Urea Cycle Flashcards
1
Q
Disposal of nitrogen - liver
A
- Ammonia is produced as a result of aa degradation and is potentially toxic – required to be converted into a non-toxic form
- Liver: principle site where most aa’s are degraded leading to the production of: NH3 is TOXIC, Carbon skeleton
- NH3 is detoxified to Urea via the Urea Cycle in liver
- Urea is the major end-product of N metabolism
2
Q
2 possible fates of carbon skeletons
A
- Used for gluconeogenesis
- Burned in TCA cycle
3
Q
Urea in kidney
A
- excreted in the urine
- kidneys also excrete ammonia as ammonium ions (important in regulation of the acid base balance)
- amount of ammonium ions produced depends on the amount of protons that need to be excreted
- other non-protein nitrogenous substances are Uric acid (end-product of purine degradation) and creatinine (end-product of creatine degradation)
4
Q
Ammonia in transport of nitrogen
A
- NH3 is neurotoxic: Creates requirement for a non-toxic transport form of ammonia
- NH3 is transported from peripheral tissues: Glutamine, Alanine
5
Q
Glutamine in nitrogen transport
A
- Incorporation of ammonia to glutamate
- Glutamate DH: incorporates NH4+ (ammonium) on a- ketoglutarate producing glutamate
- Glutamine synthetase: incorporate 2nd NH4+ ion to produce glutamine
6
Q
Alanine in nitrogen transport
A
- transport form of ammonia from muscle
- main ammonia transporter produced in muscle due to the high levels of pyruvate generated
- In liver converted back to pyruvate – glucose.
- Ammonia enters urea cycle
7
Q
Urea Cycle
A
- Converts toxic NH3 to non-toxic urea
- Takes place in liver
- Partly mitochondrial (two reactions) and partly cytosolic (three reactions)
- Requires two nitrogen atoms:
1st N atom of urea donated by ammonia (GDH & Glutaminase), 2nd N atom is donated by aspartate
8
Q
Formation of Ammonia in Liver
A
- Transamination: amino group of an amino acid is transferred to α-ketoglutarate to form glutamate
- Oxidative deamination by Glutamate dehydrogenase forms free ammonia
- Glutamate also produced from reverse reaction: removal of free NH3
- Ammonia also formed from glutamine by glutaminase reaction (free ammonia and glutamate are products)
9
Q
How many reactions are there in the Urea Cycle?
A
- 5 reactions: 2 mitochondrial, 3 cytosolic
10
Q
First reaction of Urea cycle
A
- catalysed by Carbamoyl phosphate synthetase I
- Catalyses condensation and activation of ammonia (NH3) and bicarbonate (HCO3-)
- Requires hydrolysis of 2ATP
- Rate limiting step of the urea cycle
11
Q
Second reaction of Urea cycle
A
- Ornithine transcarbamoylase condenses carbamoyl phosphate and ornithine, which forms citrulline
- citrulline is then moved out of the mitochondria into the cytosol of the cell by the transporter SLC25A15
12
Q
Third reaction of Urea cycle
A
- Argininosuccinate synthetase takes citrulline formed in the mitochondrial stage and condenses it with aspartate to form argininosuccinate
- occurs by the formation of an intermediate, citrulline-AMP
13
Q
Fourth reaction of Urea cycle
A
- Argininosuccinase breaks Argininosuccinate into arginine and fumarate
- Fumarate is used in TCA cycle, where aspartate is reformed
14
Q
Fifth reaction of Urea cycle
A
- arginine is then further broken down into urea and ornithine by arginase
- ornithine is then transported into the mitochondria by ornithine translocase
- There, it is used by ornithine transcarbamoylase again to form citrulline, cycle repeats
15
Q
Regulation of Urea cycle
A
- Need to regulate cycle in response to; changes in protein degradation, high dietary intake
- Increase in arginine indicates substrate availability i.e. NH3
- N-acetyl glutamate (NAG) is an allosteric activator of CPS I (rate limiting enzyme)
- After a protein rich meal & in presence of arginine NAG formation is increased
16
Q
Inherited disorders of Urea cycle
A
- deficiency exists in all of the 5 enzymes of the Urea Cycle
- results in the accumulation of SUBSTRATE of that enzyme
- All urea cycle disorders are characterized by increased blood ammonia levels (hyperammonaemia) and blood glutamine levels
there is also decreased urea formation - Untreated: neurological manifestations, seizures and mental retardation
17
Q
Benzoic acid (management of hyperammonaemia)
A
- combines with glycine
- Requires activation to CoA derivative
- Product is Hippuric acid
- Excretion of 1 atom of nitrogen
18
Q
Phenylbutyrate (prodrug) (management of hyperammonaemia)
A
- combines with glutamine
- Prodrug converted to active molecule by oxidation
- Conversion to CoA derivative phenylacetyl-CoA
- Product is Phenylacetylglutamine
- Excretion of 2 atom of nitrogen
- Pharmaceutical invention can be used for all deficiencies
19
Q
Dialysis (management of hyperammonaemia)
A
- acute emergency situation
- Low protein/high carbohydrate diet to minimize N intake
- Prescribe α-keto acids (expensive) - prevention of stresses that induce a catabolic state
- Long term: Liver transplantation
20
Q
L-arginine supplementation (management of hyperammonaemia due to CPSI deficiency)
A
- Arginine stimulates the formation of N-acetylglutamate
- High levels of NAG might stimulate deficient CPSI
- L-arginine is also used to treat other deficiencies – those before ariginase
- Promotes excretion of citrulline and argininosuccinate
21
Q
Acquired hyperammonaemia
A
- Liver disease due to: viral or drug induced hepatitis, alcoholic cirrhosis
- Resultant cirrhosis of the liver leads to porto-systemic shunting of blood
- Portal blood enters the systemic circulation without going to the liver
- Ammonia produced in the intestine directly enters circulation
- Results in neurotoxicity
22
Q
Intestinal formation of ammonia
A
- Bacterial ureases form ammonia in the colon (also degradation of proteins)
- Ammonia formed enters the portal circulation - delivered back to the liver
- Cirrhotic liver portal blood enters systemic circulation bypassing the liver
23
Q
Ammonia neurotoxicity
A
- mechanism is unknown - two hypotheses can be considered:
- Energy and Osmotic Effect
- Neurotransmitter Effect
24
Q
Energy and osmotic effect
A
- α-ketoglutarate converted to glutamate via glutamate DH (reductive amination direction)
- Results in consumption of TCA cycle intermediates i.e. α-KG
- Results in decreased TCA cycle activity = ATP synthesis is reduced
- Subsequent reduced Na+/K+ ATPase activity, a solute pump that pumps sodium out of cells while pumping potassium into cells, both against their concentration gradients
- This results in: Changes in osmotic pressure, Eventual neuronal cell death
25
Q
Neurotransmitters effect
A
- Glutamate is converted to glutamine via action of Glutamine synthetase
- Results in high circulating glutamine levels: high glutamine levels cause cerebral oedema
- Reduced glutamate, and hence GABA levels in the brain
- Both Glu and GABA are reduced when NH3 is abundant: GABA is major inhibitory neurotransmitter (formed from glutamate), Glu itself is a major excitatory neurotransmitter
26
Q
Treatment of Acquired hyperammonaemia
A
- Low-protein/high carbohydrate diet - intuitive: less N in, less needed to dispose
- Lactulose: disaccharide (galactose –fructose): resistant to digestion in the small intestine, normal bacterial flora in colon produce lactic acid, acid conditions favour NH4+ formation (can’t be reabsorbed so more N excreted in faeces), promotes more NH3 entering lumen from tissues
- Neomycin: (or other antibiotic treatment): reduction in bacterial urease in the gut
