GB 11. Post-Absorption Processing of Proteins Flashcards by Jennita Ariaratnam

What are amino acids the building blocks of?

protein

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What are amino acids the precursors of?

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

[1] the diet

[2] protein turnover

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What is the process of protein turnover?

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

[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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What are 2 factors that control protein degradation?

[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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Are amino acids stored?

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

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What are the 2 protein degradation systems?

[1] Ubiquitin - Proteasome System
- energy dependent

[2] Lysosomal Enzymes
- not energy dependent

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

[1] utilisation of amino acids by tissues

[2] release of amino acids following protein degradation

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

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

[2] Liver

gluconeogenesis
urea cycle (utilisation and excretion of NH3)

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

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

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

[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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What are glucogenic amino acids?

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

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What are Ketogenic amino acids?

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

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What are the 8 essential amino acids?

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

[1] Arginine (Arg)

[2] Histidine (His)

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

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

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

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

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

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What are amphibolic intermediates?

created from both anabolic and catabolic intermediates

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

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Phenylketonuria - accumulation of phenylketones in the urine

deficiency in the phenylalanine hydroxylase enzyme converting phenylalanine to tyrosine

Why is the synthesis of Glutamine (Gln) important?

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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)

What are characteristics of the Transaminase (Aminotransferase) reaction?

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between amino acid and alpha-keto acid

What is the most abundant enzyme in human tissues?

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transaminases

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

transaminases | - used in amino acid synthesis + degradation

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

Glutamate-alpha-Ketoglutarate

What are the 2 most important transaminase enzymes + reactions?

[1] Alanine Aminotransferase | [2] Aspartate Aminotransferase

How are amino acids degraded in transamination reactions?

through the removal of amino group: | - elimination from body in the form of urea (transamination + urea cycle)

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

NH3 is highly toxic | - the amino groups must be collected in a non-toxic form (primarily GLUTAMATE)

What reaction is glutamate dehydrogenase involved in?

- involved in reversible reaction | - involved in conversion between glutamate and alpha-ketoglutarate

What is oxidative deamination?

- part of the glutamate dehydrogenase reaction - converting glutamate to alpha-ketoglutarate - removal of NH3 in the process

What is reductive amination?

- part of the glutamate dehydrogenase reaction - converting alpha-ketoglutarate to glutamate - addition of NH3

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

- converts glutamine to glutamate | - important because NH3 is released from the glutamine

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

- they become intermediates of the TCA cycle | - used in gluconeogenesis

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

- they are degraded to intermediates of the TCA cycle | - used in gluconeogenesis

``` What happens to the carbon skeletons of these amino acids: tryptophan phenylalanine tyrosine isoleuce ```

- both glucogenic and ketogenic | - they can undergo gluconeogenesis or ketogenesis

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

- they are ketogenic - they are degraded to Acetyl-CoA - used in ketogenesis

Where does the urea cycle take place in?

in the liver

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

amino nitrogen

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

[1] Alanine - made from the pyruvate in the muscle [2] Glutamine - made from glutamate in most tissues

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

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

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

- urea is synthesized in the liver - it is water soluble and can be transported in the vascular circulation - it is excreted by the kidneys

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

- blood urea increases [uraemia] | - the kidneys do not excrete as much urea as they are supposed to

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

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!

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

5 to 50 umol/L

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

levels can rise over 1000 umol/L | - leads to hyperammonaemia

What are the 2 causes of hyperammonaemia?

[1] Acquired Hyperammonaemia - e.g. liver disease/failure [2] Hereditary Hyperammonaemia - e.g. an inherited urea cycle defect

What are the effects of Hyperammonaemia?

- CNS toxicity - tremors, slurred speech, vomiting, cerebral oedema - coma, death...

What is the most common urea cycle disorder in humans?

- 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

What are the symptoms of OTC Deficiency?

- vomiting - refusal to eat - progressive lethargy - coma

GB 11. Post-Absorption Processing of Proteins Flashcards

(50 cards)