DSP Biochem: Nitrogen Metabolism V-A: Urea Cycle

Typical Transamination Reaction

Aspartate (or other amino acid) to Glutamate (using OAA and alpha-ketoglutarate) *collector of nitrogen!!

What is the required cofactor for transamination reactions?

Pyridoxal Phosphate (PLP) comes from Vitamin B6

What is needed to make PLP?

Vitamin B6

What is N-acety-gluatmate (NAGS) has what affect on CPS I

allosterically activates it!

What organ is glutminase important in?

kidney (makes glutamate and NH3)

Coenzyme required for glutamate dehydrogenase

NAD+ or NADPH

What form is favored at physiological pH, ammonia or ammonium??

NH4+

NH4+ vs NH3...which exists in solution and which can cross membranes?

NH4+ in solution NH3 can cross membranes

Can ammonia, when protonated to form NH4+, cross membranes?

No!!

Serine becomes what? theoronine becomes what? whats the cofactor involved

PLP - serine = pyruvate - theoronine = alpha ketobutyrate

coenzyme required for aminotransferases

PLP (derived from vit B6)

Why cant NH4+ cross membranes?

a. NH3 is present in the body because this is the form that can cross cell membranes. For example, NH3 passes into the urine from kidney tubule cells and decreases the acidity of the urine by binding protons, forming NH4+. b. Once the NH4+ is formed → now a charged compound → can no longer freely diffuse across membranes.

Major Sxs of PLP deficiency

a. Vitamin B6 deficiency symptoms include dermatitis, a microcytic, hypochromic anemia, weakness, irritability, and, in some cases, convulsions. b. Xanthurenic acid (a degradation product of tryptophan) and other compounds appear in the urine because of an inability to metabolize amino acids completely. A decreased ability to synthesize heme from glycine may cause the microcytic anemia, and decreased decarboxylation of amino acids to form neurotransmitters may explain the convulsions. c. Although vitamin B6 is required for a large number of reactions involved in amino acid metabolism, it is also required for the glycogen phosphorylase reaction.

Benzoid Acid MOA for hyperammonemia/urea cycle

after activation, reacts with glycine to form hippuric acid, which is excreted. As glycine is synthesized from serine, the body now uses nitrogens to synthesize serine, so more glycine can be produced.

What are the two direct sources of nitrogen for the synthesis of urea?

ammonium ion and aspartate

What two molecules contribute nitrogen to urea?

aspartate and ammonium ion

how do kidneys cells differ in there managment of ammonia?

can excrete ammonium ion directly!! - doesnt have to be transferred to liver - this is why glutaminase is so important in kidney...can be converted to NH3/glutamate and be directly excreted - can form salts with metabolic acids to neutralize the urine and serve as a buffer

Glutamate

collector of nitrogens!! - recieves nitrogens from other

What happens to the carbon skeletons of the alpha-keto acids formed during transamination?

converted to common intermediates of energy producing pathways

Glutaminase does what function?

converts glutamine to glutamate and NH4

Where does the urea cycle occur?

first two steaps in mitochondrial matrix the rest = cytosol

What is the fate of amino acids in the fed state?

glucose or TGs (packaged and secreted in VLDL)

AST

glutamate to asparate (for contribution of nitrogen to urea cycle)

what is the only amino acid that undergoes rapid oxidative deamination?

glutamate!! - transamination rx shift amino groups to glutamate so it can release amino groups as free ammonia to undergo urea cycle

HOw does glutamine become glutamate?

glutaminase

What amino acids are the major carrier of nitrogen in the blood?

glutamine and alanine

What are the two ways to transport ammonia to the liver?

glutamine and alanine

How is glutamine formed? what is the purpose of the glutamine?

glutamine synthase (requires ATP) and provides way for nitrogen to be carried in the body - glutamine takes nitrogen from muscle, liver and CNS (brain) * major mechanism for ammonia removal in brain

How does muscle generally release amino acid nitrogen?

in the form of glutamine and alanine - this helps keep ammonia low in the blood

What organ is the glutaminase reaction particularly important?

kidney

What is the most common genetic problem involved in the urea cycle?

ornithin transcarbamoylase

What is the most common genetic condition affecting the urea cycle?

ornithin transcarbamoylase deficiency (causes hyperammonemia)

What two substrates form citrulline in the mitochondrial matrix?

ornithine (regnerated after urea is formed) carbomoyl phosphate (from free ammonium using CPS I) * citrulline is then transported out of the mitochondria

Biggest Problem with urea cycle disorders

ridding body of the excess ammonia that isnt being excreted via the urea cycle

Sxs of Ammonia Toxicity

slurring of speech tremors drowsiness cerebral edema blurring of vision *can cause coma and death

What two amino acids CANNOT undergo transamination?

threonine and lysine

How does glutamate collect nitrogen from other amino acids?

through transamination reactions

What does ornithine transcarbamoylase do?

transfer carbomoyl phosphate to ornithine in mitochondrial matrix to make citrulline

Waht is the fate of acmino acids in the fasted state?

used for gluconeogenesis and ketone body synthesis - carbons are oxidized to CO2 and H20 (used for energy storage or energy)

Are transamination reactions reversible?

yes!! readily reversible - can be used for amino acid removal of nitrogen or the synthesis of amino acids

List the two treatment options for hyperammonemia and urea cycle disorders

1. benzoic acid 2. phenylbutyrate

What are the two biochemical methods for removing nitrogen from amino acids?

1. transamination 2. removal as ammonia or ammonium ion (NH4)

What system is most effected by high levels of ammonia?

CNS

Explain how alanine is used to transport ammonia to liver

- alanine is produced by transamination of pyruvate (which comes from glycolyis or amino acid breakdown) - once in liver, alanine is transaminated back to pyruvate (pyruvate becomes glucose which can be used by muscle in the glucoce-alanine cycle)

What is special about arginase?

- arginase cleaves arginine to yield urea and ornithine - ONLY IN LIVER!!! therefore in other tissues such as the kidney...these enzymes produce arginine

What happens to urea that enters the gut?

- bacteria in the gut release the nitrogen of certain amino acids as ammonia or ammonium ion (NH4+)

What happens to the NH4+ produced by bacteria in the lumen of the intestinal tract?

- enters the hepatic portal vein and travels to the liver

Why is ammonia absorbed by the intestine, not immediately toxic?

- free ammonia is rapidly fixed into alpha-ketoglutarate by glutamate dehydrogenase to form glutamate or by glutamate.....(glutamine synthesase to form glutamine) - glutamnine can be used by may tissues - glutamate donates nitrogens to pyruvate to form alanine which travels to liver

how is NAG synthesized?

- from acetyl coA and glutamate by N-acetylglutamate synthase (arginine activates this reaction!!!) (more arginine means the urea cycle needs to run) - also regulated by substrate availability (short term regulation) and enzyme induction (long term)

Explain how glutamine is used to transport ammonia to liver

- gluatamine synthase combines ammonia with glutamate to form glutamine - this allows a non-toxic way to transport ammonia - once in liver, cleaved by glutaminase to produce gluatamate and free ammonia

Explain Transamination

- nitrigon is transferred as an amino group from the original AA to alpha-ketoglutarate, forming glutamate, the orginal amino acid becomes the corresponding alpha-keto acid example: aspartate can be transaminated to form oxaloacetate - amino group is transferred to alpha-ketoglutarate which is converted to its corresponding amino acid, glutamate

Phenylbutyrate MOA or hyperammonemia/urea cycle

- prodrug that is rapidly converted to pheynlacetate - rapidly combines with glutamine - it can then be exreted in the urine!! helps to clear nitrogenous waste *helps aid in excretion of ammonia (NH3)

Not related but....CPS II?

- used for pyrimidine synthesis - doesnt require NAGs - uses glutamine as the nitrogen source - occurs in cytosol

Where do asparate and ammonium ion both come from?

GLUTAMATE!! - ammonia: through oxidative deamination by glutamate dehydrogenase - aspartate: through transamination of OAA by AST

Activators/Inhibitors Allosterically of Glutamate Dehydrogenase

GTP = inhibitor ADP = activator (when energy levels are low, amino acid degradation needs to be high)

Enzyme required for oxidative deamination rx

Glutamate dehydrogenase