Lecture 5.1: Amino Acid/ Protein Metabolism

Question 1

The 9 Essential Amino Acids

Answer 1

If : Isoleucine
Learned: Lysine
This: Threonine
Huge: Histidine
List: Leucine
May: Methionine
Prove: Phenylalanine
Truly: Tryptophan
Valuable: Valine

Question 2

Where do amino acids come from?

Answer 2

Essential AAs come from diet
In addition to this, non-essential AAs can be synthesised by the body

Question 3

Where do carbon atoms for amino acid synthesis come from? (3)

Answer 3

• Intermediates of glycolysis (C3)
• Pentose phosphate pathway (C4 and C5)
• Krebs cycle (C4 and C5)

Question 4

What is transamination?

Answer 4

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

Question 5

What compounds require Tyrosine for synthesis?

Answer 5

• Catecholamines
• Melanin
• Thyroid hormones

Question 6

What compounds require Cysteine for synthesis?

Answer 6

• Hydrogen sulphide (signalling molecule)
• Glutathione

Question 7

What compounds require Tryptophan for synthesis?

Answer 7

• Nicotinamide
• Serotonin (5HT)
• Melatonin

Question 8

What compounds require Histidine for synthesis?

Answer 8

• Histamine

Question 9

What compounds require Glutamate for synthesis?

Answer 9

• GABA

Question 10

What compounds require Glycine for synthesis?

Answer 10

• Purines
• Glutathione
• Haem
• Creatine

Question 11

What compounds require Arginine for synthesis?

Answer 11

• Nitric oxide

Question 12

What compounds require Serine for synthesis?

Answer 12

• Sphingosine

Question 13

Nitrogen Containing Compounds: Major

Answer 13

• Proteins
• Amino acids
• Purines + Pyrimidines (DNA /RNA)

Question 14

Nitrogen Containing Compounds: Minor

Answer 14

• Porphyrins (haem)
• Creatine
• Neurotransmitters (e.g. dopamine)
• Some hormones (e.g. adrenaline)

Question 15

Nitrogen Balance: Zero N balance

Answer 15

(N equilibrium)
Intake = output
No change in total body protein
Normal state in adult

Question 16

Nitrogen Balance: Positive N balance

Answer 16

Intake > output
Increase in total body protein
Normal state during growth and pregnancy
Or in adult recovering from malnutrition.

Question 17

Nitrogen Balance: Negative N balance

Answer 17

Intake < output
Net loss of body protein
Never normal
Causes include trauma, illness, burns or malnutrition

Question 18

Creatinine

Answer 18

• A useful clinical marker
• Breakdown product of creatine and creatine phosphate in muscle
• Produced at constant rate depending on muscle mass
• Creatinine urine excretion over 24h proportional to muscle mass
• Also commonly used as indicator of renal function (↑ in blood → damage to nephrons)

Question 19

Creatinine: Excreted in urine per day (M/F)

Answer 19

• Men 14-26 mg/kg
• Women 11-20 mg/kg

Question 20

When does mobilisation of protein reserves occur?

Answer 20

Occurs during extreme stress (starvation)
Under hormonal control

Question 21

Mobilisation of Protein Reserves: Insulin and Growth Hormone

Answer 21

Effect on protein synthesis: Increases
Effect on protein degradation: Decreases

Question 22

Mobilisation of Protein Reserves: Glucocorticoids (e.g. cortisol)

Answer 22

Effect on protein synthesis: Decreases
Effect on protein degradation: Increases

Question 23

Cushing’s Syndrome

Answer 23

Excessive breakdown of protein
Excess cortisol
Weakens skin structure leading to striae formation

Question 24

Why is removal of Nitrogen from Amino Acids important?

Answer 24

Removal of NH2 is essential →allows carbon skeleton of amino acids to be used in oxidative metabolism

Removed N can be incorporated into other compounds or excreted from body as urea

Question 25

Pathways of Removal of Nitrogen from Amino Acids (2)

Answer 25

Two main pathways facilitate removal of N from amino acids:

• Transamination: switches NH2 from an amino acid to a ketoacid creating a
new amino acid
• Deamination: removes ammonia (NH3)

Question 26

Transamination (4)

Answer 26

The NH2 group from one amino acid is transferred to α-ketoglutarate, which becomes glutamate

Most aminotransferases use α-ketoglutarate to funnel the amino group to glutamate

Keto acid is produced from the initial amino acid

Aminotransferases require the coenzyme pyridoxal phosphate, a derivative of vitamin B6

Question 27

Aminotransferases

Answer 27

They are Important Diagnostic Markers

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 LiverDiseases
• Toxic injury

Question 28

Aminotransferases: Alanine aminotransferase (ALT)

Answer 28

Converts alanine to glutamate

Question 29

Aminotransferases: Aspartate aminotransferase (AST)

Answer 29

Converts aspartate to glutamate

Question 30

Deamination

Answer 30

Releases NH2 as free ammonia
Also important in deamination of dietary D-amino acids
Converted to urea or excreted directly in urine
Keto acids can be utilised for energy
At physiological pH, ammonia (NH3) is rapidly converted to ammonium 4 ion (NH +)

Question 31

Which enzymes can deaminate amino acids?

Answer 31

• Amino acid oxidases
• Glutaminase
• Glutamate dehydrogenase

Question 32

Where does Deamination occur? Why is it important?

Answer 32

Mainly occurs in liver and kidney
Ammonia (and ammonium ions*) is very toxic and must be removed

Question 33

Biochemical Basis of Ammonia Toxicity

Answer 33

Readily diffusible and extremely toxic to brain
Blood level needs to kept low (25 – 40 μmol/l)
Several potential toxic effects

Question 34

Several potential toxic effects of Ammonia Toxicity

Answer 34

• Interference with amino acid transport and proteinsynthesis
• pH effects(alkaline)
• Interference with TCA cycle (reacts with α-ketoglutarate toform glutamate)
• Alteration of the blood–brainbarrier
• Disruption of cerebral blood flow
• Interference with metabolism of excitatory amino acid neurotransmitters (e.g.
glutamate andaspartate)

Question 35

Urea

Answer 35

• High N content
• Non-toxic
• Extremely water soluble
• Chemically inert in humans (bacteria can break it down to release NH3)

Question 36

Urea: where made? where removed? function?

Answer 36

• Synthesised in liver by the urea cycle
• Most urea is excreted in urine via the kidneys
• Also performs useful osmotic role in kidney tubules

Question 37

Urea Cycle: where + overview

Answer 37

Occurs in liver and involves 5 enzymes
Amount of urea cycle enzymes normally related to need to dispose of ammonia as urea

Question 38

Urea Cycle: Regulation

Answer 38

Cycle is inducible but not regulated:
• High protein diet induces enzymes
• Low protein diet or starvation represses enzymes

Question 39

Re-feeding Syndrome

Answer 39

Can occur when nutritional support given to severely malnourished patients

Ammonia toxicity significant factor (urea cycle down regulated)

Re-feed at 5 to 10 kcal/kg/day

Raise gradually to full needs within a week

Question 40

Defects in the Urea Cycle

Answer 40

Autosomal recessive disorders caused by deficiency of one of enzymes in the urea cycle

Mutations cause a partial loss of enzyme function

Question 41

Defects in the Urea Cycle: Effects

Answer 41

Leads to:
• hyperammonaemia
• accumulation/excretion of urea cycle intermediates

Question 42

Ornithine Transcarbamylase (OCT) Deficiency

Answer 42

Severity depends on:
• nature of defect
• amount of protein eaten

Severe urea cycle disorders symptoms within 1 day of birth, if untreated child will die

Mild urea cycle enzyme deficiencies may not show symptoms until early childhood

Question 43

Ornithine Transcarbamylase (OCT) Deficiency: Symptoms

Answer 43

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

Question 44

Ornithine Transcarbamylase (OCT) Deficiency: Management

Answer 44

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

Question 45

Amino Acid Metabolism Disorders

Answer 45

• Either total or more commonly partial loss of enzyme activity
• If untreated, frequently lead to intellectual impairment
• Treatment involves restricting the amount of specific amino acids in diet

Question 46

Phenylketonuria (PKU): What is it?

Answer 46

• Most common inborn error of amino acid metabolism
• Deficiency in phenylalanine hydroxylase (PAH)
• Autosomal recessive (chromosome 12)
• Accumulation of phenylalanine in tissue, plasma and urine
• Phenylketones in urine
• Phenylketones oxidised to phenylacetate give musty smell

Question 47

Phenylketonuria (PKU): Treatments

Answer 47

• Strictly controlled low phenylalanine diet plus supplementation with tyrosine
• Avoid artificial sweeteners (contain phenylalanine)
• Avoid high protein foods such as meat, milk, and eggs

Question 48

Phenylketonuria (PKU): Symptoms

Answer 48

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

Question 49

Homocystinurias: What is it?

Answer 49

• Defect in cystathionine β-synthase (CBS) is the most common (defect in methionine synthase also possible)
• Autosomal recessive disorders
• Excess homocystine (oxidised form of homocysteine) excreted in urine
• Accumulation of homocysteine and methionine causes disease symptoms

Question 50

Homocystinurias: Treatments

Answer 50

• Low-methionine diet
• Avoid milk, meat, fish, cheese, eggs
• Nuts, and peanut butter also contain methionine
• Cysteine, vit B6, Betaine, B12 and folate supplements

Question 51

Homocystinurias: Symptoms

Answer 51

• Disclocation of the lens
• Long limbs and fingers
• Intellectual disability