Amino Acid Metabolism

Metabolism of Amino Acids - II

• At normal conditions, amino acids provide 10-15% of the energy for humans.

Outline

• Common catabolic pathways of amino acids will be discussed.
• The transport of ammonia will be specified.
• The pathways of amino acid degradation (the carbon skeleton) and associated disorders will be identified.
• Sources and fates of blood ammonia will be outlined.
• The urea cycle and its regulation will be explained.
• Hyperammonemia and ammonia toxicity will be clarified.
• Possible treatments for a defective urea cycle will be reviewed.

Catabolism of Amino Acids

• Amino acids are broken down into amino acids, nucleotides, and biological amines.
• Nitrogen groups degrade into NH4+.
• The carbon skeleton degrades into citrate cycle, pyruvate oxaloacetate, and Acetyl-CoA

Common Catabolic Pathways of Amino Acids

• Transamination includes the reversible transfer of an aminogroup from transaminable aminoacids to alpha-Keto acids.
• Transamination mainly occurs in the liver, kidney, brain, and heart.
• Transaminable amino acids include lysine, threonine, proline, and hydroxyproline.
• ALT and AST are predominantly found in the liver, but AST is also found in the heart, muscle, and kidneys.

Common Catabolic Pathways

• Deamination is associated with or without dehydrogenation (oxidative or non-oxidative)
• Oxidative (aerobic dehydrogenase), major sites include the liver and kidney.
• Mitochondrial glutamate dehydrogenase is a common example.
• L-amino acid oxidases & D-Amino acid oxidases within peroxisomes are other examples.
• Non-oxidative pathways mainly involve the use of dehydratases in various organs.
• Examples of this involve hydroxyamino acids.
• Serine becomes pyruvate + NH4+.
• Threonine becomes alpha -ketobutyrate + NH4+.

Transdeamination

• This involves the sequential occurrence of transamination and deamination.
• Aminotransferase degrades into alpha-Amino Acid, dehydrogenase degrades into NADH + NH4+, and the urea cycle degrades the product into urea.

Other Catabolic Pathways

• Deamidation involves glutamine & asparagine containing R group amides.
• They are released as NH4+ by deamidation
• Asparagine is deamidated by asparaginase, which produces aspartate and NH4+.
• Glutaminase acts on glutamine and is important in the liver and kidney, forming glutamate & NH4+.
• Hydrolytic deamination breaks apart nitrogenous bases.

Transport of Ammonia

• Skeletal muscle ammonia is removed and transported by the glucose-alanine cycle.
• Ammonia is transported from most other tissues to the liver in the form of glutamine.
• Other Sources of Ammonia:
• Nucleotide Catabolism
• Release of free ammonia
• Formation of glutamine by glutamine synthetase

Transport of Ammonia From Skeletal Muscle to Liver: Glucose-Alanine Cycle

• Glucose becomes Pyruvate through glycolysis.
• Pyruvate becomes Glutamate then Alanine with the help of alanine aminotransferase.
• The alanine then returns to the liver, becomes Glutamate using alanine aminotransferase, and becomes urea for the urea cycle.

Pathways of amino acid degradation – Carbon-Skeleton

• Leucine, Lysine, Phenylalanine, Tryptophan, and Tyrosine degrade into Ketone bodies.
• Arginine, Glutamine, Histidine, and Proline degrade into Glutamate.
• Isoleucine, Methionine, Threonine, and Valine degrade into Succinyl-CoA.
• Phenylalanine and Tyrosine degrade into Fumarate.
• Alanine, Cysteine, Glycine, Serine, Threonine, Tryptophan, Asparagine, and Aspartate degrade into Glucose.

Six Amino Acids Degraded to Pyruvate

• Serine can be catabolized into Glycine or pyruvate
• Tryptophan is degraded to Alanine and Acetoacetate
• Threonine catabolism occurs via two pathways via dehydrogenase (mitochondrial) and dehydratase (cytosolic)

Cytosolic Threonine Catabolism (main pathway)

• Threonine degrades into Alpha-Ketobutyrate.
• It then degrades into Propionyl-CoA.
• Next, the product degrades into Methylmalonyl-CoA.
• Finally, through Succinyl-CoA the result is fulfilled.

Glycine Synthesis and Degradation

• Glycine cleavage enzyme system is a major pathway.
• Important route for generation of one carbon unit

Degradation of aromatic amino acids

• Cleavages of aromatic rings in biological systems are catalyzed by dioxygenases and monooxygenases.

Tryptophan degradation

• Tryptophan becomes N-Formylkynurenine via Dioxygenase.
• N-Formylkynurenine becomes Kynurenine.
• Kynurenine becomes 2-Amino-3-carboxymuconate-6-semialdehyde, then Alanine with the help of Dioxygenase.

Degradation of Phenyl Alanine and Tyrosine

• Tyrosine to Homogentisate is catalyzed by 4-hydroxy-phenylpyruvate dioxygenase
• Requires O2 and Vit C
• Also generates dehydroascorbate, CO2 and H2O

Phenyketonuria (PKU) & Tyrosinemia (type II & III)

• Tyrosinemias type II & III - if untreated, weakness, liver damage, mental retardation could occur.

Phenyketonuria

• Caused by deficiency of hepatic phenylalanine hydroxylase or of its tetrahydrobiopterin cofactor.
• Neurological problems like mental retardation (major manifestation), seizures, tremors, etc. can occur due to the reduced production of catecholamines.
• Hypopigmentation (light skin, hair, blue eyes) occurs due to reduced melanin production

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:
• Defective "serotonin" formation
• Impairment of melanin synthesis
• Treatment: By giving diet having very low levels of phenylalanine

Alkaptonuria and Tyrosinaemia Type 1

• Alkaptonuria causes accumulation of Homogentisate.
• Dark urine is present when staying in air due to homogentisic acid.
• Tyrosinaemia type 1 is usually present with severe liver disease

Alkaptonuria

• Arthritis is a noted long-term complication
• Urine turns a black color upon exposure to air
• Dark spots appear on the sclera and cornea
• Darkening of the ear occurs
• Accumulation of oxidized homogentisic acid pigment happens in connective tissue (ochronosis)

Albinism

• Genetically determined lack or deficit of enzyme tyrosinase.
• Tyrosinase in melanocytes oxidize tyrosine to DOPA and DOPA-quinone
• 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

Five Amino Acids Degraded to alpha-ketoglutarate

• Arginine
• Histidine
• Proline
• Glutamate
• Glutamine

Amino Acids Degraded to Succinyl CoA

• Methionine
• Valine
• Isoleucine
• Threonine

Methionine derivative: S-adenosyl methionine (SAM)

• Is a powerful methyl donor
• Methylation is important in biosynthesis of hormones, proteins, neurotransmitters, phospholipids, L-carnitine, creatine, specific membrane function.

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 occurs using two approaches:
• by high doses of vitamin B6
• limiting intake of methionine

Cysteine Catabolism

• Nutritionally semi-essential that is present in the form of L-cystine in ECM.
• The liver regulates free cysteine pool.
• Level is regulated by increased/decreased synthesis of glutathione by glu-cysteine ligation/breakdown
• H2S is physiologically active gasotransmitter that regulates various function such as angiogenesis, synaptic transmission, transcription, etc

Degradation of the Branched-chain Amino Acids

• Branched amino acids are degraded in, muscle, adipose, kidney and brain to alpha-keto acids

Genetic Defects of Branched Chain Amino Acid Metabolism

• Maple syrup urine disease can occur

Maple Syrup Urine Disease

• This is the disorder of the oxidative decarboxylation of alpha-ketoacids derived from valine, isoleucine, and leucine due to deficiency of the branched-chain alpha-keto acid dehydrogenase
• Levels of branched-chain amino acids and corresponding alpha-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.
• Early symptoms include vomiting and loss of appetite.
• In addition, fatigue, ketoacidosis, and metal and physical retardation can occur.
• Unrecognized disease leads to seizures, coma, and death.
• Treatment plans: Dietary restriction

Catabolism of Lysine

• Catabolizes into acetyl-CoA (ketogenic)
• Pipecolic acid can be metabolized back to alpha-amino-adipic semialdehyde by reversible reaction manner.
• Trymethyllysine residue can generate carnitine upon the protein breakdown

Fate of Nitrogen From AA Catabolism

• Most tissues contain Glutamate and Glutamine.
• Livers contain Glutamate and Urea.
• Muscle contains Amino acids.

Sources and Fates of Blood Ammonia

• Main source: transamination followed by deamination of amino acids.
• Glutaminase is present in intestine
• Minor sources:
• Deamination of nitrogenous bases.
• Putrefaction of dietary proteins
• 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.

Sources of Nitrogen and Urea Formation

• NH3 in biomolecules and Amino acids turns into Glutamine.
• Citrate cycle and Amino acids turns into Glutamate.
• Gutamine and Glutamate become NH4+.
• Glutamate, NH4+ and HCO3 become Aspartate.
• All the prior products combine into the UREA cycle product UREA, while Fumarate is produced.

UREA CYCLE: Formation of Carbamoyl Phosphate

• Glutamine from extrahepatic tissues, and Amino acids turn into Alpha-Ketoglutarate.
• Alanine from muscle turns into Alpha-Keto acid.
• OOC-CH2-CH2-CH-COO comes from Glutamate.
• Glutamate then becomes carbamoyl phosphate via glutamate dehydrogenase.

UREA CYCLE: Formation of Ammonia

• Ornithine turns into carmamoyl phosphate via Ornithine transcarbamoylase.
• Citrulline becomes Urea via Argininosuccinase.

UREA CYCLE: For Removal of Ammonia

• Key regulatory enzyme: Carbamoyl Phosphate Synthetase I Allosteric Activator: N-acetyl-glutamate

Linkage of Citric Acid and Urea cycles

• Fumarate (in cytosol) becomes Citric acid cycle (in mitochondria)

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 is the key
• N-acetylglutamate (NAG) synthesis increases 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 causes an increase in urea cycle intermediate

Blood Urea Level

• Normal blood urea level occurs in the range of 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

Metabolic Disorders of Urea Biosynthesis

• Comparatively rare, but medically devastating in occurrence, showing similar clinical signs & symptoms
• rational therapy of the disorders stem from normal and impaired individuals.
• 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
• 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

Hyperammonemia (Ammonia intoxication)

• Normal blood NH3 level: 40 - 70 μg/100ml.
• Hyperammonemia: >80 µg/dL in blood leads to ammonia intoxication.
• Types of hyperammonemia:
• Acquired hyperammonemia:
• Eg is caused by liver cirrhosis, hepatitis, cancer
• Inherited hyperammonemia: Due to urea cycle disorders.
• e.g. genetic deficiencies of Urea cycle enzymes.
• Congenital hyperammonmia Type 1: carbamoyl phosphate synthetase deficiency
• Congenital hyperammonmia Type II: the enzyme involved is Ornithine transcarbamoylase.

Ammonia Toxicity: Examples

• Enhances amination of alpha – ketoglutarate to glutamate in brain causes a decrease in cellular respiration/ ATP
• An increase in Glutamine formation causes a decrease in cellular pool of glutamic acid, gamma – amino butyric acid (GABA)
• High glutamine and glutamate in brain causes brain swelling
• Out flow of glutamine from the brain ↑ in tryptophan antiport causes an increase in Serotonin synthesis
• Results in a peculiar flapping tremor, slurring of speech, blurring of vision and in severe cases follows coma and death.

Argininosuccinase Deficiency in Urea Cycle

• Arginine turns into Argininosuccinate.
• Argininosuccinase Deficiency comes after this stage.

Possible Treatments for Defective Urea Cycle

• Feeding the patients with Benzoate or phenylacectate
• Benzoate becomes Benzoyl CoA.
• Phenylacetate becomes Phenylacetyl CoA.

Test your knowledge of amino acid metabolism! This quiz covers topics such as enzyme cofactors, complications in tyrosinemia, PKU, hypopigmentation, phenylacetate accumulation, catabolism of serine, asparagine deamidation, and the glucose-alanine cycle.