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Nitrogen Metabolism & Urea Synthesis

Afronewtzz Biochemistry Degree English 66 cards

For each urea molecule synthesised in the urea cycle, what is C=O provided by?

Cards in this deck (66)

  1. 1What are the 2 main nitrogen-containing metabolites?Amino acids
    Nucleic acids
  2. 2How are amino groups normally disposed of from the body?From the breakdown of amino acids as urea in the urine.
  3. 3How is ammonia itself, toxic at high concentrations?It impairs brain metabolism.
  4. 4When does urea production occur in the body?When the dietary supply of nitrogen exceeds the body's requirements i.e. high protein diet or during starvation when the body resorts to breaking down protein to provide for its energy needs.
  5. 5Where is the major site of urea synthesis?The liver.
  6. 6When an amino acid loses its amino group, what is the name of the carbohydrate section of the group?The alpha-keto acids.
  7. 7What are 2 options of the breakdown of alpha-keto acids during amino acid break down?1. Oxidised to carbon dioxide and water (essentially providing energy).
    2. Provide carbon units for glucose synthesis i.e. rebalance glucose in the blood to continue providing glucose for the brain during starvation mode for example.
  8. 8The amino group/ NH4+ which is liberated from amino acid breakdown (as oppose to the alpha-keto group) has what 2 options?- Recycled (i.e. biosynthesis of nucleic acids and proteins)
    - Excreted as either NH4+, Urea, Uric acid... depending on the organism.
  9. 9What 2 amino acids have both the amino group (NH4+) which we need to get rid of, (common to all amino acids) but also nitrogen in their R group?Glutamine and Asparagine.
  10. 10Describe the process of the removal of these nitrogen-containing groups from Glutamine.- Glutamine is hydrolysed to Glutamate by glutaminase enzyme. Water comes in and it's oxygen displaces the side chain's Nitrogen... releasing NH4+.
    - Glutamate is then deaminated directly to alpha-ketoglutarate and ammonia using glutamate dehydrogenase (NAD+ > NADH and NH4+ ammonia group).
  11. 11Describe the process of the removal of these nitrogen-containing groups from Asparagine amino acid.- Asparagine is hydrolysed to aspartate by asparaginase enzyme - water comes in and it's oxygen replaces the R group's Nitrogen's position. NH4+ is released.
    -
  12. 12What is meant by 'deamination' in the conversion of Glutamate to alpha-keto-glutarate?Deamination is the removal of the amino group from Glutamate.
  13. 13What is meant by the hydrolysis reaction in the conversion of Glutamine to Glutamate, or Asparagine to Aspartate for example?The addition of water in order to replace the nitrogen-containing group on the 'R' side chain of the amino acid.
  14. 14When you remove the amino group from an amino acid, what is the name of the corresponding carbohydrate group left?The alpha-keto acid.
  15. 15What is the corresponding alpha-keto-acid of Alanine amino acid?Pyruvate
  16. 16What is the corresponding alpha-keto-acid of Aspartate amino acid?Oxaloacetate
  17. 17Glutamine is therefore readily converted into its alpha-keto-glutarate... but what about other amino acids?The amino group from OTHER amino acids is first transferred to glutamate by transaminases so that they become their alpha-keto-acid and alpha-keto-glutarate is reverted back to Glutamate.
    Then, glutamate dehydrogenase catalyses the removal of this amino group to form alpha-keto-glutarate again.
  18. 18What is the name of the enzyme which catalyses the transferring of an amino group from Aspartate to Glutamate? Forming Oxaloacetate.Glutamate oxaloacetate transaminase.
  19. 19What is the name of the enzyme which catalyses the transferring of an amino group from Alanine to Glutamate? Forming PyruvateGlutamate pyruvate transaminase.
  20. 20What is the role of transaminase enzymes in the removal of amino group from amino acids?Transaminases interconvert an amino acid with its corresponding alpha-keto-acid using the cofactir pyridoxal phosphate.
  21. 21What is the name of the cofactor in the transaminase reactions of the liberation of an amino group from amino acids?Pyridoxal phosphate.
  22. 22What are Glucogenic amino acids?Amino acids which can feed into the glycolytic pathway to re-synthesise glucose by gluconeogenesis.
  23. 23What are Ketogenic amino acids?Ketogenic amino acids feed into either AcetylCoA or AcetoacetylCoA rather than the glycolytic pathway.
  24. 24What 2 amino acids produce Oxaloacetate?Asparagine
    Aspartate
  25. 25Name 4 amino acids which produce Pyruvate.- Alanine
    - Cysteine
    - Glycine
    - Serine
    - Threonine
    - Tryptophan
  26. 26What 3 amino acids produce Fumarate?Aspartate, Phenylalanine, and Tyrosine.
  27. 27What 4 amino acids produce Succinyl-CoA?Isoleucine
    Methionine
    Threonine
    Valine
  28. 28Name 4 amino acids which produce alpha-keto-glutarate.- Arginine
    - Glutamate
    - Glutamine
    - Histidine
    - Proline
  29. 29Why are Glutamine and Asparagine both hydrolysed to Glutamate and Aspartate before they can undergo the removal of their amino group?Because they both contain nitrogen in their 'R' side chain as well as their amino group - this is removed by both glutaminase and asparaginase respectively.
  30. 30Can amino acids be both ketogenic and glucogenic amino acids?Yes.
  31. 31Once the amino group has been liberated from the amino acid, what is the fate of the remaining carbohydrate group, or the alpha-keto acid?It can either be completely oxidised in the citric acid cycle into carbon dioxide and water, or it can be used as the carbon backbone in glucose synthesis.
  32. 32Once ammonia has been liberated from Glutamate, whereabouts is it sent to and what does it do?It gets sent into the mitochondria where it combines with a bicarbonate ion, using the energy from the hydrolysis of 2ATPs... forming a carbamoyl phosphate.
  33. 33How is carbamoyl phosphate formed in the mitochondria during stage one of urea synthesis?Stage 1 = Carbamoyl phosphate synthetase. NH3 enters the mitochondria and uses the energy from the hydrolysis of 2 x ATP molecules to combine with a bicarbonate ion. This forms carbamoyl phosphate.
  34. 34Write the equation for the formation of Carbamoyl phosphate.2ATP + HCO3- + NH3 > Carbamoyl phosphate + 2ADP + Pi (carbamoyl phosphate synthetase).
  35. 35Once carbamoyl phosphate has been produced, what takes place in the urea cycle?Ornithine (a carrier) combines with carbamoyl phosphate - phosphate is liberated and citrulline forms.
  36. 36Once citrulline is formed, what is the next stage in the urea cycle?A citrulline carrier sotuated in the membrane of the mitochondrion carries the citrulline molecule across the membrane into the cytosol.
  37. 37Once the citrulline molecule has been carried across into the cytosol, what takes place in the urea cycle?Citrulline merges with an incoming aspartate to form Argino-succinate.
    This hydrolyses ATP to AMP + PPi.
    Uses enzyme argino-succinate synthase.
  38. 38Once Argino-Succinate has formed in the urea cycle, what happens next?Argino succinase catalyses the liberation of 4C fumarate from Argino-succinate, thus leaving Arginine in the citric acid cycle.
  39. 39What happens once Arginine has formed in the urea cycle?Arginase catalyses the formation of Urea from the input of Water.
    Ornithine is regenerated.
  40. 40Once ornithine is regenerated in the cytosol, where does it go and how?Ornithine travels back into the mitochondria via an ornithine carrier.
  41. 41For each urea synthesised, where are the NH3 groups originating from?- One NH3 group from free ammonia (from the transaminase reaction in the mitochondria).
    - One NH3 group added from aspartate (generated from transamination of oxaloacetate).
  42. 42For each urea molecule synthesised in the urea cycle, what is C=O provided by?C=O is provided by bicarbonate ion (CO2).For each urea molecule synthesised in the urea cycle, what is C=O provided by?
  43. 43How many molecules of ATP are required to drive the urea cycle forward?3 ATPs.
  44. 44Whereabouts in the cell does the Urea cycle occur?Involves both cytosolic and mitochondrial steps.
  45. 45Describe how the Citric Acid Cycle links in with the Urea Cycle.- OAA is the alpha-keto-acid of Aspartate.
    - When fumarate is released from the Urea Cycle it can be fed into the citric acid cycle where it can be converted into Malate > OAA.
    - An alpha amino acid (i.e. Glutamate) can then donate an amino group to OAA.
    - Glutamate then becomes alpha-keto-glutarate, and OAA becomes Aspartate (it is no longer an alpha-keto-acid, but an amino acid).
    - Aspartate can now donate another amino group to the Urea cycle.
    - Thus, they are linked.
  46. 46Why would citric acid cycle intermediate be used as carbon backbones for gluconeogenesis rather than be completely oxidised in the cycle?So that the body can maintain blood glucose levels for supply to the brain.
  47. 47What is Gluconeogenesis? What is this the opposite of?- Gluconeogenesis is the synthesis of Glucose de novo as opposed to glycogenolysis/ glycolysis (breakdown of glucose).
  48. 48Where does Gluconeogenesis occur?In the liver and also in the proximal convoluted tubules of the kidney.
  49. 49When does Gluconeogenesis occur?Either: late in starvation, after exercise, diabetes or even at birth whereby we are trying to maintain our blood glucose supply.
    Our carbohydrate (glycogen stores) have been removed so we are beginning to break down fat, and subsequently amino acids.
  50. 50Describe how glucose could be synthesised (Gluconeogenesis) late in starvation when amino acids are beginning to be broken down - use Glutamine as the AA example in the liver.- Glutamine enters the liver proximal tubule epithelial cell.
    - Glutamine (contains N in its 'R' side chain) so this is removed by Glutaminase to form Glutamate.
    - Glutamate is then converted into alpha-keto-glutarate by glutamate dehydrogenase (more NH3 is liberated).
    - This NH3 goes on to be released into the urine.
    - The alpha-keto-glutarate however, can now undergo GNG (gluconeogenesis) into Glucose which can then enter the blood stream for supply to the brain - maintaining blood glucose levels).
  51. 51Why do we lose muscle mass during starvation?We have depleted our Glycogen stores and fat stores, and are now breaking down protein (which go through transaminase reactions in order to enter the urea cycle where backbones can be liberated for re-entry into the citric acid).
  52. 52How is lactate used directed to the liver as a substrate for Gluconeogenesis?- During periods of strenuous exercise, the body undergoes anaerobic respiration in order to balance the redox state of NAD.
    - This produces high levels of lactate which exits the muscle tissue and via specific transporters, enters the liver tissue for GNG.
    - Lactate is converted back to pyruvate in the liver.
    - Then, using energy in the form of ATP the pyruvate is converted to Glucose via the GNG pathway and is transported back into the blood to supply glucose back to the muscle.
  53. 53What is the name for lactate being fed through the Gluconeogenesis pathway?The Cori Cycle
  54. 54In what way are certain amino acids (i.e. alanine and glutamine) substrates for GNG?Alanine and Glutamine, in the corresponding Alanine and Glutamine cycles can transfer amino groups from muscle to the liver for urea synthesis.
    i.e. Pyruvate (alpha-keto-acid) > Alanine (Amino acid) > Blood > Liver > Converted back to pyruvate (transaminase reaction- alpha-keto-glutarate converts to glutamate; removing NH3 group from the amino acid, alanine - converting it to pyruvate, the alpha-keto-acid) > GNG > Glucose (re-supply to blood).    In what way are certain amino acids (i.e. alanine and glutamine) substrates for GNG?
  55. 55In what way do certain sugars, such as Fructose (from Sucrose) act as substrates for the GNG pathway?Fructose can enter the glycolytic pathway and be used to re-synthesise glucose.
  56. 56In what way do Glycerol and propionate from lipid breakdown act as substrates for the GNG pathway?Glycerol and propionate (from odd chain fatty acid oxidation) can come in as a substrate for the GNG pathway but the fatty acids themselves can not enter the GNG pathway for glucose production.
  57. 57What is the purpose of Fructose bis-phosphatase enzyme in GNG?Binds to Fructose-1,6-bisphosphate and hydrolyses the 1 phosphate on the fructose, to form fructose 6 phosphate.
  58. 58What is the purpose of Glucose-6-phosphatase in GNG?Binds to Glucose-6-phosphate and hydrolyses the phosphate at the 6th position to generate the free glucose.
    This removes the negative charge on the molecule, and allows glucose to leave the cell easily.
  59. 59Describe one way in which equilibrium effects the influence of Gluconeogenesis.An equilibrium reaction in GNG is Glucose-6-Phosphate < > Fructose-6-Phoshate, regulated by phosphoglucose isomerase. If the amount of Fructose-6-phosphate in the cell is extremely high, this will drag the reaction in favour of making more Glucose-6-phosphate, and therefore drive the synthesis of Glucose
  60. 60Why do you need reduced forms of NAD (NADH) in order to drive GNG?Because in the Glycolysis pathway, NAD+ is reduced to NADH - therefore in order to favour GNG, this cofactor needs to be already in its reduced form ready to be oxidised back in between the steps of 1,3-bisphosphoglycerate and Dihydroxyacetone phosphate and Glyceraldehyde-3-phosphate.
  61. 61Why do you need ATP in order to drive GNG?Because in the forward Glycolytic pathway you require ADP in order to remove a phosphate from 1,3-bisphosphoglycerate to make just 3-phosphoglycerate.
    However, in the reverse pathway, in GNG, you require ATP since you need to re-add this phosphate onto 3-phosphoglycerate in order to make 1,3-bisphosphoglycerate.
  62. 62How do we convert Pyruvate back to Phosphenolpyruvate for GNG?1) Pyruvate > OAA (catalysed by pyruvate carboxylase) - uses ATP > ADP + Pi and used Bicarbonate/ HCO3-.
    2) OAA > Phosphoenolpyruvate (catalysed by phosphoenolpyruvate carboxykinase/ PEPCK) - uses GTP to drive forward > GDP - decarboxylation reaction (omits CO2).
  63. 63How many molecules of ATP are required for GNG in comparison to what Glycolysis generates?GNG = requires 2 x ATP
    Glycolysis = generates 1 x ATP
  64. 64Whereabouts does each stage of conversion of phosphoenolpyruvate > pyruvate occur?1) Pyruvate > OAA = Mitochondria
    2) OAA > PEP = Cytosol
  65. 65Why is Biotin used as a cofactor for enzymes involved in reactions that use bicarbonate/ HCO3- ?Because the reactive site within the biotin can sit and react with the bicarbonate ion.
    This means that enzymes which use biotin as a cofactor can move the carbon dioxide derived from bicarbonate around.
  66. 66How does pyruvate carboxylase use biotin as a cofactor in the formation of OAA?Pyruvate carboxylase binds to the biotin cofactor which readily reacts with HCO3-. This allows for OAA to form, since the bicarbonate group provides the carbon and oxygen in order to carry out this reaction.

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Degree Biochemistry Flashcards on Nitrogen Metabolism & Urea Synthesis, created by Afronewtzz on 08/05/2015.
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