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Metabolism, Endocrinology and Haematology > Protein and Amino Acid Metabolism > Flashcards

Protein and Amino Acid Metabolism Flashcards

1
Q

Where does stage 1 of catabolism of proteins occur and what enzymes are involved

A

Occurs in the GI tract

Proteases and peptidases break protein down into amino acids which are absorbed

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2
Q

What molecules stimulate uptake of protein into cells and what molecule stimulates breakdown of proteins

A

Uptake - insulin and growth hormone

Breakdown - cortisol

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3
Q

What are the types of amino acids

A

Ketogenic

Glucogenic

Ketogenic and Glucogenic

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4
Q

How are amino acids catabolised

A

Amino group removed and converted to urea

Remaining C-skeleton converted to one or more of: pyruvate, oxaloacetate, fumarate, alpha-ketoglutarate, succinate CoA, acetyl CoA, acetoacetyl CoA

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5
Q

Why are amino acids described as Ketogenic and what can they be used to synthesise

A

They produce acetyl CoA when catabolised (or acetoacetyl CoA)

Acetyl CoA then used to make ketone bodies or fatty acids

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6
Q

Why are amino acids described as Glucogenic and what can they be used to synthesise

A

They produce other products (not acetyl CoA) when catabolised

Synthesise glucose or glycogen

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7
Q

What are the terms used to describe the intake of nitrogen against loss of nitrogen

A

N-balance - where intake equals loss

Positive N-balance - intake is greater than loss

Negative N-balance - intake less than loss

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8
Q

When would a person have positive N-balance and when would they have negative N-balance

A

Postive N-balance - active growth, pregnancy, tissue repair, convalescence

Negative N-balance - starvation, malnutrition, trauma

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9
Q

What are the essential amino acids

A

Isoleucine

Lysine

Threonine

Histidine

Leucine

Methionine

Phenylalanine

Tryptophan

Valine

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10
Q

Which amino acids are conditionally essential and under what circumstances are they essential

A

Arginine - during periods of active growth (body can synthesise small quantity)

Tyrosine - if diet low in phenylalanine (synthesised from it)

Cysteine - if diet low in methionine (synthesised from it)

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11
Q

Why is a regular and adequate supply of protein needed in the diet

A

To replace the amino acids that are not reutilised after protein breakdown

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12
Q

What are the two signalling molecules synthesised from amino acids

A

Nitric oxide from L-arginine

Hydrogen sulphide from L-cysteine

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13
Q

What are the three features of amino acid breakdown

A

C-atoms converted to intermediates of carbohydrate and lipid metabolism

Start with removal of NH2 group by transamination of deamination)

N-atoms usually converted to urea

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14
Q

What are the two ketogenic only amino acids

A

Leucine

Lysine

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15
Q

Which amino acids are both glucogenic and ketogenic

A

Isoleucine

Tyrosine

Phenylalanine

Tryptophan

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16
Q

Which enzymes are involved in transamination and what stimulates their synthesis

A

Aminotransferases

Cortisol

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17
Q

Which molecules can be used in transamination as keto acid 2 and what are they converted to

A

Alpha-ketoglutarate - converted to glutamate

Oxaloacetate - converted to aspartate

18
Q

Which two tranaminases are clinically important and why

A

Alanine aminotransferase (ALT)

Aspartate aminotransferase (AST)

Important as used when assessing liver function - measured in serum

19
Q

What is deamination and which groups of enzyme are involved

A

Removal of the NH2-group as a free NH3

L and D-amino acid oxidases - convert amino acids to keto acids and NH3

20
Q

What are the importance of D-amino acid oxidases

A

Convert D-amino acids into keto acids as D-amino acids cannot be used for protein synthesis as they are the wrong optic isomer

21
Q

What does glutaminase do

A

Converts glutamine to glutamate and NH3

22
Q

Why is glutamate dehydrogenase important

A

Involved in disposal of amino acids and synthesis of non-essential amino acids

Produces alpha-ketoglutarate from glutamate

23
Q

When do disorders of amino acid metabolism usually have a clinical consequence

A

When affected enzyme is involved in amino acid breakdown

This is because it allows the amino acid to accumulate and the AA may be toxic itself and/or be metabolised to toxic products

Restrict amount of amino acid in diet

24
Q

What is phenylketonuria/PKU

A

Inherited metabolic disorder where urine contains large amounts of phenylketones produced from phenylalanine

Usually there is defect in phenylalanine hydroxylase and as a result phenylalanine accumulates in tissues and blood

Phenylalanine is metabolised by other pathways to produce other products, including phenylpyruvate

25
Q

What are the consequence if PKU is left untreated

A

Inhibition of brain development as phenylpyruvate inhibits pyruvate uptake into mitochondria, interfering with energy metabolism in the brain

Associated with seizures, microcephaly

26
Q

How do you test for PKU

A

Heel prick test shortly after birth

27
Q

What is homocystinuria

A

Inherited autosomal recessive defect in methionine metabolism, in which Type 1 is cauesd by deficiency in cystathionine beta-synthase (CBS) enzyme

CBS normally converts homocysteine to cystathionine which is then converted to cysteine

Homocysteine levels rise and some can be converted to methionine

28
Q

How is homocystinuria usually detected

A

Elevated levels of homocysteine and methionine in the plasma

Homocytine in the urine

29
Q

What can homocystinuria cause

A

Disorders of connective tissue, muscle, CNS, cardiovascular system

Due to chronic elevated plasma levels of homocysteine

30
Q

What is the mechanism behind ammonia’s toxicity

A

Reacts with alpha-ketoglutarate to form glutamate in mitochondria via glutamate dehydrogenase

This removes alpha-ketoglutarate from TCA cycle, slowing it

This disrupts energy supply to brain cells

May affect pH inside cells of CNS and may interfere with neurotransmitter synthesis and release

31
Q

What are the clinical effects of hyperammonaemia

A

Blurred vision

Tremors

Slurred speech

Coma

Death

Interference with amino acid transport and protein synthesis

Disruption of cerebral blood flow

Alkaline effects

Interference with metabolism of excitatory amino acid neurotransmitters

Alteration of blood brain barrier

Interference with TCA cycle

32
Q

How can ammonia be detoxified

A

Used in synthesis of N-compounds - glutamate

Conversion to urea and excreted

Ammonia can be excreted directly in urine

33
Q

Describe glutamine synthesis

A

Synthesised from ammonia and glutamate via glutamine synthetase

34
Q

How is glutamine synthesis used to combat ammonia toxicity

A

Glutamine is synthesised using ammonia in cell and is then released where it is transported to the liver and kidney

Glutamine is then hydrolysed by glutaminase releasing ammonia which can be excreted in urine or converted to urea

35
Q

How is alanine used in ammonia detoxification

A

Pyruvate transaminated in tissue by glutamate to produce alanine

Alanine travels in blood to liver

In liver amine group transferred to glutamate and pyruvate produced from reaction which can travel back to tissues to be used again

36
Q

How is urea synthesis regulated and what condition is associated with giving too much protein quickly to malnourished patients

A

Enzymes of the cycle are inducible with high protein diet inducing the enzyme and vice versa

Refeeding syndrome is caused by giving too much protein too soon to malnourished as the excess amino acids are degraded leading to hyperammonaemia

37
Q

Describe the inherited diseases of the urea cycle

A

A set of autosomal recessive diseases affecting one of the enzymes in the cycle

Can only be partial loss of the enzyme as full loss is fatal

38
Q

What are the symptoms of inherited diseases of the urea cycle

A

Vomiting

Lethargy

Irritability

Mental retardation

Seizures

Coma

39
Q

What is the treatment for diseases of the urea cycle

A

Low protein diets

Diets where keto acids of essential amino acids are used to replace the amino acids themselves

40
Q

When might hyperammonaemia develop as a secondary consequence

A

Due to liver disease like cirrhosis

Liver’s ability to remove NH3 from portal blood is impaired

41
Q

What happens to urea in the body

A

Most is filtered and excreted in the urine

A small amount crosses the intestinal wall, entering intesine where bacteria then break it down releasing ammonia that can be reabsorbed

Metabolism, Endocrinology and Haematology (27 decks)

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