Nitrogen and Amino Acid Metabolism Flashcards
Urea vs. Uric Acid
Urea is formed from ammonia in the liver as an end product of nitrogen metabolism (urea cycle)
Uric Acid is the end product of purine degredation
Transamination and Deamination
2 ways to remove amino groups from AA in the process of removing AA from the overall “amino pool” (AA degredation)
Transamination transfers an amino group to another alpha-keto acid. Requires Vitamin B6 (PLP) as a cofactor
Deamination removes an amino group from the AA forming free ammonia
Alanine Aminotransferase
ALT
Glucogenic reaction that converts Alanine to pyruvate via transamination transferring the amino group to alpha-ketogluterate forming glutamate.
requires PLP (vitamin B6)
3 Enzymes that can link free ammonia
Glutamate sythatase - forming glutamine from glutamate
Glutamate Dehydrogenase - reversible reaction converting between Glutamate (tissue) and Alpha-Ketogluterate (Liver)
CPT1 - Converts Ammonia (NH2) and Bicarb (HCO3) to Carbamyl-P in the urea cycle. Requires 2ATP and N-acetylglutamate (NAG) as a cofactor
Essential Amino Acids
Phenylalanine
Valine
Trypophan
Threonine
Isoleucine
Methionine
Histadine
Arginine (conditionally essential)
Leucine
Lysine
Glutamate Dehydrogenase
Reversible reaction
In Liver:
forms alpha-ketogluterate and free NH3 from Glutamate. Requires NAD+
In Peripheral Tissue:
Forms Glutamate from Alpha-ketogluterate and free NH3 . Requires NADH
Glutamate synthatase
In Peripheral Tissue:
Forms Glutamine from glutamate and free NH3
Glutaminase
In Renal Tubule and Liver:
Forms Glutamate and free NH3 from Glutamine
Cystinurea
Tubular reabsorption of cystine is decreased (along with ornithine, arginine, lysine) Due to a genetic deficiency of the cystine transporter
Increased cystine in the urine
Cystine precipitates in renal tubules forming stones
One of the causes of renal stones in children.
Hartnup’s Disease
Inherited defect in the transport of the neutral amino acids such as Tryptophan
Decreased absorption and increased excreation of tryptophan.
Most patients are normal as enough dibasic AA to meet the bodies needs. however some may develop NAD+ deficiency (Pallegra)
Pallegra
NAD+ deficiency resulting in decreased Tryptophan &/or Niacin in the body. (niacin is synthesized from Tryptophan)
Results in: (4 D's) Diarrhea Dermatitis Dementia Death
Alanine Metabolism
Major transport form of AA from the muscle (important during starvation)
Alanine is the major precursor for gluconeogenesis during starvation (forms pyruvate)
converts Alanine to pyruvate via transamination transfering the amino group to alpha-ketogluterate
Glucose-Alanine Cycle
Pyruvate is formed in the muscle by glycolysis and is transaminated (ALT) to Alanine.
Alanine is transported to the liver and converted back to pyruvate (via ALT) for gluconeogenesis
Glucose is then released into the blood for use in other tissues.
Aspartate Metabolism
Asparagine is converted to Aspartate via aspariginase.
Aspartate is then converted to OAA via AST, a transaminase that requires PLP as a cofactor.
Aspartate can be converted back to Asparagine with the addition of NH3 from a glutamine donor.
This takes place with the enzyme Asparagine Synthetase
Amino Acid Pool
Contributors/sources of Amino Acids:
Dietary Protein from Digestion
Synthesis of non essential Amino Acids
Tissue Protein Catabolism
Utilization of Amino Acids:
Synthesis of Tissue Proteins
Amino Acid Catabolism
Synthesis of Nitrogen containing compounds (purines, pyrimadines, neurotransmitters, etc.)
Asparagine Synthetase
Forms Asparagine from Aspartate. Requires glutamine as a NH3 donor.
Asparaginase
Forms Aspartate and free NH3 from Asparagine.
Used to treat leukemia as the lack of asparagine will starve the tumor.
Aspartate Transaminse
AST
Reversible reaction
Interconverts Aspartate and OAA. Requires transfer of NH3 to Alpha-ketoglutarate forming glutamine (or the reversible)
Phenylketonuria
PKU
Type I:
Deficiency of phenylalanine hydroxylase that converts Phe to Tyr. (BH4 as a cofactor)
Type II: (more severe)
Deficiency of dihydrobiopterin synthesis or dihydrobiopterin reductase(BH2/BH4)
Elevated phenylalanine levels in the blood.
Can cause: Mental problems (low IQ), Seizures, Spasticity, Autistic behaviors, Hypopigmentation, and Skin rashes
Presents with mousey odor of urine (phenylpyruvic acid), decreased skin and hair pigment (tyr converted to melanin)
Treated by a low Phe diet. (no eggs, milk, meat, aspartame) and sapropterin (synthetic BH4) for mild forms of PKU
Alkaptonuria
Deficiency of Homogentistic Acid Oxidase that converts homogensistic acid maleylacetoacetate in phenylalanine-tyrosine catabolism to fumarate and Acetoacetate
homogentistic acid deposits in connective tissue and cartilage (Ochronosis). Causes severe arthritis
presents with brown urine and Ochranosis (Acid pigment deposits in cartilage)
treatment by low Phe and Tyr in diet. (difficult due to restriction of 2 essential amino acids)
Tyrosinemia Type I
Deficiency of Fumaryl Acetoacetate Hydrolase
that converts fumaryl acetoacetate to fumarate and acetoacetate in phenylalanine-tyrosine catabolism
Manifestations are severe and usually fatal
–Liver failure
–Renal failure
–Cabbage like odor of the urine
Treatment by dietary restriction of Phe& Tyr
Maple Syrup Urine Disease
Deficiency in Branched Chain Alpha-Keto Acid Dehydrogenase that catabolises BC Keto Acids to corresponding Acyl CoA using TPP (B1) as a cofactor
BCAA build up in the blood and BC Keto Acids in the urine.
Presents with poor feeding, vomiting, poor weight gain and increasing lethargy.
Ketosis and the characteristic odor of maple syrup in the urine are usually present when the first symptoms develop
Can result in coma and death if not treated.
Treated by dietary restriction of BCAAs
Methylmalonic Aciduria
Deficiency of Methylmalonyl CoA Mutase that converts methymalonyl CoA to Succinyl CoA in the catabolism of Odd chain FA, Val, Ise and Met.(requires cobalamin (B12) as a cofactor)
neurological manifestations: seizures, encephalopathy
In some cases methylmalonyl CoA mutase enzyme has a reduced affinity for the B12 coenzyme. therefore can be treated with B12.
Homocystinuria
Deficiency of cystathionine synthase that converts homocysteine to Cystathione in the conversion of Met to Cys. Requires PLP as a cofactor.
Characterized by dislocation of lens (ectopialentis), Skeletal abnomalities, Mental retardation, premature arterial disease (lipid oxidation and platelet aggregation that leads to fibrosis & calcification of atherosclerotic plaques)
Fates of Homocysteine
Converted back to Met via Vitamin B12 (Cobalaine) and folate (adds a methyl group)
Converted to Cysteine for excretion (multistep) requiring 2 Vitamin B6 (PLP) reactions.
Urea Cycle (overview)
Forms urea from ammonia in the liver.
2 reactions take place in liver cell mitochondria and 3 reactions take place in the cytosol.
The first nitrogen of urea is donated by ammonia, the second by aspartate (formation of asp from OAA)
Carbamoyl Phosphate Synthetase I
CPS I
Forms Carbamoyl Phosphate from ammonia and HCO3 in the mitochondria. Requires N-acetylglutmate (NAG) Requires 2 ATP
First step in Urea Cycle. Rate limiting step.
N-acetylglutamate
NAG
Allosteric activator of CPS I in the urea cycle
Ornithine Transcarbamoylase
OTC
Forms Citruline in the mitochondria from Carbamoyl Phosphate and Ornithine.
Second step in Urea Cycle
Citruline is then transported out of the mitochondria into the cytosol
Argininosuccinate Synthetase
ASS
Forms Argininosuccinate from Aspartate and Citrulline in liver cytosol. Requires ATP
Third step in Urea Cycle
Argininosuccinate Lyase
ASL
Frees Arginine from Argininosuccinate forming Fumarate (enters TCA or oxidized back to OAA)
Fourth step in Urea Cycle
Arginase
ARG
Releases Urea from arginine and forms Ornithine
Fifth step in Urea Cycle
Ornithine is transported into the mitochondria to complete the Urea Cycle
Hyperammonemia Type I
CPS I deficiency
Characterized by hyperammonemia and neurological manifestations
May be able to treat with arginine (stimulate the formation of NAG)
Hyperamonemia Type II
OTC Deficiency
X-linked recessive
Characterized by hyperammonemia, increased Orotic acid in urine
Elevated carbamoyl phosphate drives pyrimidine biosynthesis –orotic acid is elevated in the serum & urine
Most common Urea Cycle disorder
Management of Hyperammonemias
Dialysis
Administration of Benzoic Acid: Combines with glycine to form Hippuric Acid that is excreted in urine (1 nitrogen per molecule)
Phenylbutyrate: drug that combines with glycine then condenses with glutamate to remove 2 nitrogens per molecule
Minimize nitrogen by a low protein diet.
Liver transplant
Acquired Hyperammonemia
Liver disease due to viral or drug induced hepatitis, alcoholic cirrhosis
In cirrhosis, there is porto-systemic shunting of blood
–Portal blood enters the systemic circulation without going to the liver
–Ammonia produced in the intestine directly enters circulation and results in neurotoxicity
Treatment of acquired hyperammonemia
Low protein diet.
Lactulose: Normal flora digest in the colon, produce lactic acid, lactic acid is neutralized by NH4+
Neomycin: Reduction of bacterial ureases in the gut.
Citrullinemia
Argininosuccinate synthetase (ASS) deficiency
hyperammonemia, with very high levels of serum citrulline, and citrullinein the urine
Treatment mayinclude arginineAdministration:
–Enhances urinary citrullineexcretion
–sometimes allows the urea cycle to progress due to high levels of substrate
Argininosuccinic aciduria
ASL deficiency
Hyperammonemia, Increased argininosuccinate levels in plasma and CSF
Sometimes treated with surplus of arginine
–May allow urinary excretion of argininosuccinate
–May raise levels of intermediates to allow urea cycle to function (km)
Argininemia (Hyperargininemia)
Arginase deficiency
Elevated serum ammonia and elevated arginine
Fate of Urea
Urea is transported to the kidney where it is excreted in urine
•Some urea is degraded in the gut, the ammonia reenters circulation and the liver must detoxify it
•Kidney failure leads to elevated BUN
–(blood urea nitrogen)
Effects of elevated blood ammonia
Glutamate is converted to glutamine
–Glutamine synthetase
–Results in high circulating glutamine levels
•Reduced GABA and glutamate levels in the brain
–GABA is major inhibitory NT (formed from glutamate)
–GLU itself is a major excitatory NT
Tyrosine Hydroxylase
Forms DOPA from Tyrosine. Requires BH4
Found in Epinephrine formation from Phe and Tyr
DOPA Decarboxylase
Forms Dopamine from DOPA. requires PLP
Found in Epinephrine formation from Phe and Tyr
Dopamine Hydroxylase
Forms norepinephrine from Dopamine. Requires Vitamin C
Found in Epinephrine formation from Phe and Tyr
Phenylethanolamine N methyl transferase
Forms Epinephrine from Norepinephrine. requires SAM
Found in Epinephrine formation from Phe and Tyr
Parkinson’s Disease
Neurodegenerative disorder
•Loss of dopamine producing cells in the basal ganglia
- Characterized by movement disorders: spasticity, tremors, loss of memory, mood disturbances, postural instability
- Symptoms are improved by administration of L-DOPA. L-Dopais converted to Dopamine in the brain, that improves the symptoms
Catacholamine Degradation
Norepinephrine and epinephrine are degraded by Monoamine Oxidase (MAO) and Catechol O-methyl transferase (COMT) to form Vanillyl Mandelic acid (VMA) that is excreted in urine.
•Urinary VMA levels may be measured to estimate levels of epinephrine & norepinephrine produced
Pheochromocytoma
High urinary VMA and catecholamines
Characterized by overproduction of catecholamines (epi, norepi)
–Adrenal medulla tumor
•Predominant episodic symptoms include:
–Headache, Sweating, Tachycardia, hypertension
•Must use 24 hour urinary measurement
–During a symptom episode
Serotonin Synthesis and Metabolism
Serotonin is synthesized in the gut, platelets and CNS
Tryptophan Hydroxylase Forms 5 hydroxy tryptophan from tryptophan. requires BH4
Amino acid Decarboxylase forms SEROTONIN (5-hydroxy tryptamine). Requires PLP
Serotonin is metabolized to 5-hydroxyindole acetic acid (5-HIAA) by Monoamine Oxidase (MAO)
Carcinoid syndrome
Tumor of serotonin producing cells in Gastrointestinal tract (APUD cells)
•Cutaneous flushing, sometimes accompanied by sweating, Gastrointestinal hypermotility causing diarrhea, Bronchospasm
Increased serotonin metabolite 5-HIAA in urine
Melatonin
Sleep inducing hormone produced via small modifications to serotonin
Regulates Circadian rhythm,Light / dark cycles
GABA
Gamma Amino Butyric Acid
Inhibitory neurotransmitter in central nervous system
Formed from Glutamic acid via Glutamate decarboxylase (PLP as cofactor)
Reactions requiring tetrahydrobiopterin
- Phenylalanine hydroxylase(converts Phe to Tyr)
- Tyrosine hydroxylase (converts Tyr to DOPA)
- Tryptophan hydroxylase (converts Trpto 5-hydroxy tryptophan)
Deficiency of BH2 synthase or BH2 reductase results in hyperphenylalaninemia & decreased synthesis of neurotransmitters (catecholamines, serotonin).
–Results in delayed mental development and seizures
Creatine
Found in muscle, cardiac, brain
- Synthesized from Arginine, Glycine & SAM (S-Adenosyl Methionine)
- Accepts Phosphate groups from ATP when the muscle is resting and donates phosphate groups to ADP when muscle is contracting. (via Creatine phosphokinase(CPK))
- Creatine is converted to Creatinine (spontaneous event) and excreted in urine
Histamine
Produced during allergic & inflammatory reactions by mast cells
•Histamine is a vasodilator
Formed from histadine via AA Decarboxylase (PLP as cofactor)
antihistamines do not reduce the formation of histamine, they reduce the ability of histamine to function as signal to other pathways
–Receptor antagonist
Nitric Oxide synthase
Forms NO from Arginine and releases citrulline
NO causes local vasodilation
•Nitroglycerin used in treatment of myocardial ischemia is converted to NO that results in vasodilation of coronary blood vessels and improvement of blood flow to heart
Albinism
Deficiency of the TYROSINASE enzyme
–Deficient conversion of Tyrosine to MELANIN
Results in lower visual acuity and photophobia
•light colored skin and hair
•Increased risk of skin damage on exposure to sunlight and increased risk of skin cancer
Glutathione
Tripeptide composed of Glutamate, Cysteine, and Glycine
an intracellular reducing agent (antioxidant)
•Important for detoxification of toxic hydrogen peroxide (H2O2) especially in the red blood cells
