Urea Cycle

Differentiate between a positive and negative nitrogen balance and what conditions could lead to this? What percentage of excreted nitrogen is in urea?

+: intake > out; growth and recovery from illness and malnutrition

-: intake < out; illness, malnutrition, burns

Why does BUN levels decrease when pregnant? What else decrease with BUN? What happens when there is kidney damage? gout? Liver Damage? and other urea cycle disorders?

During pregnancy, the glomerular filtration rate increases

what are the three major steps in amino acid degradation?

1. removal of nitrogen (transamination, deanimation, deamidation)

2. clearance of NH4+ (urea cycle, ammonia itself is toxic)

3. utilization of carbon backbone (Fed state: synthesis of glycogen and triglycerides; Fast state: energy production)

Where does Urea gets its nitrogen and carbon from?

Urea gets one nitrogen from aspartate and one from NH4+; it gets its carbon from CO2

Differentiate deamination and deamidation in the context of nitrogen removal from amino acids? (definition? Which AA? Enzyme used?) Why is deamidation important?

• deamination: removal of amino group of some amino acids (glycine, threonine, histidine, serine, glutamate)

  • Uses glutamate dehydrogenase (reversible); glutamate —> alpha-ketoglutarate in liver, kidney; opposite way in other tissues

• Deamidation: removal of amide group from glutamin and asparagine ; enzyme = glutaminase

  • most important in kidney

  • provides most of the NH4+ in urine

    • NH3 helps balance pH

describe transmination as it relates to nitrogen transfer. General Pathway?

• main pathway of amino acid nitrogen removal

• It uses transaminases (ALT, AST) and requires PLP (B6) as coenzyme.

• general pathway: amino acid loses its NH4 to become a-keto acid; a-ketoglutarate gains NH4 to become glutamate.

Draw out the reaction including AST and ALT; what is the difference and smilarity between them?

• ALT > AST in specificity;

• AST is used for diagnosing myocardial infarcation;

• both: elevated levels = liver damage

What are the four central roles Glutamate have in nitrogen removal?

1. gets NH4 from other amino acids via transamination

2. provides NH4 to Urea cycle through deamination by glutamate dehydrogenase

3. it can transaminate OAA into aspartate. Aspartate can then provide an amino group to Urea

4. precursor of the allosteric activator of Urea

Where does the Urea cycle take place? How does nitrogen from other tissues get transported to this place? Where does urea go?

take place in liver

Amino acid nitrogen is carried to liver in the form of Alanine and Glutamine

Urea go to kidney to be excreted in urine. some urea also leave through intestines

describe how Glutamine transport Amino Acid Nitrogen to the liver? What is the net glutamine producer? main glutamine users?

Describe the transport of Amino Acid Nitrogen to the liver via Alanine:

1. what is its two purpose?

2. Draw out how the muscle and liver is connected via this

3. What are the main sources of Alanine

1. GNG; removal of nitrogen for Urea Cycle

2. Sources:

• Main: muscle (results of protein degradation/ transamination)

• other = kidney, intestines (conversion of glutamine to alanine)

  • glutamine → (via glutaminase) glutamate → (via ALT) Alanine

Why is ammonia dangerous? What are the two mechanism in which the body control this? What is the main detoxification reaction in the brain?

Ammonia is toxic

Two ways:

1. transamination reaction collect nitrogen on glutamate rather than release free ammonia

2. three eyzmes can fix free ammonia into organic molecules

   1. glutamate dehydrogenase

   2. glutamine synthetase

   3. carbamoyl phosphate synthetase I

Draw out the Urea Cycle

Describe the Carboamoyl Phosphate synthesis step in the Urea Cycle. How is it allosterically regulated

1. HCO3 + NH4+ → Carbamoyl Phosphate

2. catalyzed by carbamoyl phospahte synthetase I (CPSI)

3. Takes place in mitochondria

4. rate-limiting, committed step

5. requires 2 ATPS

6. allosterically regulated by N-acetylglutamate (NAG)

Describe the synthesis reaction of NAG

Glutamate + acetyl CoA —> NAG which activates CPSI

this reaction above is activated by Arginine

describe the reaction for citrulline synthesis

carbamoyl phosphate + ornithine —> Citruline

reaction is catalyzed by ornithine transcarbamoylase (OTC); takes place in mitochonria

What is ornithine translocase?

this enzyme helps transport ornithine from the mitochondria into cytosol

what causes type II hyperammononemia

deficiency of ornithine transcarbamoylase

what causes type I hyperammonemia?

deficiency of CPSI

what causes HHH syndrome?

deficiency in ornithine translocase

aka: hyperornithinemia-hyperammonemia- homocitrullinuria syndrome

describe the reaction for argininosuccinate synthesis

citrulline + aspartate —> arginosuccinate

• catalyze by argininosuccinate synthtase;

• requires 1 atp

• the aspartate nitrogen will be incorporated into urae

• location: cytosol

what is citrullinemia type I? What is citrullinemia type II?

I: deficiency in arginosuccinate synthetase

II: deficienct citrin (mitochonrial glutamate/aspartate antiporter)

describe the reaction for arginine synthesis

argininosuccinate —> fumerate and arginine

• catalyzed by argininosuccinate lyase

• cytosol

• only way arginine is produced in the human body

what causes argininosuccinyl acidemia?

deficiency in argininosuccinate lyase

describe the reaction of cleavage of Arginine into Urea and Ornithine

what other pathways can arginine go into?

arginine —> urea + ornithine

• catalyzed by arginase

• cytosol

• protein synthesis and nitric oxide synthesis

what causes arginiemia?

deficient in arginase

how many phosphate bonds are broken in the Urea cycle?

4

how does a high protein diet and excessive degradation affect the Urea cycle?

these causes increases in urea synthesis because an increase in arginine levels = increase in NAG = more urea produced because increase activity of CPSI

what are the symptoms of hyperamonemia? Compare and contrast primary an seconary hyperammonemias

hepatic encaphalopathy, cerebral edema, seizures, nausea, vomiting, lethargy, coma and death

primary: enzyme in urea cycle is defective

seconary: main cause is hepatic failure, can be caused by genetic defects not relate to urea cycle enzymes

what are the five primary hyperammonemias diseases? which ones are autosomal recessive?

1. type 1 (CPSI or NAG synthase deficiency_

2. type II (ornithine transcarbamoylase)

3. citrullinuria type I (argininosuccinate synthetase deficiency)

4. argininosuccinic aciemia (argininosuccinate lyase deficiency)

5. argininemia (arginase deficiency)

All are autosomal recessive except type II (x-linked)

What is the consequences of Type II hyperammonia in regards to carbamoyl phosphate

• OTC is defective → increases in carbamoyl phosphate

• accumulate leads to leakage out of mitochondria

• accumulation of carbamoyl phosphate → increasess in Orate → pyrimidines and urine

what are the three treatments for reducing nitrogen load on urea cycle

1. low protein diet, avoid fasting (to avoi AA degradation)

2. scavengers drugs (conjugates amino acids and target them for urinary excretion)

   1. benzoate: benzoate + glycine → hippuric acid → excretion

   2. phenylbutyrate : phenylbutyrate + glutamine → phenylacetylglutamine → excretion

3. reducing ammonia from microflora: lactolose laxative decreases pH → gut ammonia = not reabsorbed;

   1. also urease inhibitors can reduce nitrogen from this source as well

describe the amino acid supplementation to treat urea cycle deficiency

1. arginine: in all primary hyperammonemias, except for arginase deficiency, this is low; also promotess production of NAG → activates CPSI

2. citruline: in type I and II, low citruline; also captures aspartate so at least one nitrogen can be excreted

3. N-carbamoylglutamate: agonist for CPSI in case NAG synthase deficiency