Degradation of Amino Acids 1:Urea Cycle Flashcards
What are the products of the reactions catalyzed by the following enzymes:
- Glu DH
- Gln synthetase ( main brain detox)
- Carbomyl phosphate synthetase I (1 step of urea synthesis)
- Glu
- Gln
- Carbomyl phosphate
The levels in type II hyperammonemia
- Urine orotate
- Blood citrulline
- Blood Arginine
- Blood NH3
- High
- low
- low
- high
The reaction catalyzed by carbomyl phosphate synthetase I
- Formation of carbomyl phosphate
- reactants: CO2 and NH4+
- uses 2 ATPs
- 1st step in urea synthesis
- one of 3 enzymes that can “fix” free ammonia into organic molecules
- controls blood ammonia level
Citruline synthesis
- enzyme
- location
- transport of ractant and product
- enzyme deficiency
catalyzed by ornithine transcarbamoylase
takes place in mitochondria.
ornithine is transported into the mitochondria from the cytosol by ornithine translocase.
citrulline is transported to the cytosol.
deficiency causes Type II Hyperammonemia.
Glutamine transport to the liver
- Gln producers
- Gln users
Net glutamine producers: Muscle and brain.
Main glutamine users: Kidney, gut, immune cells and liver
Glucose/Alanine Cycle
muscle: Glucose → pyr → Ala
Liver: Ala → C and N*
N converts to urea and excreted in urine
C convers to glucose and enters the glycolytic tissue
* ALT converts the Alanine to glutamate (liver)
Sources of alanine:
Main: muscle (result of protein degradation and transamination)
Other: kidney, intestines (conversion of glutamine to alanine)
1- Glutamine →1 Glutamate →2 Alanine
- rxn 1 catalyzed by glutaminase
- rxn 2 catalyzed by ALT
Reaction catalyzed by glutamate dehydrogenase
- a-ketoglutarate and ammonia are reactants
- Glu is the product
- reversible raction
- utlizes NADH/NADPH
- one of 3 enzymes that can “fix” free ammonia into organic molecules
- controls blood ammonia level
Primary hyperammonemias
- names and causes
- Mode of inheritance
- metabolites as a diagnostic tool
- Type I hyperammonemia (CPSI or N-acetylglutamate synthase deficiency)
- Type II hyperammonemia (ornithine transcarbamoylase) (X-linked recessive)
- Citrullinuria Type I (argininosuccinate synthetase deficiency)
- Argininosuccinic acidemia (argininosuccinate lyase deficiency)
- Argininemia (arginase deficiency)
All autosomal recessive except type II.
Metabolites before the deficiency accumulate in blood/urine.
Metabolites after the deficiency have much lower levels.
Regulation of mt. CPSI
- High [Arg]
- High [N-acetyl-Glutamate
- H. Arg leads to:
- High level of N-acetyl glutamate
- increased ornithine production ► inc urea production.
- H. N-acetyl glutamate ► inc. CPSI activity ► inc urea production
Reason for Ala transport to the liver
- Removal of nitrogen through the urea cycle
- Gluconeogenesis
Argininosuccinate synthesis
- enzyme
- location
- energy requirement
- reactant formation
- deficiency
- catalyzed by argininosuccinate synthetase.
- cytosol.
- 1 ATP molecule.
- aspartate is produced by transamination of oxaloacetate (AST); the aspartate nitrogen will incorporate into urea.
- deficiency causes Citrullinuria Type I.
Blood ammonia levels in:
- Urea cycle disorders
- liver damage
Both increased
Two major mechanisms for the control of blood ammonia levels:
I.Transamination reactions “collect” nitrogen on glutamate rather than release free ammonia.
II.Three enzymes can “fix” free ammonia into organic molecules.
- Glu DH
- Gln synthetase ( main brain detox)
- Carbomyl phosphate synthetase (1 step of urea synthesis)
Major source of alanine as a result of protein degradation and transamination:
Muscle
The rxn catalyzed by glutamine synthetase
- major detox rxn in brain
- Glu and ammonia are reactants
- forms Gln
- requires energy in the form of ATP
- one of 3 enzymes that can “fix” free ammonia into organic molecules
- controls blood ammonia level
Disorders of urea cycle and blood ammonia levels
- Deficiency in the urea cycle leads increased ammonia levels in the blood (hyperammonemia)
- Ammonia is especially toxic for the nervous system.
Symptoms:
encephalopathy, cerebral edema, seizures, nausea, vomiting, lethargy, coma and death if untreated.
The condition treated with arginine (Arg as an intermediate of urea cycle)
The mechanism of action
if argininosuccinate lyase is deficient.
It generates more ornithine for the urea cycle to continue.
It also catalyzes the production of N-acetyl-glutamate so accelerates CPSI.
Transamination
Transfer of an a-amino group from an amino acid to an a-keto acid (reversible, aminotransferase, PLP as coenzyme)
q The main pathway of amino acid nitrogen removal is transamination
qThe amino acid nitrogen is generally collected on glutamate.
The levels in arginosuccinic acidemia
- Urine orotate
- Blood citrulline
- Blood Arginine
- Blood NH3
- -
- high (200 ug)
- low
- high
Carbamoyl phosphate synthesis (for urea cycle)
- enzyme
- rxn characteristics
- location
- energy reguirement
- regulation
- enzyme deficiency
- Alt way for biosynthesis
- Carbamoyl phosphate synthetase I (CPSI).
- rate-limiting, committed step, irreversible.
- mitochondria.
- 2 ATPs.
- allosterically regulated also by N-Acetyl-glutamate.
- deficiency causes Type I hyperammonemia.
- From glutamine and CO2 by CPSII in the cytosol. ⇒ pyrimidine synthesis
Major steps of amino acid degradation:
1- N removal
- Transamination
- deamination
- deamidation
2- NH4+ clearance - Urea formation
- 1 N from NH4
- 1 N from Asp
- C from Co2
3- C backbone utilization
- Fed: glycogen and TAG synthesis
- Fasting: energy production
Three enzymes capable of fixing N into organic molecules
Three enzymes can “fix” free ammonia into organic molecules.
Glu DH
Gln synthetase ( main brain detox)
Carbomyl phosphate synthetase (1 step of urea synthesis)
Energetics of the urea cycle
4 high energy phosphate bonds are broken, which makes the cycle irreversible.
Part of the energy can be recovered if fumarate enters the (TCA) cycle.
Asp+NH3+CO2+3ATP → Urea+Fumarate +2ADP+AMP+3H2O
The levels in citrullinuria
- Urine orotate
- Blood citrulline
- Blood Arginine
- Blood NH3
- -
- high (>1,000 ug)
- low
- high
Blood uric acid level in:
1- gout
2- pregnancy
- increased
- decreased in pregnancy
Major source(s) of alanine as a result of the conversion of glutamine to alanine:
Kidney and intestine
Glutamine →1 Glutamate →2 Alanine
rxn 1 catalyzed by glutaminase
rxn 2 catalyzed by ALT
The condition treated with citruline as an intermediate of urea cycle
Mechanism of action
- Type I and II hyperammonemia
- Captures Asp, so at least one N can be excreted.
Blood Urea Nitrogen (BUN) levels:
- Kidney damage
- liver damage
- Pregnancy
- Inc- kidney damage
- dec- liver damage
- dec- pregnancy
The central role of glutamate in nitrogen removal
1.It collects nitrogen from other amino acids trough transamination.
● Transamination (N collector)
2.It can provide nitrogen (NH4+ ) to the urea cycle through deamination by glutamate dehydrogenase.
●deamination bu glutamate dehydrogenase
3.It can provide nitrogen to urea indirectly by transaminating oxaloacetate into aspartate. Aspartate then provides an amino group to urea.
●OXA→ Asp (transamination)
4.It is a precursor for the allosteric activator of the urea cycle.
How to reduce the ammonia load on the urea cycle
- Low protein diet
- avoid fasting
- scavenger drugs
Deamidation and its importance
Removal of amide group from Asn and Glu
Gln → Glu (glutaminase)
§ important in kidney.
§ provides most of the NH4+ in the urine.
§ NH3 aids H+ secretion into the urine and indirectly helps to balance blood pH.
Serum ALT level after poisoning
- ~ 36 hr pst poisioning the max level of ALT (x20)
Regulation of the urea cycle
All the factors, as well as the activators and diet
- N-Acetylglutamate is the allosteric activator (CPSI) (i.e. the committed rate limiting step of the cycle).
- The synthesis of N-Acetylglutamate is activated by high arginine levels.
- High protein diet and excessive degradation a body proteins (during fasting) will generally increase urea synthesis. (increases the synthesis of urea cycle enzyme)
- Concentration of substrates and intermediates
Reactions catalyzed by ALT and AST
4 diagnostically important N metabolites in blood
Urea Nitrogen- BUN
Creatinine
Uric acid
Ammonia
Scavenger drugs and their function in treatment of urea cycle disorders
A way to reduce the ammonia load on the urea cycle.
conjugate and target AA’s for urinary excretion
e.g. benzoate and phenylbutyrate
Hepatic encephalophaty and its main causes:
a form of secondary hyperammonemias, when hepatocytes cannot carry out the urea cycle. (hepatic encephalopathy)
Name 2 scavenger drug and their rxn’s
Benzoate + glycine → Hippuric acid ⇒ excretion
Phenylbutyrate + glutamine → Phenylacetylglutamine ⇒ excretion
Blood creatinine levels:
- Kidney damage
- Pregnancy
- Inc- kidney damage
- dec- pregnancy
Major regulators of mt. CPSI
Arginine and N-acetyl glutamate
Primary and secondary hyperammonemias
Primary hyperammonemias:
- Enzymes involved in the urea cycle are defective.
Secondary hyperammonemias:
- Main cause is hepatic failure (such as cirrhosis, hepatitis or hepatotoxins). Hepatocytes cannot carry out the urea cycle. (hepatic encephalopathy)
PLP
Pyrodoxal phosphate
B6 (pyridoxine) → PLP
diagnostic serum aminotransferases
ALT- AST
ALT and AST are present at high levels in liver.
High levels of ALT and AST in plasma indicates liver damage (cirrhosis, hepatitis, liver toxicity).
ALT is more specific than AST.
AST has been used for diagnosing myocardial infarction in the past.
Deamination
Deamination: removal of the a-amino group of certain amino acids as NH4+.
Glutamate → a-ketoglutarate + NH4+
Liver and kidney: Glu →a-ketoglutrate (in other tissues the reverse rxn is favored.
Both rvs and fwd rxns are catalyzed by glutamate dehydrogenase
Arginine synthesis
- enzyme
- location
- spc inf.
- fate of by-products
- enzyme deficiency
- argininosuccinate lyase.
- cytosol.
- the only way arginine is produced in the human body.
- the released fumarate can enter the TCA cycle and regenerate oxaloacetate and then aspartate.
- deficiency causes Argininosuccinyl acidemia
Urea cycle only takes place in the liver. But amino acids are degraded in all tissues.
How does N from other tissues get to the liver?
Nitrogen is carried to the liver by:
Alanine and Glutamine (other aa in smaller extent)
Cleavage of Arg into urea and ornithine
- enzyme
- locaiton
- fate of arginine
- fate of ornithine
- deficiency
- arginase.
- cytosol.
- arginine can be used by other pathways
- Protein synthesis
- nitric oxide synthesis
- ornithine moves back to the mitochondrium and can react with another molecule of carbamoyl phosphate
- deficiency causes Argininemia
Major N metabolite in urine
Urea
Urine N containing metabolites: urea, NH4+, creatinine, and uric acid.
Major sources:
- urea cycle (liver)
- glutamimne (kidney)
- Creatine phosphate (muscle)
- Purines
The main pathway of aminoacid nitrogen removal
transamination
Urea cycle starts in ——- and ends in ——- .
Starts in mitochondria and is completed in the cytosol
Urea Cycle During Fasting
- During fasting muscle proteins are degraded and amino acids are transported to the liver and converted to glucose.
- The nitrogen of the used amino acids are converted to urea
- During starvation the brain switches from glucose to ketone bodies (mainly from fatty acids) as energy source.
- The utilization of amino acids decreases ► decreased urea production.
The levels in argininemia
- Urine orotate
- Blood citrulline
- Blood Arginine
- Blood NH3
- -
- -
- high
- moderately high
The levels in type I hyperammonemia
- Urine orotate
- Blood citrulline
- Blood Arginine
- Blood NH3
- low
- low
- low
- high
2 major ways to treat urea cylce disorders
A- reduce ammonia load on the urea cycle
B- provide urea cycle intermediates
