Protein And AA Metabolism

Question 1

What are major nitrogen containing compounds

Answer 1

Major Nitrogen containing compounds:
• Amino acids
• Proteins
• Purines + Pyrimidines (DNA / RNA)
Smaller amounts of others – e.g.
• Porphyrins (haem) 
• Creatine phosphate 
• Neurotransmitters (e.g. dopamine) 
• Some hormones (e.g. adrenaline)

Question 2

What is creatinine?

Answer 2

• Breakdown product of creatine & creatine phosphate in muscle
• Usually produced at constant rate depending on muscle mass (unless muscle is wasting)
• Filtered via kidneys into urine
• Creatinine urine excretion over 24h proportional to muscle mass
• Provides estimate of muscle mass
• Also commonly used as indicator of renal function (raised level on damage to nephrons)

Question 3

What happens when N intake = output

Answer 3

No change in total body protein. Normal state in adult.

Question 4

What happens when N intake > output

Answer 4

Increase in total body protein. Normal state in growth & pregnancy or in adult recovering from malnutrition.
`

Question 5

What happens when N intake < output

Answer 5

Net loss of body protein. Never normal. Causes include trauma, infection, or malnutrition.

Question 6

Describe protein turnover

Answer 6

See slide

Question 7

Give an example of a glucogenic, ketogeinc and both AA

Answer 7

Gluco: ala, gly, cys, ser, asp, asg, arg, pro, his, glu
Keto: lys, leu
Both: thr, try, tyr, phe,
One of each

Question 8

When does mobilisation of protein reserves occur?

Answer 8

• Occurs during extreme stress (starvation)

* Under hormonal control

Question 9

What effect do insulin/growth hormones and glucocorticoids have on protein synthesis and degradaion?

Answer 9

Insulin and growth hormone: increase protein synthesis, decrease protein degradation

Glucocorticoids: decease protein synthesis, increase protein degradation

Question 10

What is Cushing syndrome

Answer 10

Excessive breakdown of protein can occur in Cushing’s syndrome (excess cortisol). Weakens skin structure leading to striae formation.

Question 11

What are the 9 essential amino acids

Answer 11

Isoleucine, lysis, threonine, histidine, leucine, methionine, phenylalanine, tryptophan, valine

If learned this huge list may prove truly valuable

Cannot be made by the body so must be obtained form food

Question 12

What are the conditionally esential amino acids

Answer 12

• Certain amino acids are Conditionally essential
• Children & Pregnant women = high rate of protein synthesis. Also require some arginine, tyrosine & cysteine in diet

Question 13

What is the difference between plant and animal protein

Answer 13

• Protein of animal origin considered “High quality” (Contain all essential amino acids)
• Proteins of plant origin generally considered “lower quality” since most are deficient in one or more essential amino acids.
• Therefore essential that vegetarian diet obtains protein from a wide variety of plant sources

Question 14

Where do non essential amino acids come from?

Answer 14

• In addition to dietary intake, body can synthesise
some amino acids it requires (the non-essential amino acids)
• Carbon atoms for non-essential amino acid synthesis come from:
• Intermediates of glycolysis (C3)
• Pentose phosphate pathway (C4 & C5)
• Krebs cycle (C4 & C5)

• Amino group provided by other amino acids by the process of transamination or from ammonia.

Question 15

What are amino acids required to synthesise?

Answer 15

In addition to protein synthesis (requires all 20 amino acids) amino acids also required for synthesis of other important compounds (requires specific amino acids).

Tyrosine
• Catecholamines
• Melanin
• Thyroid hormones

Histidine
• Histamine

Arginine
• Nitric oxide

Cysteine
• Hydrogen sulphide (signalling molecule)
• Glutathione

Glutamate
• GABA

Serine
• Sphingosine

Tryptophan
• Nicotinamide
• Serotonin (5HT)
• Melatonin

Glycine
• Purines 
• Glutathione 
• Haem 
• Creatine

Question 16

Describe removal of nitrogen from amino acids

Answer 16

• Removal of amino group is essential to allow carbon
skeleton of amino acids to be utilised in oxidative
metabolism
• Once removed nitrogen can be incorporated into other compounds or excreted from body as urea
• Two main pathways facilitate removal of nitrogen from amino acids

Transamination and deamination

Question 17

What are the 2 main aminotransferase enzymes?

Answer 17

Alanine aminotransferase ( A LT ) Converts alanine to glutamate
Aspartate aminotransferase (AST) Converts glutamate to aspartate

Plasma ALT and AST levels measured routinely as part of liver function test Levels particularly high in conditions that cause extensive cellular necrosis such as:
• Viral hepatitis
• Autoimmune Liver Diseases
• Toxic injury

Death cap mushrooms cause acute liver failure if ingested as plasma ATl levels up top 20x normal

Question 18

What is deamination?

Answer 18

Liberates amino group as free ammonia
• Mainly occurs in liver & kidney
• Keto acids can be utilised for energy
• Also important in deamination of dietary D-amino acids (found in plants and microorganisms) (need it in the L form as enzymes cant deal with D form)
• Ammonia (and ammonium ions) very toxic and must be removed. Ultimately converted to urea or excreted directly in urine

At physiological pH, ammonia (NH3) is rapidly converted to ammonium ion (NH4+)

Question 19

Which enzymes can deamination amino acids `

Answer 19

AA oxidases
Glutaminase
Glutamate dehydrogenase

Question 20

What is urea

Answer 20

NH2CONH2

• High nitrogen content
• Non-toxic
• Extremely water soluble
• Chemically inert in humans (bacteria can break it down to release NH
• Most urea is excreted in urine via the kidneys
• Also performs useful osmotic role in kidney tubules

Question 21

What is the urea cycle?

Answer 21

Ornithine cycle
• Occurs in liver and involves 5 enzymes
• Amount of urea cycle enzymes normally related to need to dispose of ammonia
• High protein diet induces enzyme levels
• Low protein diet or starvation represses levels
• Cycle is inducible but not regulated

Question 22

What is refeeding syndrome

Answer 22

• Can occur when nutritional support given to severely malnourished patients
• Ammonia toxicity significant factor (urea cycle down regulated)
• Re-feed @ 5 to 10 kcal/kg/day. Raise gradually to full needs within a week

Enzymes in urea cycle not sufficiency to deal with amie groups in big meal - leads to ammonia toxicity

Question 23

Describe defects in the urea cycle

Answer 23

• Autosomal recessive genetic disorders caused by deficiency of one of enzymes in the urea cycle
• Occur ~1 in 30,000 live births
• Mutations cause a partial loss of enzyme function
• Leads to:
- hyperammonaemia
- accumulation/excretion of urea cycle intermediates

Clinical picture (NH3 toxicity)
• Severity depends on:
- nature of defect
- amount of protein eaten
• Severe urea cycle disorders show symptoms within 1 day after birth. If untreated, child will die.
• Mild urea cycle enzyme deficiencies may not show symptoms until early childhood

Symptoms
• Vomiting 
• Lethargy 
• Irritability 
• Mental retardation 
• Seizures 
• Coma

Management:
• Low protein diet
• Replace amino acids in diet with keto acids

Question 24

Why does ammonia concentration in the blood need to be kept low?

Answer 24

• Readily diffusible and extremely toxic to brain
• Blood level needs to be kept low (25-40 µmol/L)
• Several potential toxic effects:
• Interference with amino acid transport and protein synthesis
• Disruption of cerebral blood flow
• pH effects (alkaline)
• Interference with metabolism of excitatory amino acid neurotransmitters (e.g. glutamate and aspartate)
• Alteration of the blood–brain barrier
• Interference with TCA cycle (reacts with α-ketoglutarate to form glutamate)

Question 25

What are the 2 mechanisms of safe transport of amino acid nitrogen from tissues to the liver for disposal?

Answer 25

Glutamine • Ammonia combined with glutamate to form glutamine • Glutamine transported in blood to liver or kidneys where it is cleaved by glutaminase to reform glutamate and ammonia. • In liver ammonia fed into urea cycle. In kidney excreted directly in urine Alanine • Amine groups transferred to glutamate by transamination • Pyruvate then transaminated by glutamate to form alanine • Alanine transported in blood to liver where it is converted back to pyruvate by transamination. • Amino group fed via glutamate into urea cycle for disposal as urea whereas pyruvate is used to synthesise glucose which can be fed back to tissues

Question 26

What are the clinical problems of amino acid metabolism?

Answer 26

* Over 50 inherited diseases involving defects in amino acid metabolism * Either total, or more commonly partial loss of enzyme activity * Rare (many diseases <1:250,000) although collectively as a group constitute a significant portion of paediatric genetic disease * If untreated frequently lead to intellectual impairment * Treatment involves restricting specific amino acids in diet

Question 27

What does the heel prick test test for?

Answer 27

Heel prick test • Sickle cell disease • Cystic fibrosis • Congenital hypothyroidism ``` Inborn errors of metabolism • Phenylketonuria (PKU) • Maple syrup urine disease • Isovaleric acidaemia (IVA) • Glutaric aciduria • Homocystinuria ```

Question 28

What is phenylketonuria?

Answer 28

* Most common inborn error of amino acid metabolism (~1 in 15,000 births) * Deficiency in phenylalanine hydroxylase so cant convert phenylalanine to tyrosine * Autosomal recessive. Affected gene is on chromosome 12 * Accumulation of phenylalanine in tissue, plasma & urine - it’s hers transaminated to phenylpyruvte and then converted to phenylketones * Phenylketones in urine * Musty smell

Question 29

What is the treatment of PKU?

Answer 29

Strictly controlled low phenylalanine diet enriched with tyrosine Avoid artificial sweeteners as they contain this Avoid high protein foods

Question 30

What are the pathways affected by PKU?

Answer 30

* Noradrenaline * Adrenaline * Dopamine * Melanin * Thyroid hormone * Protein synthesis

Question 31

What are the symptoms o PKU?

Answer 31

* Severe intellectual disability * Developmental delay * Microcephaly (small head) * Seizures * Hypopigmentation

Question 32

What is homocystineuria

Answer 32

* Problem breaking down methionine * Excess homocystine (oxidised form of homocysteine) excreted in urine * Autosomal recessive disorders. * Incidence ~1 in 344,000 * Defect in cystathionine β -synthase most common 9requires vita as a co factor) * cant convert homocysteine to cystathione - build up of homocysteine (this can be converted back to methionine) and build up of these 2 causes disease symptoms * Affects connective tissue, muscles, CNS, CVS * Elevated homocysteine associated with cardiovascular disease

Question 33

What is the treatment for homocystinuria?

Answer 33

Low methionine diet Avoid meat, fish, milk, cheese, eggs, nuts and peanut butter Take cysteine, vit b6, betaken, b12 and folate supplement