Nitrogen metabolism Flashcards
Phenylalanine hydrolase
- PAH
- Phenylalanine + tehtrahydrobiopeterin (BH4) (cofactor)–>Tyrosine + dihydroiopeterin (BH2)
Defect in PKU I (classic PKU)
PKU I
- defect in phenylalanine hydroxylase
- can’t convert phenylalanine to tyrosine (tyrosine becomes essential AA)
- decreased melanin synthesis (from tyrosine)–>decreased pigmentation of skin (phe–|tyrosinase)
- CNS symptoms: delay in milestones, low IQ (untreated, major IQ reduction in early development); Seizures @ high blood [phe]
- Phe metabolized to phenylpyruvate, phenylacetate, phenyllactate–>excrete in urine–>mousey odor
- blood [Phe] elevated
-IQ decreases after cessation of Phe free diet
Treatment:
- dietary restriction of Phe
- Sapropterin–>synthetic BH4 (mild or moderate forms of disease) (mutant enzyme with low cofactor affinity)
Sapropterin
synthetic tetrahydrobiopterin (BH4)
Maternal PKU syndrome
High Phe in mothers is tratogenic–>leads to birth defects
- microcephaly, mental retardation, congenital heart defects
- kid isn’t PKU + (ususally heterozygous)
PKU II
- malignant PKU
- deficiency of diydrobiopterin reductase (BH2/BH4) or BH2 synthesis
- more severe/worse prognosis than PKU I
- CNS symptoms worse–>decreased neurotransmitter synthesis (serotonin, dopamine, catecholamines)
- treatment: Phe restriction, dietary biopterin and precursors of neurotransmitters (blood/brain barrier limits efficacy)
- elevated Phe and metabolites
- phenylpyruvic acid in urine leads to mousey odor
dihydrobiopterin reductase
Dihydroiopterin (BH2)–>tetrahydrobiopterin (BH4)
deficient in PKU II
Homogentisic acid oxidase
Full name homogentisate 1,2-dioxygenase
homogentisic acid—> maleylacetoacetate
used in pathway from tyrosine to fumarate and acetoacetate
Alkaptonuria
- defect in homogentisate 1,2-dioxygenase
- homogentisic acid–>maleylacetoacetate
- ^ is a brown pigment
- relatively benign
- Ochronosis: homogentisic acid deposits in cartilage and connective tissue–>leads to severe arthritis
- homogentisic acid is excreted in urine–>brown color on standing (oxidation of homogentisic acid)
- dietary restriction of Phe and Tyr may reduce deposition
- bluish-black discoloration of sclera and auriculum)
Tyrosinosis
- Tyrosinemia type I
- deficinecy of fumarylacetoacetate hydrolase
- build up of fumaryl acetoacetate leads to kidney and liver damage
- severe usually fatal
- cabbage-like odor of urine
- attempt dietary restriction of Phe and Tyr (difficult b/c both essential and needed for neurotransmitter synthesis)
Fumarylacetoacetate Hydrolase
fumarylacetoacetate–>fumarate + acetoacetate
defective in Tyrosinemia Type I
Maple Syrup Urine Disease (MSUD)
- deficiency of branched chain α-ketoacid dehydrogenase (branched chain keto acid–>___acyl-CoA)
- rare
- symptoms develp @ 4-7 days
- presents with poor feeding, vomiting, poor weight gain, increasing lethargy
- neurological signs (alternating muscular hypotonia and hypertonia, seizures, encepalopathy)
- ketosis–>maple syrup odor
- coma and death in early infancy if untreated
- Treatment: dietary restriction of branched chain amino acids (leu, ile, val)–>all essential, difficult (improves neurological manifestations; dietary supplementation with TPP (vit B1 useful in some pts with low coenzyme affinity)
Branched chain α-ketoacid dehydrogenase
- Requires TPP
- deficiency causes MSUD
- branched chain keto acid–> corresponding acyl-CoA
- paralog of αKgDH, PDH
Methylmalonic Aciduria
- deficiency in Methylmalonyl-CoA Mutase
- elevated levels of methylmalonic acid (methylmalonic acidemia)
- metabolic acidosis
- Methylmalonic Aciduria
- neurological symptoms: seizures, encephalopathy
- some pts improve with B12 (cobalmin) supplementation–>those with reduced coenzyme affinity
Methylmalonyl-CoA Mutase
- methylmalonyl-CoA to succinyl-CoA
- reuqires vit B12 cofactor dervived cofactor-adenosylcobalamin
- deficiency causes methymalonic aciuria
Homocystinuria/Hyperhomocysteinemia
- group of disorders involving homocysteine metabolism–>commonly Cystathionine β-synthase
- homocysteine accumulates in connective tissue and disrupts the structure (forms disulfide bridges with collagen cys residues)
- dislocation of lens (ectopia lentis) after age 3 (and other ocular abnormalities)
- skeletal abnormalities (childhood osteoporosis)
- mental retardation
- premature arterial disease (lipid deposits–>atheromas; lipid oxidation and platelet aggregation–>fibrosis & calcification of atherosclerotic plaques)
- some pts respond to oral vit B6
- special diet
-Hyperhomocysteinemia is typically managed with vitamin B6, folic acid, and vitamin B12 supplementation. [3] Taurine supplementation also has been found to reduce homocysteine levels.[4] (http://en.wikipedia.org/wiki/Hyperhomocysteinemia)
Cystathionine β-synthase
- part of transulfuration pathway (methionine to cys)
- homocysteinine–>cystathionine
- requires PLP (vit B6)
Fate of homocysteine
- recycled to methionine (required THF and vit B12)
- sulfur transfered to serine by Cystathionine β-synthase in transulfuration rxn forming cysteine and a carbon skeleton (require B6=PLP), carbons–>Succinyl-CoA (needs B12)
Amino acid pool
- circulating free amino acids
- sources: dietary, tissue protein catabolism, synthesis of non-essential
Uses:
- Amnio acid catabolism: ammonia–>urea
- Carbon skeleton: glucose/lipid synthesis, oxidation in TCA (pyruvate)
- stynthesize nitrogen containing compounds: creatine, heme, neurotransmitters, purines, pyrimidines, etc
- build proteins
Essential Amino Acids
- Phenylalanine (F), Valine (V), Tryptophan (W), Threonine (T), Isolucine (I), Methionionine (M), Arginine* (R), Leucine (L), Lysine (K)
- Arg is essential only during high demand in childhood
The Whole Food Ladder Really Must Have Various Key Ingredients
Conditionally Essential AA
- Cysteine-when met is insufficient
- Arg-conditional during rapid growth when in high demand
- Tyrosine: Y and F can be interconverted, one is needed
Protein degredation pathways
- Lysosomal-extracellular or membrane proteins
- ubiquitin/proteasome-proteins made by the cell
Ubiquitin/Proteasome degredation
- used for oxidatively damaged, or denatured proteins
- PEST sequences-rich in pro, blu, ser, and the have short t1/2
Lysosome functions
- normal degredation fo some cellular components
- material from phagocytosis
- receptor mediated endocytosis (i.e.: LDL receptor)
- autophagy
- extracellular digestion: acrosome, inflammatory processes can cause inappropriate release of lysosomes from WBC’s (i.e.: gout)
Liver in N metabolism
- deamination (AA–>NH3 + carbon skeleton)
- NH3–>urea (urea cycle)
- carbon skeleton–>gluconeogenesis, TCA, ketogenesis, probably FA de novo synthesis
Kidney in N metabolsim
- excretes urea from liver in urine
- excrete NH3 as NH4+ (regulation of acid/base balance; source is gluatmine, glutaminase)
- excretes uric acid (end product of purine degradation), creatinine (end product of creatine degradation), nitrogenous non-protein substances
Cystinuria
- caused by decreased tubular reabsorption of cystein (and other dibasic aa’s ornitine, arg, lys)
- inherited deficiency of cystine transporter
- Cystine excreted in urine
- Cys precipitates in renal tubules (systine stones)–>cause of renal stones in children
Hartnup’s Disease
- inherited defect in transport of neutral amino acids (ex: tryptophan)
- decreased absorption and increased excretion (due to decreased reabsorption)
- Some pts experience niacin def (low protein diet)–>NAD+ deficiency aka pellagra (niacin is usually synthesized from W)
- 4D’s of Pellagra: diarrhea, dermatitis, dementia, death
Ketogenic Amino acids
exlusively: Leu, Lys
also glucogenic: Ile, Phe, Tyr, Trp
Ile, Leu—>acetyl-CoA
Leu, Lys, phe, trp, tyr–>acetoacetyl-CoA
Glucogenic AA
everything except Leu and Lys
also Ketogenic: Ile, Phe, Tyr, Trp
*Asn, Asp-->OAA Phe, Tyr--->fumarate Ile, Val (branched chain)-->Succinyl-CoA **Ala-->pyruvate **Glu, Gln-->α-ketogluterate
ALT
- Alanine aminotransferase/transaminase
- Requires PLP (B6)
Ala + αKG Pyruvate + Glu
reversable, driven by concentration
-in liver, entrance of asp into TCA, also another enzyme that can interconvert Glu and α-KG
Importance of Alanine
- major transport Amino acid from muscle to liver (glucose/alanine cycle)
- major precursor of glucose genesis (ALT to pyruvate)
Glucose/Alanine cycle
a/k/a Cori Cycle:
- pyruvate formed in muscle converted to Ala and transported to liver
- in liver turned back to pyruvate, used as substrate for glugoneogenesis, glucose put back into blood to muscle
Glutamate dehydrogenase
Glutamate + NAD+ α-ketogluterate + NH3 + NADH
- Oxidative deamination/reductive amnination (backwards)
- Goes forwards in liver to deliver α-ketogluterate to TCA under starvation, also release NH3 for urea cycle
-Goes reverse in peripheral tissues to sequester NH3 for transport to liver for urea cycle when tissue [NH3] is high
1 of 3 enzymes that can incoporate free NH3
AST
Aspartate aminotransferase a/k/a serum glutamic oxaloacetic transaminase (SGOT)
Requires PLP (B6)
Aspartate + α-ketogluterate Oxaloacetate + Glutamate
-in liver, entrance of asp into TCA, also another enzyme that can interconvert Glu and α-KG
enzymes that do: α-ketogluterateglutamate
ALT, AST, and glutamine transaminase
Glutamine Synthetase
Glutamate + NH3 + ATP—->Glutamine + ADP + Pi
- present in peripheral tissues, especially important in Brain (neurotransmitter synthesis I belive uses many deaminations, maybe) and endothelium of hepatic vein (prevent free NH3 from getting to the rest of the body)
- 1 of 3 enzymes that can incorporate free NH3
- Glutamine used to transport NH3 to liver
Glutaminase
- Glutamine + H2O—>Glutamate + NH3
- present in renal tubules and liver
- renal: used to maintain acid/base balance, excrete as NH4+
- liver: NH3 goes into urea cycle, glutamate gives off another NH3 using GDH, and α-KG goes into TCA
Asparagine Synthetase
Aspartate + Glutamine + ATP —->Asparagine + AMP + PPi
-uses glutamine as source of Nitrogen
Asparaginase
Asparagine + H2O —> Aspartate + NH3
- ASP is used to make OAA by AST for TCA/gluconeogenesis
- NH3 to Urea cycle
- Used to treat leukemia pts (leukemia cells need Asn, reduce availability to limit growth)
General transaminase rxn
α-Amino acid + α=ketogluterate α-keto acid + Glutamate
- All transaminases require PLP (vitamin B6)
- α-Kg is usually the amino acceptor but there are exceptions
- glutamate becomes important N carrying group
Ammonia transport
- Alanine: from muscle; transamination of pyruvate buy ALT
- Glutamine: most tissues; from glutamine by glutamine synthase (glutamate from protein breakdown, glutamate dehydrogenase, ALT, AST)
NH3 formation in liver
- Glutamate: Glutamate dehydrogenase, get NH3
- Glutamine: Glutaminase forms NH3 and glutamate
- Alanine and other AAs: transaminases (using PLP) form glutamate
Sources of N in urea cycle
- 1st from NH3
- 2nd from Asp (remember asp forms OAA by AST using Glu as N source)
Urea cycle enzymes in order
- Carabmoylphosphate synthetase I, CPS-1 (mito) (rate limiting step)
- Ornithine Transcarbamyolase, OTC (mito)
- Arginosuccinate Synthetase, ASS (cyto)
- Arginosuccinate Lyase, ASL (cyto)
- Arginase, ARG1 (cyto)
Urea cycle intermediates in order
Starting material: HCO3-, NH4+, 2 ATP
- Carbamyl phosphate
- Citrulline (+Aspartate)
- Argininosuccinate
- Arginine (+fumerate)
- Ornithine (+Urea)
CPS-I
Carbamoyl Phosphate Synthetase I
HCO3- + NH4+ 2 ATP —> Carbamyl Phosphate + 2 ADP + Pi
- 1st and rate limiting step
- Mitochondria
- N-acetylglutamate is required as an absolute activator
- 1 of 3 enzymes that incorporate free NH3
OTC
Ornithine Transcarbamyolase
Carbamyl Phosphate + Ornithine –> Citrulline + Pi
- 2nd step
- mitochondria
ASS
Argininosuccinate Synthetase
Citrulline + ATP + Aspartate –> Argininosuccinate + AMP + PPi
- 3rd step
- cytosol
ASL
Argininosuccinate lyase
Argininosuccinate –> Arginine + Fumarate
- 4th step
- cytosol
- Fumerate my enter TCA or be oxidized back to oxaloacetate
ARG-I
Arginase
Arginine–>Urea + Ornithine
- 5th step
- cytoplasm
Enzymes that incorporate free NH3
Glutamate Dehydrogenase, Glutamine synthase, Carbamyl Phosphate Synthase I
Energy used in urea cycle
4 high energy bonds: 2 ATP–>ADP, 1 ATP–>AMP
Inherited Urea Cycle disorders
- accumulation of substrates of deficient enzyme (any complete absence would be miscarriage)
- Increased blood ammonia (hyperammonemia) and elevated blood glutamine
- urea formation decreased altough levels will be approximately normal
- Hyperammonemia symptoms appear in first few days: lethargy, irritability, feeding difficulties
- nerurological symptoms if untreated i.e.: seizures, mental retardation
- CPS1 and OTC (first two) are most severe
Hyperammonemia Type I
- Carbamoyl-phosphate synthetase deficiency
- hyperammonemia
- neurological symptoms
- elevated glutamine
- sometimes respons to Arg intervention (arg stimulates NAG formation, might stimulate CPS-I)
Hyperammonemia Type II
- Ornithinie Transcarbamyolase deficiency
- X-linked (more in male, more severe in male)
- most common
- hyperammonemia
- increased orotic acid in urine and serum (orotic aciduria/emia) (mostly worry about the urine)
- diagnose by elevated serum ammonia and urine orotic acid
Citrullinemia
- Argininosuccinate synthetase deficiency
- diagnosis: hyperammonemia, very increased citrulline levels (blodd and urine)
- treatment may include arginine–>enhance urinary citrulline excretion. drive urea cycle with more substrate
Argininosuccinic aciduria
- Argininosuccinate Lyase deficiency
- hyperammonemia
- diagnosis: argininosuccinate elevated in plasma and CSF, increased argininosuccinate levels in urine, moderately high citrulline
- can sometimes treat with arginine–>enhance urinary excretion, push urea cycle with substrates/intermediates
(Hyper)argininemia
- Arginase deficiency
- increased arginine levels
- diagnosis: high serum arg and NH3 (NH3 not as high as other UCDs)
- restrict diet to essential amino acids excluding arginine
- frequently adult onset: neurological problems
Treatment of hyperammonemia
- dialysis-emergency
- benzoic acid–>benzoyl-CoA + glycine–> Hippurate (remove 1 mol N per mol of drug)
- low protein/high carb diet (minimize N intake)
- avoid stresses leading to catabolic state
- long term: liver transplant
- phenylbutrate–>phenylacetate–>phenylacetyl-coA–>phylacetylglutamine (2 N per molecule removed in urine)
Phenylbutrate
phenylbutrate–>phenylacetate–>phenylacetyl-coA–>phylacetylglutamine (condense with glutamine)
-2 N per molecule removed in urine
benzoic acid
benzoic acid–>benzoyl-CoA + glycine–> Hippurate (remove 1 mol N per mol of drug)
Fate of Urea
- transported to kidney, eliminated in urine
- 25% degraded by bacterial ureases in gut–>ammonia returns to circulation (85% portal, 25% systemic) and must go through urea cycle again
- kidney failure–>elevated blood urea nitrogen (BUN) wherease UCD leads to elevated NH3
Aquired hyperammonemia
- liver disease (viral or drug induced hepatitis, alcoholic cirrhosis, I think some GSDs)
- cirrhosis–>portocaval anastamosis leads to higher [NH3] in systemic circulation b/c blood doesn’t go through liver–>neurotoxicity
Treatment:
- low protein/high carb diet (less N ingested)
- Lactulose-disaccharide, resistant to digestion–>used by intestinal flora–>produce lactic acid–>neutralized by NH4+–>N excreted in feces
- Neomycin (or other antibiotic)–>kill your flora, less urease
- other treatments
Hypothesises for NH3 neurotoxicity:
Energy and osmotic effect:
α-ketogluterate–>glutamate requires reducing equivalents and consumes TCA cycle intermediates–>less TCA–>less ATP synthesis–>reduced K+/Na+ ATPase–>up osmotic pressure–>neuronal death
Neurotransmittler effect:
Glutamate to glutamine (using glutamine synthase–>high serum glutamine)–>reduced glutamate and GABA (made from glutamate in brain) (both reduced at high NH3) (GABA is an inhibitory NT, GLU is a major excitatory NT)
Tyrosine hydroxylase
Tyrosine—>L-DOPA (dihydroxyphenylalanine)
requires tetrahydropiopterin (BH4) (–>BH2)
DOPA Decarboxylase
L-DOPA—>Dopamine + CO2
Requires PLP (vit B6)
Dopamine β-decarboxylase
L-DOPA + O2 —>Norepinepherine + H2O
Requires Vitamin C (absorbic acid—>dehydroxyascorbic acid)
Phenylethalnolamine-N-methyltransferase
Norepinepherine + S-adenosyl-methionine (SAM)–>Epinepherine + homocysteine
Parkinson’s Disase
- neurodegenerative disorder
- loss of dopamine producing cells
- movement disorders: spasticity, tremors, loss of memory, mood distrubances, postural instability
- improve with administration of L-DOPA–>converted to dopamine in brain (inc [substrate])
- Inhibit peripheral dopamine formation (so that L-DOPA is use preferentially in brain)–>DOPA decarboxylase inhibitor (presumably can’t cross blood brain barrier)
Norepinephrine and Epinephrine degredation
2 enzymes: Monoamine Oxidase A (requires FAD), then Catechol-O-methyltransferase (COMT, requires S-adenosyl-methionine)
(Nor)epinephrine—->——>Vanillyl Mandelic Acid (VMA)
VMA excreted in urine, used to measure (nor)epinepherine levels
Dopamine Degredation
2 Enzmes:
Monoamine oxidase (A or B don’t matter) (FAD)
Catechol-O-methyltransferase (COMT) (SAM)
Dopamine—>—>homovanillic acid (HVA)
-HVA in urine, presumably can beassayed for dompamine levels
Pheochromocytoma
- tumor of adrenal gland medulla leading to overprocudtion of catecholamines
- predominant symptoms: headache, sweating, tachycardia
- also palpitations, anxiety, panic attacks, hypertension
- symptoms are episodic
- Diagnosis: high urinary VMA and catecholamines–>24 hr measurement during symptom episode
Serotonin synthesis and degradation
Tryptophan—>5-hydroxytryptophan (5-HTP)—–>Serotonin (5-hydroxytryptamine)—–>5-hydroxyindole acetic acid (5-HIAA)
- Tryptophan Hydroxylase (BH4)
- (Aromatic L-) Amino acid Decarboxylase (PLP/B6)
- MAO-A (FAD)
Produced in gut, platelets and CNS
Tryptophan Hydroxylase
Tryptophan—->5-hydroxytryptophan
requires BH4–>BH2
Aromatic acid decarboxylase
5-hydroxytryptophan—>Serotonin
-PLP
Wikipedia:
Full name: Aromatic L-amino acid decarboxylase
Same as Dopa decarboxylase
L-DOPA to dopamine - a neurotransmitter
5-HTP to serotonin (5-HT) - also a neurotransmitter
tryptophan to tryptamine - a precursor to many alkaloids found in plants and animals
Phenylalanine to phenethylamine - a trace neurotransmitter
Carcinoid Syndrome
- tumor of serotonin producing cells in GIT (APUD cells)
- Cutaneous flushing
- gastrointestinal hypermotility–>diarrhea
- bronchospasm
- increase 5-HIAA in urine
Melanin
-derived from Serotonin (which was derived from trypophan)
Tetrahydrobiopterin
- BH4
- required cofactor in many amino acid hydroxylations
- Penylalanine/tyrosine/tryptophan hydroxylases
- Also Nitric oxide synthase (NOS) and Alkylglycerol monooxygenase
Tetraydrobiopterin Deficiency
- caused by deficiency of dihydrobiopterin synthase of BH2 reductase
- hyperphenylalaninemia and decreased synthesis of neurotransmitters (catecholamines and serotonin)
- delayed mental development and seizures
- Managed with dietary BH4, dietary neurotransmitter precursors, restriction of Phe
Glutamate Decarboxylase
Glutamic Acid—>Gamma amino butyric acid
requires PLP
GABA
- inhibitory neurotransmitter in CNS
- formed from glutamate by glutamate decarboxylase
Histidine Decarboxylase
Histidine——>histamine
PLP
(Sobering doesn’t care if we know something this simple)
Histidine
- synthesized from Histidine by histidine decarboxylase (with PLP) in Mast Cells
- vasodialator
- antihistimine drugs reduce adverse affects of allergic reactions
- antihistimies are typically receptor antagonists or otherwise interrupt signalling pathway, do not normally interere in synthesis
Creatine
- reservoir of high energy bonds in muscle
- synthesized from Arg, Gly, and SAM
- accepts Pi groups from ATP when resting (Creatine Kinase)
- dontates Pi to ADP when contracting (Creatine Kinase)
- spontaneously converted to Creatinine–>end product of creatine metabolsm
Creatinine
- spontaneously formed from Creatine–>end product of creatine metabolsm, eliminated in urine
- dependent on muscle mass of individual
- serum creatinine used as marker of kidney fxn–>not efficiently filtered in renal failure, serum creatinine rises
Creatine Kinase
- or Creatine Phosphokinase
- creatine + ATPcreatine-Pi + ADP
- total CK/CK-MB used as marker of MI
Nitric Oxide
- Synthesized from Arginine–>NO⋅ + citrulline
- Nitric Oxide Synthase requires heme, NADPH, FAD, FMN, BH4
- made in enothelial cells
- local vasodialation
- nitroglycerin–>converted to NO⋅ causes vasodialiation of coronary blood vessels–>improvement of blood flow to dart
- short half life
- 2nd messenger of cGMP pathway (activates soluble guanylate cyclase)
Albinism
- caused by deficiency of Tyrosinase (oxidase)
- defcient conversion of tyrosine to melanin
- group of disorders (partial to complete)-severe form affects eyes (oculocutaneous albinism)
- lower visual acuity adn photophobia
- light colored skina dn hair
- increased risk of sun damage from sun and skin cancer
Melanin
- herogenous group of tyrosine derived pigments
- tyrosinase is rate limiting step in synthesis
Thyroid hormone
- T3 or T4
- formed on tyroglobulin protein
- tyrosine acceps iodione (iodination)
Glutathione
- tripeptide made from glutamate, cysteine, and glycine
- reducing agen
- detox of H2O2
- protects from membrane damage, mainatin free -SH ends of proteins
- conjugated to drugs to make them H2O soluble
- cofactor in some enzymatic rxns (glutathione peroxidase)
- aid in rearrangment of disulfide bonds
- protects RBC from oxidative stress
- glutathione reductase needs NADPH (from PPP)
