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Gastrointestinal Biology > GB 11. Post-Absorption Processing of Proteins > Flashcards

GB 11. Post-Absorption Processing of Proteins Flashcards

1
Q

What are amino acids the building blocks of?

A

protein

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2
Q

What are amino acids the precursors of?

A

[1] haem
- glycine (succinyl CoA)

[2] nucleic acids

  • purine + pyrimidines
  • aspartate, glycine + glutamine
  • aspartate + glutamine

[3] hormones
- e.g. thyroxine

[4] neurotransmitters
- e.g. dopamine, catecholamines

[5] biologically active peptides
- e.g. gonadotropins, PTH, insulin

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3
Q

Where are the 2 places that circulating amino acids come from?

A

[1] the diet

[2] protein turnover

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4
Q

What is the process of protein turnover?

A
  • proteins are degraded into amino acids
  • amino acids can be catabolized (can be used as a source of fuel for prolonged fasting and exercised)
  • amino acids can also be used for de-novo synthesis to create proteins once again
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5
Q

What is the half lives of most proteins and the half lives for some enzymes?

A

[1] MOST PROTEINS

  • half life of 2 to 3 days
  • e.g. cytochromes

[2] SOME ENZYMES

  • half life of around 30 minutes
  • e.g. HMG-CoA Reductase
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6
Q

What are 2 factors that control protein degradation?

A

[1] N-Terminal Residue
- it determines the protein half-life

[2] “PEST” Amino Acid Sequences

  • Pro (P), Glu (E), Ser (S), Thr (T)
  • they are rapidly degraded
  • degraded by lysosomal peptidases
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7
Q

Are amino acids stored?

A

No!
They are either:
[1] re-used in protein synthesis
[2] broken down

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8
Q

What are the 2 protein degradation systems?

A

[1] Ubiquitin - Proteasome System
- energy dependent

[2] Lysosomal Enzymes
- not energy dependent

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9
Q

There is a homeostasis and balance in maintaining circulating amino acids. This is dependent on….[2]

A

[1] utilisation of amino acids by tissues

[2] release of amino acids following protein degradation

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10
Q

What are the 2 principles organs involving in maintaining circulating amino-acid levels?

A

[1] Muscle
- generates more than 50% of circulating amino acids

[2] Liver

  • gluconeogenesis
  • urea cycle (utilisation and excretion of NH3)
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11
Q

Compare the amio acid transport in fasting states and a protein load state.
SLIDE 12

A

FASTING:
- all go towards liver (to create energy)

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12
Q

What is the Glucose/Alanine Cycle? (in other words, hepatic gluconeogenesis)

A

[1] in MUSCLE, ALANINE is created by the TRANSAMINATION of PYRUVATE

[2] Alanine sent from muscle to the liver

[3] Liver uses ALANINE to create GLUCOSE

[4] Glucose from the liver is sent to the muscle

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13
Q

What are glucogenic amino acids?

A

The breakdown of these amino acids yield TCA cycle intermediates which can be used to make glucose

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14
Q

What are Ketogenic amino acids?

A

The breakdown of these amino acids yields acetoacetate (Acetyl-CoA)

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15
Q

What are the 8 essential amino acids?

A 
[1] Isoleucine (Ile)
[2] Leucine (Leu)
[3] Valine (Val)
[4] Lysine (Lys)
[5] Methionine (Met)
[6] Phenyalanine (Phe)
[7] Threonine (Thr)
[8] Tryptophan (Try)
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16
Q

What are the 2 semi-essential amino acids? (the ones that are essential for children)?

A

[1] Arginine (Arg)

[2] Histidine (His)

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17
Q

What are the 3 non-essential amino acids that are formed from the essential amino acids?

A

[1] Cystine (Cys)
[2] Tyrosine (Tyr)
[3] Hydroxylysine (Hyl)

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18
Q

How many non-essential amino acids are formed from amphibolic (anabolic + catabolic) intermediates?

A

9

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19
Q

What are the 3 enzymes that play a central role in the synthesis of the non-essential amino acids that come from amphibolic intermediates?

A

[1] Transaminases*
[2] Glutamine Synthetase
[3] Glutamate Dehydrogenase

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20
Q

What are amphibolic intermediates?

A

created from both anabolic and catabolic intermediates

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21
Q

What is phenylketonuria? It is a deficiency of what enzyme?

A

Phenylketonuria - accumulation of phenylketones in the urine

  • deficiency in the phenylalanine hydroxylase enzyme converting phenylalanine to tyrosine
22
Q

Why is the synthesis of Glutamine (Gln) important?

A

in addition to creating glutamine, the reaction is useful for STORING FREE AMMONIA in a NON-TOXIC FORM
(NH3 becomes NH2 in the glutamine structure)

23
Q

What are characteristics of the Transaminase (Aminotransferase) reaction?

A
  • between amino acid and alpha-keto acid
24
Q

What is the most abundant enzyme in human tissues?

A

transaminases

25
Q

What is the most abundant enzyme in human tissues? What are they used for?

A

transaminases

- used in amino acid synthesis + degradation

26
Q

In transaminases, which pair is the most widely used “donor-acceptor couple”?

A

Glutamate-alpha-Ketoglutarate

27
Q

What are the 2 most important transaminase enzymes + reactions?

A

[1] Alanine Aminotransferase

[2] Aspartate Aminotransferase

28
Q

How are amino acids degraded in transamination reactions?

A

through the removal of amino group:

- elimination from body in the form of urea (transamination + urea cycle)

29
Q

When the amino groups are removed from amino acids, what must be done/taken into consideration?

A

NH3 is highly toxic

- the amino groups must be collected in a non-toxic form (primarily GLUTAMATE)

30
Q

What reaction is glutamate dehydrogenase involved in?

A
  • involved in reversible reaction

- involved in conversion between glutamate and alpha-ketoglutarate

31
Q

What is oxidative deamination?

A
  • part of the glutamate dehydrogenase reaction
  • converting glutamate to alpha-ketoglutarate
  • removal of NH3 in the process
32
Q

What is reductive amination?

A
  • part of the glutamate dehydrogenase reaction
  • converting alpha-ketoglutarate to glutamate
  • addition of NH3
33
Q

What does the enzyme glutaminase do? Why is this reaction important?

A
  • converts glutamine to glutamate

- important because NH3 is released from the glutamine

34
Q

What happens to the carbon skeletons of most amino acids (other than some exceptions)?

A
  • they become intermediates of the TCA cycle

- used in gluconeogenesis

35
Q

What happens to the carbon skeletons of most amino acids (other than some exceptions)?

A
  • they are degraded to intermediates of the TCA cycle

- used in gluconeogenesis

36
Q 
What happens to the carbon skeletons of these amino acids:
tryptophan
phenylalanine
tyrosine
isoleuce
A
  • both glucogenic and ketogenic

- they can undergo gluconeogenesis or ketogenesis

37
Q

What happens to the carbon skeletons of these amino acids:
leucine
lysine

A
  • they are ketogenic
  • they are degraded to Acetyl-CoA
  • used in ketogenesis
38
Q

Where does the urea cycle take place in?

A

in the liver

39
Q

For the urea cycle to take place in the liver, what must be transported there first?

A

amino nitrogen

40
Q

What are the 2 ways that amino nitrogen is transported in the circulation?

A

[1] Alanine
- made from the pyruvate in the muscle

[2] Glutamine
- made from glutamate in most tissues

41
Q

Free ammonia (NH3) is produced in the body in small quantities by…[4]

A

[1] breakdown of purines + pyrimidines
[2] breakdown of amines (dietary) + monoamines
[3] breakdown of glutamine
[4] bacterial digestion of urea in the gut

42
Q

Where is urea synthesized? How is urea excreted from the body?

A
  • urea is synthesized in the liver
  • it is water soluble and can be transported in the vascular circulation
  • it is excreted by the kidneys
43
Q

In renal failure, what may appear in regards to urea?

A
  • blood urea increases [uraemia]

- the kidneys do not excrete as much urea as they are supposed to

44
Q

What is the chemical structure of urea? In the biosynthesis of urea, what is required?

A

it contains 2 amino groups

  • one comes from carbamoyl phosphate (synthesized from glutamate)
  • the other comes from the amino acid Aspartate
  • 3 ATP molecules are required per urea molecule produced!
45
Q

Normally, what are the levels of free ammonia in the blood serum?

A

5 to 50 umol/L

46
Q

What happens if ammonia levels exceed the capacity of the urea cycle?

A

levels can rise over 1000 umol/L

- leads to hyperammonaemia

47
Q

What are the 2 causes of hyperammonaemia?

A

[1] Acquired Hyperammonaemia
- e.g. liver disease/failure

[2] Hereditary Hyperammonaemia
- e.g. an inherited urea cycle defect

48
Q

What are the effects of Hyperammonaemia?

A
  • CNS toxicity
  • tremors, slurred speech, vomiting, cerebral oedema
  • coma, death…
49
Q

What is the most common urea cycle disorder in humans?

A
  • OTC deficiency
  • Ornithine Transcarbamylase is the final enzyme in the proximal portion of the urea cycle - responsible for converting carbamoyl phosphate and ornithine into citrulline
  • X linked disorder (approx. 20% of female carriers of the OTC gene are symptomatic)
  • lack of the OTC enzyme results in excessive accumulation of nitrogen, in the form of ammonia (hyperammonemia) in the blood
  • excess ammonia, which is a neurotoxin, travels to the CNS through the blood (resulting in symtpoms and physical findings associated with OTC deficiency)
  • symptoms include: vomiting, refusal to eat, progressive lethargy, coma
  • severity and age of onset of OTC deficiency vary
50
Q

What are the symptoms of OTC Deficiency?

A
  • vomiting
  • refusal to eat
  • progressive lethargy
  • coma

Gastrointestinal Biology (15 decks)

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