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BIOC2005 - Metabolism > 5 - Protein Metabolism, The Fasting State and Starvation > Flashcards

5 - Protein Metabolism, The Fasting State and Starvation Flashcards

1
Q

What is the primary aim of the metabolism regulation in the fasting state and starvation?

A

Conserving glucose for the tissues that rely on it as a fuel, particularly the brain.

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2
Q

How long do the body’s glycogen stores last?

A

In around two days.

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3
Q

What fuels are used as alternatives to glucose during fasting?

A

Fats and proteins.

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4
Q

Which alternative fuel is primarily responsible for supporting the body during fasting? Why is this?

A

Fats, as proteins have an integral structural role so using them as fuel is disadvantageous.

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5
Q

How can proteins be used as a fuel source?

A

In gluconeogenesis or in direct oxidation.

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6
Q

What are the processes of conserving carbohydrate stores and functional peptides called?

A

Glucose Sparing and Protein Sparing.

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7
Q

What first signals the body to enter the fasting state, and what organs sense this?

A

Low blood glucose, sensed by the pancreas and the liver and effected across other tissues by pancreatic response.

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8
Q

How does the pancreas respond to low blood glucose?

A

By decreasing insulin secretion and starting to produce glucagon.

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9
Q

What tissues does glucagon effect?

A

Primarily the liver.

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10
Q

Why do some of the liver functions respond to the decrease in blood glucose without signalling?

A

To enable immediate response.

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11
Q

What is the primary signal for most tissues to enter the fasting state?

A

Low insulin signalling.

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12
Q

List the sequence of events that occur in the liver the fasting state begins and progresses to starvation.

A

1 - Glycogenolysis until stores depleted.
2 - Glucooneogenesis
3 - Ureagenesis
4 - Ketogenesis

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13
Q

What effect does the lower insulin levels have on muscle metabolism?

A

+ Glycogenolysis
+ Fatty Acid Oxidation
+ Proteolysis (Starvation)
- Glycolysis

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14
Q

What effect does the lower insulin levels have on adipose tissue?

A

+ Lipolysis/FA release

- Lipogenesis

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15
Q

What effect does insulin have on proteostasis?

A

Causes an increase in protein synthesis in order to make use of the resources currently present in the body.

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16
Q

What tissues rely on glucose for their fuel?

A

Brain, Renal Medulla, Erythrocytes.

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17
Q

Which proteins are the first to be broken down during fasting?

A

Liver proteins.

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18
Q

Protein digestion of proteins in which tissue dominates in the later stages of fasting?

A

Muscle tissue.

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19
Q

What issue is created by using proteins as fuel?

A

Disposal of the toxic waste nitrogen products.

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20
Q

What is the fate of waste nitrogen in extrahepatic tissues?

A

It must be transported to the liver as only hepatic cells are capable of performing the Ornithine Cycle.

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21
Q

How is waste nitrogen transferred to the liver?

A

The Glucose-Alanine cycle.

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22
Q

Describe the step of the G-A cycle in which the nitrogenous group is removed from the amino acids.

A

Amino Transferase transfers the amine group onto a pyruvate molecule, creating a keto-acid AA remnant and alanine from the pyruvate.

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23
Q

What is the fate of the alanine produced in extrahepatic tissues in the G-A cycle?

A

It is used to shuttle the nitrogen from the tissues to the liver.

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24
Q

What happens to the alanine used in the G-A cycle upon reaching the liver?

A

It undergoes another amino transferase reaction, being converted back to pyruvate by donating its amino group to a-ketoglutarate to produce glutamate.

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25
Q

How is the pyruvate produced in the liver used further in the G-A cycle?

A

It is used in gluconeogenesis and the resulting glucose is released back into the blood for extrahepatic tissues to take up.

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26
Q

What happens to the glutamate produced in the liver by the G-A cycle?

A

This is the final recipient of the waste nitrogen group, so it is fed into the ornithine cycle.

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27
Q

What functions does the G-A cycle provide?

A

Transfer of waste nitrogen to the liver.

Conservation of gluconeogenic potential.

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28
Q

How is the G-A cycle used to regulate carbohydrate metabolism?

A

Alanine buildup in the liver inhibits pyruvate kinase and hence glycolysis, ensuring that glucose is not wasted in the fasting state.

29
Q

Why is ammonia toxic to the body?

A

The toxicity is not fully understood, but may be due to a-ketoglutarate depletion, Glu/Gln accumulation and neurotransmitter interference.

30
Q

At what blood concentration is ammonia lethal?

A

50uM

31
Q

How are the three primary mechanisms that organisms use to remove ammonia from their systems classed?

A

Ammoniotelic, Uricotelic, Ureatelic

32
Q

What is Ammoniotelic nitrogen secretion?

A

Direct secretion of ammonia, only suitable for marine animals due to its toxicity.

33
Q

What is Ureotelic nitrogen secretion?

A

Conversion of ammonia to urea, which is more soluble and less toxic to allow for water-mediated excretion.

34
Q

What is Uricotelic nitrogen secretion?

A

Conversion of ammonia to Uric Acid, a highly insoluble and often almost solid product that can be excreted with minimal water loss. Useful for water-sparing organisms.

35
Q

Other than the three primary classes of nitrogen secretion, what other substances is ammonia converted into for secretion?

A

Timethyamine oxide, Creatine, Creatinine, Guanine, amino acids.

36
Q

Give to examples of organisms that change their mechanism of ammonia secretion.

A

Frogs after metamorphosing from tadpoles, ammonio- to ureotelic.

Humans in utero and after birth, ammonio- to ureotelic.

37
Q

Describe the structure of liver tissue.

A

Hexagonal blocks of cells clustered around a central vein. The bile duct, arteries and hepatic portal veins cluster at the corners and drain into the central vein via sinusoids.

38
Q

Which cells are capable of carrying out the Ornithine Cycle?

A

Periportal hepatocytes - the liver cells that line around the hepatic portal vein.

39
Q

Where in the cell does the Ornithine cycle occur?

A

Its stages are split between the mitochondrial matrix and the cytosol.

40
Q

In what two ways can the glutamate enter the Ornithine Cycle?

A

It diffuses into the mitochondrial matrix where it is deaminated, either producing free ammonia which is incorporated via Carbamoyl-P Synthetase or in transamination onto oxaloacetate to form aspartate that enters via Argininosuccinate Synthetase.

41
Q

What is the primary enzyme by which waste nitrogen enters the Ornithine Cycle?

A

Via Carbamoyl-P Synthetase

42
Q

Describe the action of Carbamoyl-P Synthetase.

A

NH3 + HCO3-
(2x ATP) ->
Carbamoyl Phosphate

43
Q

Which enzyme is used to regulate the Ornithine Cycle?

A

Carbamoyl Phosphate Synthetase.

44
Q

Describe the deamination of Glutamate in the mitochondrial matrix.

A

Glu + NAD -> a-KG + NH3 + NADH

45
Q

What are the energy/coenzyme requirements of the Ornithine Cycle?

A

1 double hydrolysis of ATP to AMP.

46
Q

What are the products of the Ornithine Cycle?

A

Urea and Fumarate.

47
Q

What is the fate of the urea produced in the Ornithine Cycle?

A

being made in the cytosol, it is immediately transported in the blood to the kidneys for excretion.

48
Q

What is the fate of the fumarate produced in the Ornithine Cycle?

A

Released into the cytosol where it is converted to malate and used in gluconeogenesis.

49
Q

How are proteins able to be used for gluconeogenesis?

A

By production of fumarate through the Ornithine Cycle.

50
Q

What enzyme is the primary control point for the Ornithine Cycle?

A

Carbamoyl Phosphate Synthetase.

51
Q

Outline the method of Carbamoyl-P Synthetase (CPS).

A

CPS activity is dependent upon N-Acetyl Glutamate (NAG), which is produced from glutamate and Acetyl CoA by NAG Synthase. NAG Synthase is also activated by arginine, so [NAG] mostly depends on amino acid concentration and hence protein catabolism.

52
Q

List the indirect regulatory methods that act to change [NAG] concentration.

A

+ Level of proteolysis ([Arg]/[Glu])

- Hormone triggered cytosol sequestration/degradation.

53
Q

How active is the Ornithine Cycle in the fed state?

A

Often high as the diet will include many amino acids to break down and store as glucose.

54
Q

What state other than Fed or Fasting would proteolysis be high in?

A

Hypercatabolic states induced by stress, trauma or infection.

55
Q

Explain the rate of infant mortality due to genetic ornithine cycle defect.

A

Defects in nitrogen waste disposal are one of the main causes of infant mortality, as because humans only express the genes in postnatal development they are born before the hyperammonemia is caused.

56
Q

What are common causes of hyperammonemia?

A

Genetic defects that reduce the functionality of ornithine cycle enzymes, or in the production of NAG (which can be treated with N-carbamyl glutamate)

57
Q

Why is the ornithine cycle less active as starvation progresses?

A

Proteolysis decreases due to protein sparing, and the ammonia produced from amino acid catabolism is actually useful for neutralising the acidosis that starvation metabolism causes.

58
Q

How does the body respond to starvation-induced acidosis?

A

Increased amino acid catabolism through increased glutaminase, PEPCK and a-KG Dehydrogenase expression.

59
Q

Where does ketogenesis occur?

A

Exclusively in the liver.

60
Q

What advantage does ketogenesis offer to extrahepatic tissues?

A

Most tissues have limited beta oxidation capacities due to low enzyme concentrations, the liver turns the FAs into ketone bodies for them which can be used as fuel far easier.

61
Q

What a ketone bodies most useful for?

A

As a supplementary energy source for the brain to limit glucose loss.

62
Q

What is the maximum proportion of energy the brain can gain from ketone bodies?

A

50%

63
Q

What is used to produce ketone bodies?

A

Fatty acids.

64
Q

Ketone bodies are like a soluble version of…

A

Acetyl CoA

65
Q

What are the substrates of Ketogenesis?

A

The reaction requires three Acetyl CoA molecules per ketone body, but releases one.
2x Acetly CoA
1x NADH
1x H2O

66
Q

What efficiency problem does use of ketone bodies cause?

A

Ketone bodies exist in equilibrium with their oxidised form - acetoacetate. Acetoacetate is prone to decarboxylating to form highly volatile Acetone which evaporates from the mouth leading to loss of potential energy.

67
Q

How is ketogenesis stimulated?

A

It is the natural fate of hepatic Acetyl CoA when the citric acid cycle is inactive, which during the fasting state will be as its intermediates will have been used for gluconeogenesis.

68
Q

How can diabetes cause issues through ketogenesis?

A

Lack of insulin response leads to high plasma fatty acid concentration, which is taken up by the liver and oxidised. The excess Acetyl CoA is used for ketogenesis but as they are not needed the ketone bodies just cause acidosis.

BIOC2005 - Metabolism (8 decks)

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