4.2 - Protein + AA Metabolism

Question 1

Major nitrogen containing compounds

Answer 1

• Amino acids
• Proteins
• Purines and pyrimidines (DNA + RNA)
• Polyphorins (haem)
• Creatine phosphate (ensures sources of ATP readily available)
• Neurotransmitters
• Some hormones

Question 2

Creatinine

Answer 2

• Useful clinical marker
• Breakdown product of creatine and creatine phosphate in muscle
• Usually produced at a constant rate depending on muscle mass, unless muscle is wasting
• Filtered via kidneys into urine
• Creatinine urine excretion is proportional to muscle mass
• Provides estimate of muscle mass
• Also used as an indicator of renal function (ie whether it is filtered out properly)

Question 3

Nitrogen balance

Answer 3

• Intake is via dietary protein, where it enters the AA pool
• Output is in urea, faeces, everyday loss of skin + hair cells
  N equilibrium where intake = output. This is normal in adult
  positive N balance where intake > output. Increased total body protein. Normal state in growth, pregnancy or recovering from malnutrition
  negative N balance where intake < output. Net loss of total body protein. Never normal. Eg in starvation where skeletal muscle used as source of energy, or also malnutrition, infection or trauma

Question 4

Protein turnover

Answer 4

• Proteins are constantly being recycled and remade from free AA pool
• Free AA sources:
  ☞ from synthesis of AA (ie from dietary protein)
  ☞ from recycling of proteins
  ☞ utilisation of carbon skeleton (need to deal with amine group of AA so that ammonia is not liberated → amine converted to urea which is excreted in urine).
  ▶︎ Two main types of AA: glucogenic + ketogenic (covered on another slide, which is used to describe the way in which we derive energy from them)

Question 5

Glucogenic and ketogenic amino acids (remember just one of each type)

Answer 5

Ie using carbon skeleton from AA for energy production…
glucogenic
- Derive energy (glucose) from them by gluconeogenesis
- Eg alanine, aspartate and asparagine
ketogenic
- Derive energy from them via ketone bodies
- Eg lysine and leucine + phenylalanine
some can be both K+G
Eg threonine, tryptophan and tyrosine

Question 6

Mobilisation of protein reserves

Answer 6

Main energy sources are adipose tissue and glycogen (liver)
- In times of starvation, can use skeletal muscle to provide energy
- This ability to draw upon skeletal muscle is limited without compromising breathing + movement etc
- note: there is glycogen in skeletal muscle, but this is used solely by the skeletal muscle itself for energy production
- Ability of mobilising protein reserves is under hormonal control
☞ insulin + growth hormone increase protein synthesis, and inhibit protein degradation
☞ glucocorticoids, eg cortisol decrease protein synthesis, and increase protein degradation (this is seen in Cushing’s where excess cortisol)

note: excessive breakdown of protein can weaken skin structure, leading to striae formation, eg seen in Cushing’s

Question 7

Essential amino acids

Answer 7

• Need all 20, but some can be synthesised by the body
• There are 9 that the body can’t synthesise: Isoleucine, Lysine, Threonine, Histidine, Leucine, Methionine, Phenylalanine, Tryptophan and Valine
• Certain AAs are conditionally essential (ie during increased demand, such as pregnancy or rapid growth. Need to intake extra by diet during these times)

Question 8

Why are some proteins from animal origin considered better than plant proteins

Answer 8

• Protein of animal origin (high quality) as they contain all essential AAs
• Proteins of plant origin (lower quality) since most sources are deficient in one or more essential amino acids
• Therefore essential that plant-based individuals obtain protein from a wide variety of plant sources

Question 9

What are the 9 essential amino acids (just names)

Answer 9

• Isoleucine
• Lysine
• Threonine
• Histidine
• Leucine
• Methionine
• Phenylalanine
• Tryptophan
• Valine
  If Learned THis Huge List May Prove TRuly VALuable

Question 10

Amino acid synthesis

Answer 10

• In addition to dietary intake, body can synthesise some AAs (not essential AAs)
• Amino group is provided by other AAs by the process of transamination or from ammonia
• Carbon atoms for non-essential AA synthesis come from:
  ☞ intermediates of glycolysis
  ☞ pentose-phosphate pathway
  ☞ kreb’s cycle

Question 11

Removal of nitrogen from AAs

Answer 11

• Removal of amino group is essential to allow carbon skeleton of AAs to be utilised in oxidative metabolism
• Once removed, nitrogen can be used in other compounds or excreted
• Excreted: nitrogen → urea (removed in urine)
• This prevents it forming ammonia, which is highly toxic to tissues
• Two main pathways facilitate removal of N from AAs: transamination and deamination

Question 12

Transamination

Answer 12

• Aminotransferase enzyme is responsible for AA → ketoacid
• Mainly, this is done by transferring the amine group and adding it to α-ketoglutarate → glutamate
• All aminotransferases require the coenzyme pyridoxal phosphate. This is a derivative of vitamin B6

Question 13

Key aminotransferases

Answer 13

alanine aminotransferase (ALT)
Alanine → pyruvate
α-ketoglutarate → glutamate

aspartate aminotransferase (AST)
Aspartate → oxaloacetate
α-ketoglutarate → glutamate

• ie in both of these, the amine group is added to α-ketoglutarate
• Plasma ALT and AST levels are routinely measured as part of liver function test
• Very high in conditions that cause extensive cellular necrosis, such as viral hepatitis, autoimmune liver diseases, toxic injury, ingestion of death cap mushrooms

Question 14

What can ingestion of death cap mushrooms cause

Answer 14

• Acute liver failure
• Aka amanita phalloides
• Plasma ALT levels up to 20x normal

Question 15

Deamination

Answer 15

• Removal of amine group (other is transamination)
• Liberates amino group as free ammonia
• At physiological pH, ammonia → ammonium ions, which are also toxic
• Mainly occurs in liver and kidney (so that ammonia can be dealt with immediately via urea cycle or excretion)
• Also important in deamination of dietary D-amino acids (found in plants and microorganisms)
• Ammonia (and ammonium ions) are very toxic, and therefore must be removed
• Ammonia is converted to urea or excreted directly in urine

Question 16

What are some enzymes that can deaminate AAs

Answer 16

• Amino acid oxidases
• Glutaminase
• Glutamate dehydrogenase

Question 17

Urea

Answer 17

• High nitrogen content
• Non-toxic, removing ammonia
• Very water soluble (ie efficient for removal)
• Chemically inert in humans
• Most urea is excreted in urine via kidneys
• Useful osmotic role in kidney tubules

Question 18

Urea cycle

Answer 18

• Occurs in liver
• Involves 5 enzymes
• Requires ATP to convert ammonia → urea
• Amount of urea cycle enzymes depends on need to dispose of ammonia
  ☞ high protein diet increases enzyme levels
  ☞ low protein diet or starvation represses enzyme levels

Question 19

Defects in urea cycle

Answer 19

• Autosomal recessive genetic disorders caused by deficiency of one of the enzymes in the urea cycle
• Mutations can cause a partial loss of enzyme function (ie less stable or less effective)
• Leads to hyperammonaemia or accumulation/excretion of urea cycle intermediates
• Can also cause ammonia toxicity (symptoms on another card) ☞ severity of this depends on the nature of the defect and the amount of protein eated
• Severe urea defects will show symptoms within 1 day of birth, but milder defects may only show in early childhood
• Management: low protein diet ☞ replace AAs in diet with ketoacids, in order to limit amount of ammonia produced

Question 19

Defects in urea cycle

Answer 19

• Autosomal recessive genetic disorders caused by deficiency of one of the enzymes in the urea cycle
• Mutations can cause a partial loss of enzyme function (ie less stable or less effective)
• Leads to hyperammonaemia or accumulation/excretion of urea cycle intermediates
• Can also cause ammonia toxicity (symptoms on another card) ☞ severity of this depends on the nature of the defect and the amount of protein eated
• Severe urea defects will show symptoms within 1 day of birth, but milder defects may only show in early childhood
• Management: low protein diet ☞ replace AAs in diet with ketoacids, in order to limit amount of ammonia produced

Question 20

Symptoms of ammonia toxicity

Answer 20

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

Question 21

Biochemical basis of ammonia toxicity + possible effects
ie why is ammonia toxic

Answer 21

Ammonia is readily diffusible and extremely toxic to brain.
Potential toxic effects:
* interferes with AA transport and protein synthesis
* distruption of cerebral blood flow
* pH effects as alkaline
* interference with metabolism of excitory AA neurotransmitters
* alteration of the blood-brain barrier
* interference with the TCA cycle, and therefore energy metabolism interference (by reacting with α-ketoglutarate → glutamate)

Question 22

Transport of AAs

Answer 22

two mechanisms are utilised for the safe transport of AA nitrogen from tissues to the liver for disposal

glutamine
- ammonia + glutamate → glutamine
- glutamine transported in blood to liver/kidneys
- here it is cleaved by glutaminase to reform glutamate + ammonia
- in liver, the ammonia is fed into urea cycle
- in kidneys, ammonia is excreted directly into urine

alanine
- amine groups transferred to glutamate by transamination
- pyruvate then transaminated by glutamate → alanine
- alanine transported in blood to liver where it is converted back to pyruvate by transamination (glucose-alanine cycle)
- amino group fed via glutamate into urea cycle for disposal as urea
- pyruvate is used to synthesise glucose, which is fed back to tissues

Question 23

what are inborn errors of metabolism

Answer 23

• over 50 inherited diseases involving defects in AA metabolism
• either total or partial loss of enzyme activity
• rare, but collectively constitute a significant portion of paediatric genetic disease
• if untreated → intellectual impairment
• treatment: restricting specific amino acids from diet
• detected using heel prick test (screening leads to early diagnosis, therefore early treatment and therefore better outcomes)

☞ defects include:
- phenylketonuria (PKU)
- maple syrup urine disease
- isovaleric acidaemia (IVA)
- glutaric aciduria
- homocystinuria

Question 24

phenylketonuria (PKU) – treatment and symptoms on another card

Answer 24

• inborn error of AA metabolism
• detected using heel prick test on newborn babies
• autosomal recessive (on chr 12)
• deficiency of phenylalanine hydroxylase
• this means phenylalanine → tyrosine cannot occur
• this causes phenylalanine to build up in tissue, plasma + urine, and with transamination, so do phenylketones in urine (as phenylalanine is a ketogenic AA)
• the lack of tyrosine produced affects noradrenaline, adrenaline, dopamine, melanin, thyroid hormone and protein synthesis pathways

Question 25

symptoms, signs and treatment of phenylketonuria (PKU)

Answer 25

signs and symptoms ☞ can be avoided with early intervention
- characteristic musty smell to urine, breath or skin
- severe intellectual disability
- developmental delay
- microcephaly
- seizures
- hypopigmentation (ie lighter skin and hair)
treatment
- strict low phenylalanine diet enriched with tyrosine
- avoid artificial sweeteners as these contain phenylalanine
- avoid high protein foods such as meat, milk and eggs

Question 26

homocystinurias

Answer 26

• autosomal recessive disorders
• defect in cystathionine β-synthase often
• cystathionine β-synthase requires active form of vit B6 as co-factor
• problem breaking down methionine, and therefore forms homocysteine
• this causes homocystine (an oxidised form of homocysteine) to be excreted in urine
• this is because homocysteine cannot be broken down to cystathionine
• affects the connective tissue, muscle, CNS and CVS
• the conversion of homocysteine → methionine is promoted by betaine, vit B12 and folate

Question 27

what are the treatments for homocystinurias

Answer 27

• low methionine diet
• avoid milk, meat, fish, cheese, eggs and nuts
• cysteine, vit B6, betaine, B12 and folate supplement

Question 28

what is the difference between cysteine and cystine

Answer 28

cysteine is an AA
cystine is 2 x cysteine bound together with a disulphide bond
(both of these are unrelated to homocystinuria)

Question 29

What is the difference between homocysteine and homocystine

Answer 29

homocysteine is made from methionine
homocystine is 2 x homocysteine (oxidised form)