Liver: Nitrogen and Protein Metabolism

Question 1

1) What are the uses of amino acids?

Answer 1

• Building blocks/synthesis of proteins
• Synthesis of neurotransmitters, creatine, carnitine, haem, purines (C,T) and pyrimidines (A,G)
• Act as a source of blood glucose (provided by liver) during fasting and starvation

Question 2

2) Describe the pathways of dietary protein inside the body

Answer 2

• Dietary protein joins the free amino acid pool
• Amino acid pool and body proteins are shared, and some of the amino acid pool is used for the synthesis of creatine, purines and pyrimidines
• creatine, purines and pyrimidines produce uric acid (from nucleic acid breakdown) and creatinine (from metabolism of creatine)
• Free AA also are excreted as urea and ammonia

Question 3

3) Define protein turnover and what is the average turnover in an adult?

Answer 3

• Body proteins are continuously degraded to amino acids and re-synthesised
• Average turnover: 300-400g/day

[No storage forms of protein so protein needed in diet to replace lost AA and allow for tissue repair]

Question 4

4) Which variables determine protein turnover?

Answer 4

• Protein half lives (most have half lives of several days)
• Structural proteins may have half lives of years
• Hormones and digestive system enzymes are degraded very rapidly with half lives of minutes

Question 5

5) Describe the amino acid pool

Answer 5

• Source of free amino acids
• very low concentrations inside cells/bloodstream
• mixing and exchange with other free AA throughout the body

Question 6

6) What is the daily protein recommendation?

Answer 6

50-70g protein per day

Question 7

7) Why is high protein intake in a well-fed individual wasted?

Answer 7

Surplus AA are rapidly catabolised and the nitrogen is excreted as urea in urine

Question 8

8) Define essential amino acids and list the 10 essential amino acids for humans

Answer 8

• 10 AA we cannot synthesize, so we rely on dietary sources (we only synthesize the other 10)
  [Plants and microorganisms can synthesize all 20]
• Valine, Lysine, Methionine, Phenylalanine, Leucine, Isoleucine, Threonine, Tryptophan (Histidine, arginine)

Question 9

9) Define nitrogen balance

Answer 9

Total amount of Nitrogen in the diet as protein = amount of nitrogen excreted from body as urea, uric acid, creatine and ammonia [in a healthy adult]

• N intake = N excretion,
• Protein synthesis = degradation
• Subject is then described as being in nitrogen balance

Question 10

10) What is positive and negative nitrogen balance and when do they occur?

Answer 10

+ve: N intake > N excretion

• synthesis of protein exceeds rate of breakdown
• during normal growth in children, convalescence after a serious illness, after immobilisation after an accident, in pregnancy
• ve: N intake < N excretion
• In starvation, during serious illness, in late stages of some cancers, injury + trauma
• May lead to irreversible loss of essential body tissue –> death

Question 11

11) Outline the pathway of protein degeneration for most cellular proteins

Answer 11

• protein recognised as ‘old’ or ‘damaged’
• removed by ubiquitin breakdown system (breaks down AA for reuse)
• Gives a mixture of 20AA

Question 12

12) Outline the pathway of protein degeneration for foreign, ‘exogenous’ proteins

Answer 12

• ‘old’ or ‘damaged’ sub cellular organelles
• taken into vesicles by endocytosis or autphagocytosis
• vesicle fuses with lysosomes
• proteolytic enzymes degrade proteins -> AA

Question 13

13) Give two triggers of the protein degeneration pathway

Answer 13

• Starvation

- Hormones : cortisol increases rate of protein breakdown in muscle

Question 14

14) State the general amino acid degradation reaction

Answer 14

AA –> NH2 + oxo acid (keto acid)

Question 15

15) Define transamination and deamination

Answer 15

Transamination: the transfer of an amino group from one molecule to another (e.g. from amino acid to keto acid)
Deamination: removal of an amino group (e.g. from an amino acid)

Question 16

16) Describe the oxidative deamination reaction

Answer 16

• Amino acid + H2O -> Keto Acid + Ammonia
• 2H are removed by NAD/NADP
• Coenzyme (e.g. NAD) -> reduced coenzyme (+2H)

Question 17

17) Describe the transamination reaction

Answer 17

• Amino acid1 + Keto acid1 Keto acid2 + amino acid2
• reversible reaction: interconversion of two forms
• swapping amino group, to form different amino acids and keto acids

[glutamate/ glutamic acid often formed]

Question 18

18) What is the purpose of transamination?

Answer 18

• To balance the dietary AA with the AA your body needs
• Dietary AA can be converted into AA you need, a more efficient utilisation would be consuming an AA source similar to your body needs

Question 19

19) Give an example of a transamination reaction and which enzyme catalyses this?

Answer 19

L-AA + 2-oxoglutarate (keto acid) -> oxo acid + L-glutamate

- catalysed by aminotransferase (containing a group derived from vitamin B6 which is a carrier of the amino group)

Question 20

20) What are the two forms the NH2 group can also be released as?

Answer 20

NH3

NH4+

Question 21

21) What happens to the amino acids that have become oxo acids?

Answer 21

• After loss of the amino group, most AA become keto/oxo acids
• These acids can be metabolised by the TCA pathway, to CO2 and H2O, providing an ATP source
• during starvation, the carbon skeleton of 13 of the AA can also be converted back to glucose by the liver, these AA are ‘glucogenic’
  [remember Acetyl CoA cannot be converted back to glucose due to irreversible reactions]

Question 22

22) What are the 2 ketogenic (non-glucogenic) amino acids?

Answer 22

Leucine and lysine: can only be degraded to Acetyl CoA and cannot be converted back to glucose

Question 23

23) Which amino acids are both ketogenic and glucogenic, and so how are they catabolised?

Answer 23

• Phenylalanine, tyrosine, tryptophan, isoleucine, threonine

- Part of their chemical structure is converted to glucose

Question 24

24) What are the 6 main roles of the liver in nitrogen metabolism?

Answer 24

• Removal of AA, glucose and fats from the portal blood supply (e.g. AA from muscle breakdown)
• Absorbed AA used for synthesis of cellular proteins
• Synthesis of plasma proteins (albumin, clotting factors, lipid transport proteins etc)
• Synthesis of haem, purines and pyrimidines for RNA and DNA
• Degradation of excessive AA by transamination
• Conversion of NH3 to urea (transported to kidneys) for excretion [ornithine cycle]

Question 25

25) How are the amino groups and ammonia transported to the liver?

Answer 25

• As glutamine in the bloodstream -> can transport 2 amino groups to the liver at a time

[Liver is the only organ that can convert the amino groups of the AA to urea for excretion]

Question 26

26) Describe the importance of glutamine

Answer 26

• Safe carrier of ammonia in the blood
• (NH3 is toxic to the brain)
• Can carry 2 amino groups (ammonia equivalents) to the liver for urea formation
• Can deliver ammonia to the kidney for pH regulatino (buffering H+)

Question 27

27) State the two reactions between glutamine and glutamate of glutamine metabolism

Answer 27

• Breakdown: Glutamine -> Glutamate
• H2O -> NH3, enzyme glutaminase
• Synthesis: Glutamate -> Glutamine
• ATP + NH3 -> ADP + Pi, enzyme glutamine synthase

Question 28

28) Name the 4 important amino acids in inter-organ transport of nitrogen

Answer 28

• Alanine (from pyruvate)
• Glutamate and glutamine (from oxoglutarate)
• Aspartate (from oxaloacetate)

Question 29

29) Describe the urea cycle

Answer 29

• CO2 and NH4 enter the cycle and react to make carbamoyl phosphate (Mg-ADP + Pi produced)
• Loss of Pi –> L-Citrulline
• addition of L-aspartate and loss of AMP + Mg-PPi from Mg-ATP produces Argininosuccinate
• loss of fumarate produces L-Arginine
• addition of water and removal of urea produces L-Ornithine and cycle continues with addition of carbamoyl phosphate

Question 30

30) What are the sources of nitrogen in urea synthesis?

Answer 30

Glutamine - transports ammonia to the liver as an amine group
L-aspartate - brings the second amino group to the urea cycle

Question 31

31) Summarise the roles of glutamate, glutamine, alanine and aspartate in nitrogen metabolism

Answer 31

Glutamine - ammonia transport (to liver for urea cycle)
Glutamate - receives nitrogen from amino acids, ammonia is released in liver mitochondria (ATP and NH3 added to glutamate forms glutamine)
Alanine - ammonia transport from muscle to liver
Aspartate - donate amino group to the urea cycle

Question 32

32) State the reaction that adds the first amino group into the ornithine cycle

Answer 32

NH4+ + CO2 –> NH2COP (carbamoyl phosphate)
- ATP –> ADP + Pi
ornithine + NH2COP –> citrulline

Question 33

33) State the 4 end products of nitrogen metabolism

Answer 33

• Urea (protein breakdown)
• Creatinine (creatine phosphate breakdown)
• Uric acid (DNA and RNA breakdown)
• Ammonia (NH4+) - for control of body pH