Amino Acid Metabolism

Question 1

Amino acids

Answer 1

• Monomeric units for proteins; therefore important for growth, metabolism

Question 2

Amino acids are a precursor for what?

Answer 2

• biologically important nitrogen containing compounds: haem, creatine, purines and pyrimidines

Question 3

Dietary intake of amino acids

Answer 3

• Dietary protein intake is main source of nitrogen
• Excess dietary AA’s not required for protein synthesis can’t be stored (in mammals) & aren’t excreted
• Converted to energy metabolites

Question 4

Amino acids as source of energy

Answer 4

• Glucose and fatty acids are the major metabolic fuels
• Oxidation of aa’s play a role in energy requirements of organisms when there is excessive protein in the diet
  when the normal metabolic fuels are unavailable due to starvation or disease
• Most humans normally get 10-25% of their energy requirements from amino acids
• In carnivores it can be up to 50%

Question 5

Sources of amino acids

Answer 5

• dietary protein digestion (60-100gm/day)
• Synthesis of non-essential amino acids
• tissue protein synthesis/ catabolism (~400gm/day)

Question 6

Utilisation of amino acids

Answer 6

• amino acid catabolism: ammonia → urea, carbon skeleton: glucose/lipid synthesis
• Synthesis of nitrogen containing compounds: haem, neurotransmitters, creatine, purines, pyrimidines

Question 7

Essential and Non-essential Amino Acids

Answer 7

• Essential: cannot be synthesized by the body; obtained via diet
• Non-essential: can be synthesized by the body

Question 8

Glucogenic amino acids

Answer 8

• catabolism yields pyruvate or an intermediate of the citric acid cycle –> Substrates for gluconeogenesis
• carbon skeletons broken down to glucose precursors

Question 9

Ketogenic amino acids

Answer 9

• yields acetoacetate, acetyl CoA or acetoacetyl CoA –> not substrates for gluconeogenesis
• carbon skeletons broken down to ketone body or fatty acid precursors

Question 10

Major site of amino acid catabolism

Answer 10

• liver

Question 11

3 main stages of amino acid catabolism

Answer 11

• 1. Deamination/Transamination– removal and conversion of amino group to ammonia or amino group of aspartate
• 2. Incorporation of nitrogen from ammonia/aspartate to urea
• 3. Conversion of amino acid carbon skeletons (a-keto acids) to one of the 7 common metabolic intermediates

Question 12

Enzymes involved in deamination/transamination

Answer 12

• Aminotransferases – these are specific for each amino acid

- Enzyme requires pyridoxal phosphate (Vit B6) as coenzyme

Question 13

Pyridoxine in deamination/transamination

Answer 13

• in the form of pyridoxal-5’-phosphate (PLP) is required to help carry the amino group
• PLP is covalently attached to enzyme via Schiff base formation through condensation of the PLP’s aldehyde group with the amino group

Question 14

Other products of transamination

Answer 14

• remaining carbon skeleton of the amino acid exists as a keto acid - fate depends on the amino acid, but is either glucose or ketone/FA precursor
• Reactions also form glutamate (or aspartate) which feeds excess NH4+ to the urea cycle

Question 15

Deamination

Answer 15

• involves the release of free NH3 from glutamate by Glutamate dehydrogenase and Glutamine synthetase

Question 16

Additional minor mechanism involving L-AA’s and D-AA’s

Answer 16

• two isomeric forms L- or D-
• Animals only utilse L-AA’s
• D-AA’s are found in bacterial cell walls, but are present in the human body where they have passed through the bloodstream from bacteria in the colon
• D-AA’s can interfere with metabolism of L-aa’s
• Defence mechanism is therefore to remove them
  The enzyme D-amino acid oxidase (D-AAO) does this

Question 17

Products of glucogenic amino acid catabolism

Answer 17

• pyruvate
• oxaloacetate
• a-ketoglutarate
• fumerate
• or succinyl-CoA

Question 18

Alanine

Answer 18

• converted to pyruvate via transamination
• Enzyme is Alanine aminotransferase (ALT)
• Enzyme requires PLP (pyridoxal phosphate) (Vit B6)
• a-Ketoglutarate acts as an amino acceptor - Glutamate

Question 19

Aspartate

Answer 19

• converted to oxaloacetate via transamination – Aspartate aminotransferase (AST)
• Reaction similar to alanine i.e. formation of glutamate
• Enzyme also requires PLP (pyridoxal phosphate) (Vit B6)
• Oxaloacetate can be used in gluconeogenesis or TCA cycle
• Reaction is reversible – produce aspartate from oxaloacetate

Question 20

Asparagine

Answer 20

• firstly deaminated to aspartate by Asparaginase, produces ammonium
• Therefore ultimately asparagine is converted to oxaloacetate
• used for synthesis of Asparagine – Asparagine synthetase.
• Glutamine is the N-donor (not free ammonia (NH3))
• used in the treatment of some leukaemia patients
• Reduced availability of asparagine to the tumour cells inhibits growth/proliferation of leukemic cells

Question 21

Glutamate

Answer 21

• required for the Urea Cycle
• special aa - can be formed or degraded by aminotransferases or by glutamate dehydrogenase (mitochondrial enzyme)
• Both reactions are reversible
• Aminotransferases need pyridoxal phosphate (PLP) (Vit B6) as cofactor, and glutamate dehydrogenase needs either NAD+ or NADPH

Question 22

Oxidative deamination by Glutamate dehydrogenase

Answer 22

• readily reversible
• Requirement for NAD+ or NADP+ depends on the direction of the reaction
• Glutamate DH is key for both disposal and synthesis of aa’s
• In humans, its one of three enzymes capable of incorporating free ammonia into an organic compounds

Question 23

Glutamine

Answer 23

• non-toxic, transport form of ammonia from the peripheral tissues (especially important in brain)
• In peripheral tissues glutamate is converted to glutamine – Glutamine synthetase
• Requires ATP and free ammonia
• converted back to glutamate by Glutaminase, releases ammonia

Question 24

Catabolism of both glutamine and glutamate

Answer 24

• produces a-ketoglutarate

- Combined actions of: Glutamate dehydrogenase, Glutaminase

Question 25

Methionine and cysteine

Answer 25

• Share a common catabolic pathway

- Methionine converted to homocysteine via SAM & SAH intermediates

Question 26

2 fates of homocysteine

Answer 26

• Conversion back to methionine by methionine synthase via reaction requiring Vit B12 as a coenzyme & THF substrate
• Transfer of sulphur group to serine by Cystathionine-b synthase with cofactor PLP

Question 27

Products of homocysteine reactions

Answer 27

• cystathionine is converted to cysteine and a- ketobutyrate by Cysthionase with cofactor PLP
• Subsequently, a-ketobutyrate is converted to succinyl-CoA for use in TCA cycle

Question 28

Final stage of cysteine catabolism

Answer 28

• converted to sulphate
• Pathway produces glutamate and pyruvate
• End product of sulphate can be excreted or combined with ATP to produce PAPS (3’-phosphoadenosine 5’-phosphosulfate).
• Serine, Glycine and Threonine also generate pyruvate (Threoine also acetyl-CoA)

Question 29

Products of ketogenic amino acid catabolism

Answer 29

• Acetyl-CoA
• Acetoacetate
• Acetoacetyl-CoA

Question 30

Lysine

Answer 30

• Converted to acetoacetate by a multi-step reaction.

- Includes standard reactions of fatty acyl-CoA oxidation (reactions 6,8,9) and ketone body formation (reactions 10, 11)

Question 31

Phenylalanine and Tyrosine

Answer 31

• Phenylanine and tyrosine share a common catabolic pathway

- Phenylalanine converted to tyrosine – Phenylalanine hydroxylase (tetrahydrobiopterin (BH4) required)

Question 32

Utilisation and regeneration of tetrahydrobiopterin (BH4)

Answer 32

• In cyclic series of reactions BH4 is regenerated after use in the phenylalanine hydroxylase reaction
• BH4 is also formed in a priming reaction by dihydrofolate reductase, but utilising a different form of 7,8-Dihydrobiopterin

Question 33

Tyrosine to homogentisate

Answer 33

• Tyrosine transaminated by Tyrosine aminotransferase (PLP required)
• Glutamate and p-Hydroxyphenylpyruvate is produced
• Decarboxylation by Hydroxyphenylpyruvate decarboxylase produces Homogentisate

Question 34

Homogensitate to fumarate and acetoacetate

Answer 34

• Oxidation followed by hydrolysis produces fumarate and acetoacetate
• Enzymes are Homogentisate oxidase and fumarylacetoacetate hydrolase
• End products: fumerate (TCA Cycle) and acetoacetate (ketone body)

Question 35

Phenylketonuria (PKU)

Answer 35

• Autosomal recessive, occurs 1/10000 in UK
• Deficiency of Phenylalanine hydroxlase
• Elevation in serum phenylalanine and reduction in tyrosine
• Metabolized to phenylpyruvate, phenylacetate & phenyllactate (excreted in urine) resulting in mousey/musty odour of urine
• Tyrosine becomes and essential amino acid

Question 36

Effects of PKA

Answer 36

• Decreased pigmentation of skin and hair as tyrosine conversion to melanin is inhibited by the elevated phe levels (inhibits tyrosinase)
• Infants appear normal until a few months old. If untreated, leads to permanent intellectual disability, seizures, delayed development, behavioural problems, & psychiatric disorders

Question 37

PKU treatment

Answer 37

• amino acid restriction via low-protein diet avoiding high-protein foods(i.e. meat, eggs, dairy)
• avoid aspartame (artificial sweetener) which is converted to phenylalaine
• supplementation w/Tyrosine
• regular monitoring of blood phenylalaine

Question 38

PKU in pregnancy

Answer 38

• If women with PKU have high phenylalanine during pregnancy this can harm the fetus or cause miscarriage

Question 39

PKU complications at birth

Answer 39

• Low birth weight
• Delayed development
• Facial abnormalities
• Abnormally small head
• Heart defects and other heart problems
• Intellectual disability
• Behavioural problems

Question 40

Tetrahydrobiopterin (BH4) deficiency

Answer 40

• Rare, autosomal recessive disorder – 600 cases worldwide
• Deficiency of dihydrobiopterin reductase
• Elevation in serum phenylalanine
• Treatment: BH4 supplementation (2-20mg/kg per day) or diet

Question 41

BH4 deficiency complications

Answer 41

• Mild: temporary low muscle tone
• Severe: intellectual disability, movement disorders, difficulty swallowing, seizures, behavioural problems, progressive problems with development, inability to control body temperature

Question 42

Branched chain amino acids

Answer 42

• Valine, Leucine, Isoleucine: branched, non-polar, glucogenic and/or ketogenic
• First is transamination
• Oxidative decarboxylation: pyruvate dehydrogenase (PDH) complex
• Generation of acyl-CoA derivatives
• 3rd step, dehydrogenation: C=C bond formation

Question 43

Maple Syrup Urine disease (MSUD)

Answer 43

• Rare, autosomal recessive disorder - 1/185 000 worldwide
• Deficiency in Branched-chain a-ketoacid dehydrogenase
• Accumulation of BCAA & associated a-ketoacids.

Question 44

Symptoms of MSUD

Answer 44

• first symptom: Ketosis and the characteristic odour of maple syrup in urine
• metabolic crisis: lack of energy, vomiting, irritability, breathing difficulties
• cause brain damage and death if untreated

Question 45

Treatment of MSUD

Answer 45

• dietary restriction of branched chain amino acids
• Problems: Very difficult to treat, BCAA are very abundant in most protein sources, 3 BCAA are all essential
• Supplementation of dietary thiamine (coenzyme) may be useful in patients that have an enzyme with low coenzyme affinity