Liver: Nitrogen and Protein Metabolism Flashcards Preview

4BBY1013 Biochemistry > Liver: Nitrogen and Protein Metabolism > Flashcards

Flashcards in Liver: Nitrogen and Protein Metabolism Deck (33)
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1

1) What are the uses of amino acids?

- 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

2

2) Describe the pathways of dietary protein inside the body

- 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

3

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

- 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]

4

4) Which variables determine protein turnover?

- 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

5

5) Describe the amino acid pool

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

6

6) What is the daily protein recommendation?

50-70g protein per day

7

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

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

8

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

- 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)

9

9) Define nitrogen balance

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

10

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

+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

11

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

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

12

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

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

13

13) Give two triggers of the protein degeneration pathway

- Starvation
- Hormones : cortisol increases rate of protein breakdown in muscle

14

14) State the general amino acid degradation reaction

AA --> NH2 + oxo acid (keto acid)

15

15) Define transamination and deamination

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)

16

16) Describe the oxidative deamination reaction

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

17

17) Describe the transamination reaction

- 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]

18

18) What is the purpose of transamination?

- 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

19

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

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)

20

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

NH3
NH4+

21

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

- 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]

22

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

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

23

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

-Phenylalanine, tyrosine, tryptophan, isoleucine, threonine
- Part of their chemical structure is converted to glucose

24

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

- 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]

25

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

- 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]

26

26) Describe the importance of glutamine

- 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+)

27

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

- Breakdown: Glutamine -> Glutamate
- H2O -> NH3, enzyme glutaminase

- Synthesis: Glutamate -> Glutamine
- ATP + NH3 -> ADP + Pi, enzyme glutamine synthase

28

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

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

29

29) Describe the urea cycle

- 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

30

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

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