glutamine and glutamate in brain  brain swelling  Out flow of glutamine from the brain  in tryptophan antiport   Serotonin synthesis  Results in a peculiar flapping tremor, slurring of speech, blurring of vision and in severe cases follows coma and death. 47 Argininosuccinase Deficiency in Urea Cycle 48 Possible treatments for defective urea cycle  Feeding the patients with Benzoate or phenylacectate 49 50