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Metabolism > Amino Acid Metabolism > Flashcards

Amino Acid Metabolism Flashcards

1
Q

Describe proteins

A
  • Raw material for amino acids
  • Carbon, Oxygen and Nitrogen (16% by weight)
  • Can be used to form ATP
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2
Q

What can proteins be broken down and reassembled to form?

A
  • Muscle protein
  • enzymes,
  • Structural proteins
  • Membrane channels and pump
  • Immunity
  • Hormones
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3
Q

Describe amino acids in the fed state (liver)

A
  • Liver to synthesise hepatic and serum proteins, biosynthesis of N-containing proteins like haem, hormones, purine/ pyramiding bases
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4
Q

What is excess amino acid converted to?

A
  • Glycogen

- Or triacylglycerol

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5
Q

What is the function of broken down amino acids in the peripheral circulation?

A
  • Periphery for protein synthesis, biosynthetic pathways and oxidation
  • Low carb and fat diet–> AA used for ATP source
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6
Q

What happens to amino acids in the basal state?

A
  • Recycling of AA not needed
  • Glucose/ ketone body synthesis
  • Urea synthesis needed for AA breakdown- allows safe removal of N
  • Amino acids used as C source for gluconeogenesis
  • Muscle protein preferentially degraded
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7
Q

What is urea?

A
  • Very soluble, non-toxic nitrogen containing substance that is readily excreted by the kidneys
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8
Q

What happens to amino acids in the prolonged fasting state?

A
  • Similar to basal
  • Spare use of muscle protein to maintain health
  • Switch to ketone bodies, less reliance on glucose to spare AA
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9
Q

What is transamination?

A
  • Funnelling of nitrogen-containing amino group from an amino acid to an acceptor
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10
Q

What is transamination essential for?

A
  • Degradation of most amino acids
  • Synthesis of non-essential amino acids
  • Exchange amino groups between amino acids
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11
Q

Describe what is happening in a transamination reaction

A
  • Swaps amino group to a different keto acid
  • Generates a new pair of AA and keto acid
  • Readily reversible
  • AA usually glutamate and corresponding keto acid is α-glutamate
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12
Q

What enzyme facilitates transamination?

A
  • Aminotransferase (or transaminase enzymes)
  • Different for different amino acid substrates
  • (E.g. alanine aminotransferase)
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13
Q

Give two examples of transamination reactions

A

1) Alanine –> Pyruvate
- α-ketoglutarate–> glutamate
- Alanine aminotransferase
2) Oxaloacetate –> Aspartate
- Glutamate–> α-ketoglutarate
- Aspartate aminotransferase
- -> Both reactions are reversible

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14
Q

How does glutamate incorporate into the urea cycle?

A
  • Glutamate or aspartate
  • N can only be incorporated into urea cycle via those two
  • Glutamate releases ammonia via carbamoyl phosphate (enters cycle)- oxidative deamination
  • Regenerates α-ketoglutarate
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15
Q

How does aspartate incorporate into the urea cycle?

A
  • Combines with citrulline
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16
Q

Where does the urea cycle occur and what does it do?

A
  • Liver

- Converts nitrogen into urea for excretion by the kidneys

17
Q

How do other amino acids incorporate themselves in the urea cycle?

A
  • Transamination passing amino groups to α-ketoglutarate or oxoaloacetate to produce glutamate and aspartate
  • They become keto acids that can be used in gluconeogenesis
18
Q

What happens in oxidative deamination?

A
  • NAD–> NADH
  • Glutamate –> amino + α- ketoglutarate
  • Ammonia becomes carbamoyl phosphate
  • Enzyme condenses ornithine and carbamoyl phosphate producing citrulline
  • Citrulline condenses with aspartate –> arginosuccinate–> Arginine + fumarate
  • Arginine–> ornithine + urea
  • Fumarate can be converted to oxaloacetate to join TCA cycle
19
Q

What enzyme condenses carbonyl phosphate and ornithine in the urea cycle?

A

ornithine transcarbomaoylase (OTC)

20
Q

What enzyme converts glutamate into ammonia?

A

glutamate dehydrogenase

21
Q

What is meant by nitrogen balance?

A

– Any normal healthy adult will ingest and secrete same amount of nitrogen, maintain nitrogen balance

22
Q

What is positive nitrogen balance?

A
  • Taking in more nitrogen
  • Need to increase protein synthesis (after accident, pregnancy, rapidly growing baby)
  • Not excreting same amount nitrogen as taken in
23
Q

What is negative nitrogen balance?

A
  • Muscle protein/ haemoglobin degraded to provide N for synthesis of other proteins
  • Needing to use up excess N
  • Amount of N excreted more than ingested, no amino acids ingested
  • Urea created when amino acids metabolised
24
Q

What is hyperammonaemia?

A
  • Accumulated toxic NH₄⁺ travels- brain (coma, damage and death)
  • Causes: hepatocyte loss (e.g. liver cirrhosis) or urea cycle enzyme deficiency
25
Q

What enzyme in the urea cycle is more likely to be deficient leading to hyperammonaemia?

A
  • Orthenine transcarbamoylase
  • Mutant enzyme protein impairs reaction and condenses carbonyl phosphate + ornithine to form citrulline
  • X-linked gene, high male mortality
  • Treatment is low protein diet or liver transplant
26
Q

Describe Alanine in inter-organ C and N shuttle

A
  • Simplest AA- muscle protein
  • Simplest transport method
  • Amino groups from broken down AA passed onto pyruvate to produce alanine
  • C shuffles in blood as alanine- converted back to pyruvate in liver
  • Glutamate transfers N into urea cycle (oxidative de-am.)
  • Meanwhile, pyruvate enters gluconeogenesis
27
Q

What enzyme is used to convert alanine to pyruvate?

A
  • Alanine transaminase (ALT)

- This enzyme is also elevated (released) during liver damage

28
Q

What is meant by the term carbon skeletons?

A
  • AA broken down, amino group shuffled by either aspartate or glutamate into urea cycle- remains= carbon skeleton
  • Can be integrated into TCA cycle
29
Q

What are glucogenic amino acids?

A
  • Carbon skeletons of AA used to produce glucose through gluconeogenesis
  • Mostly resemble different things in TCA cycle (e.g. alanine, cysteine etc similar to pyruvate)
30
Q

What are ketogenic amino acids?

A
  • Carbon skeletons degraded to form acetyl CoA- metabolised immediate precursors to lipid or ketone bodies
  • Mostly resemble acetyl CoA
31
Q

What are the possible fates of carbon skeleton?

A
  • Depends on physiological state and tissue
  • Fasting- converted to glucose or ketone bodies released in blood as fuel
  • Fed- converted to fuel such as glycogen/ triacylglycerols (only in liver)
  • If tissue needs energy- oxidised to produce ATP via TCA cycle
  • Precursors to synthesise other proteins needed by cells
32
Q

What are the metabolic relationships among amino acids?

A
  • Make body proteins, which can also break down–> AA
  • Ingested broken down to get constituent AA
  • Used as fuel source:
    • Urea cycle
    • Acetyl CoA make ketone bodies/ other fats
    • Glucose- glycolysis
    • Ketone bodies- β- oxidation
33
Q

What other things can amino acids be converted to?

A
  • Purines and pyrimidines
  • Porphyrins
  • Thyroxine
  • Neurotransmitters
  • Phospholipids
  • Coenzymes
34
Q

What clinical consequences of errors in amino acid metabolism are there?

A
  • Phenylketonuria
  • Albinism
  • Alkaptonuria
  • Maple Syrup Urine Disease (MSUD)
35
Q

Describe what happens in phenylketonuria (PKU)

A
  • phenylalanine–> tyrosine–> melanin (normal)
  • phenylalanine hydroxylase not functional
  • Phenylalanine degrades
  • Detected by increased serum phenylalanine and excess urinary excretion of phenylpyruvate & phenylactate
  • Early detection & treatment- protein restricted diet and tyrosine supplement avoid severe mental retardation
  • Autosomal recessive
36
Q

Describe what happens in albinism

A
  • Autosomal recessive
  • Tyrosine–> melanin (normal)
  • Tyrosinase mutation- lack melanin pigment
  • Skin, hair and eyes hypo pigmented
  • UV vulnerability, increased cancer risk
  • Visual problems
  • No cure, complications managed by dermatology/ ophthalmology
37
Q

Describe what happens in Maple Syrup Urine Disease (MSUD)

A
  • Absent α-ketoacid dehydrogenase- build up of α-ketoacids
  • Specific to valine, leucine and isoleucine metabolism
  • May develop 4-7 days after birth- lethargy, weight loss, metabolic derangement, progressive neurologic signs of altering hypo/hypertonia- severe encephalopathy
  • Seizures and comas possible
  • Death possible
  • Autosomal recessive

Metabolism (9 decks)

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