Protein & Amino Acid Metabolism Flashcards
What are major nitrogen containing compounds
- Amino acids
- Proteins
- Purines + Pyrimidines (DNA / RNA)
smaller amounts in
* Porphyrins (haem)
* Creatine phosphate
* Neurotransmitters (e.g. dopamine) * Some hormones (e.g. adrenaline)
How is creatinine formed
Breakdown product of creatine & creatine phosphate in muscle
What is the rate that creatinine is produced
Usually produced at constant rate depending on muscle mass (unless muscle is wasting)
Why is Creatinine a useful clinical marker
- Creatinine urine excretion over 24h
proportional to muscle mass - Provides estimate of muscle mass
- Also commonly used as indicator of renal function (raised plasma level and low urine level when there is damage to nephrons)
How much creatinine is excreted per day by males and females
Excreted in urine per day
* Men 14-26 mg/kg
* Women 11-20 mg/kg
What happens in nitrogen equilibrium
N equilibrium
Intake = output
No change in total body protein.
Normal state in adult.
What happens in positive Nitrogen balance
Positive N balance
Intake > output
Increase in total body protein.
Normal state in growth & pregnancy or in adult recovering from malnutrition.
What happens in negative Nitrogen balance
Negative N balance
Intake < output
Net loss of body protein. Never normal. Causes include trauma, infection, or malnutrition.
how do you loose Nitrogen in the body
Loss of nitrogen from Skin, Hair, Nails
Nitrogen waste products in fences and urine
What are glucogenic and ketogenic amino acids
A glucogenic amino acid is an amino acid that can be converted into glucose through gluconeogenesis. This is in contrast to the ketogenic amino acids, which are converted into ketone bodies
What happens to our dietary proteins in the body
(protein turnover)
- Dietary protein digestion in to
- Free amino acids these can be turned into cellular proteins via synthesis
- Free amino acids go to liver where they are either turned into carbon skeletons or Amino groups (-NH2)
Carbon Skeleon — (synthesis of glucose or ketone bodies)
- Turns in to Either Glucogenic amino acids or ketogenic
amino acids
- Gluconeogenesis occurs
- Production of glucose and ketone bodies
Production of energy
Amino groups (-NH2)
- Turns to Urea
- Goes to Urine where is excreted
What is proteolysis and synthesis
Free amino acids turn to cellular proteins via synthesis
Cellular proteins turn to free amino acids via proteolysis
Glucogenic and Ketogenic amino acids and both Glucogenic and Ketogenic combined
Alanine - Glucogenic - Pyruvate
Threonine - Ketogenic - Pyruvate
Both - Lysine - Actyl-CoA
When is there a mobilisation of protein reserves to produce energy
- Occurs during extreme stress (starvation)
- Under hormonal control
Hormones involved in mobilisation of protein reserves and there effect on protein-sythesis and protein degradation
Hormones / Effect on protein synthesis / Effect on protein degradation
Insulin &Growth hormone/ Increases / Decreases
Glucocorticoids (e.g. Cortisol)/ Decreases. / Increases
What is Crushing syndrome
Excessive breakdown of protein can occur in Cushing’s syndrome (excess cortisol). Weakens skin structure and causes obesity leading to striae formation.
How many different amino acids are there
20
What are the 9 essential amino acids
Isoleucine Lysine Threonine Histidine Leucine Methionine Phenylalanine Tryptophan Valine
When do some amino acids get Conditionally essential
- Certain amino acids are Conditionally essential
- Children & Pregnant women = high rate of protein synthesis. Also require some arginine, tyrosine & cysteine in diet
Other than diet how can we synthesise Amino Acid pathways
- 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.
What other compounds are amino acid important for
-Tyrosine
Catecholamines
Melanin
Thyroid hormones
- Cysteine
- Hydrogen sulphide (signalling molecule)
- Glutathione
- Tryptophan
- Nicotinamide
- Serotonin (5HT)
- Melatonin
- Histidine
Histamine - Glutamate
- GABA
- Glycine
- Purines
- Glutathione
- Haem
- Creatine
- Serine
- Sphingosine
- Arginine
Nitric oxide
Why is the removal of amino groups essential from nitrogen
What happens to nitrogen after removed from amino group
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
What are the two main pathways for the removal of nitrogen from amino acids
- Transamination
- Deamination
What is Transamination
This is the transferring of the amine group from from an amino acid to another component, using the enzyme aminotransferase.
What does Aminotransferase use to help in transamination
What happens to the byproduct
Whilst amino acid transfers its amine group
-Aminotransferase enzymes uses α-ketoglutarate which turns onto glutamate.
Glutamate can be easily fed into to the Urea Cycle.
Which substances are used to allow Aminotransferase enzymes to work
All aminotransferases require the coenzyme pyridoxal phosphate which is a derivative of vitamin B6
What are the types of Aminotransferase Enzyme
WHAT IS THE DIFFERENCE BETWEEN THEM
-Alanine aminotransferase (ALT)
Catalyses interconversion of alanine and α-ketoglutarate to
pyruvate and glutamate
-Aspartate aminotransferase (AST)
Catalyses Interconversion of aspartate and α-ketoglutarate to oxaloacetate and glutamate
What is ALT and AST levels used to measure
What diseases cause this
Plasma ALT and AST levels measured routinely as part of liver function test
Levels particularly high in conditions that cause extensive cellular necrosis (death of body tissue)
Diseases that cause this are …
* Viral hepatitis
* Autoimmune Liver Diseases
* Toxic injury
or even
* Amanita phalloides, mushrooms (commonly known as death cap cause acute liver failure if ingested.)
What is Deamination
Liberates amine group as free ammonia from the amino acid
What happens during Deamination
There is the removal of the amine group
This releases the Carbon skeleton ( Keto Acid)
The Keto Acid can be utilised for energy
Where does Deamination occur and why
Mainly occurs in liver & kidney
Occurs in the liver do ammonia produced can be dealt with by the Urea Cycle
Occurs in the Kidney so ammonia can be released into our urine
What are the two isomers for amino acids and what’s the difference cause
D and L isomer
D isomer can’t be used in protein synthesis so used for production of energy
dietary D-amino acids (found in plants and microorganisms)
Why does ammonia have to be removed quickly from the body
How are they excreted
Ammonia (and ammonium ions) very toxic and must be removed. Ultimately converted to urea or excreted directly in urine
When is ammonia converted to Ammonium
At physiological pH (PH 7.4), ammonia (NH3) is rapidly converted to ammonium ion (NH4+)
What enzymes deaminate amino acids
Several enzymes can deaminate amino acids
* Amino acid oxidases
* Glutaminase
* Glutamate dehydrogenase
Structure and facts about Urea
-Amine groups can be turned into Urea which is non toxic / inert
-Urea has two amine groups
-It is extremely water soluble
- Most urea is excreted in urine via the kidneys
- Also performs useful osmotic role in kidney tubules
Can Ammonia be released from Urea
bacteria can break it down to release NH3
What does the urea cycle do
The urea cycle allows us to safely convert amine groups (from amino acids) to Urea (which is non-toxic) so it can safely be excreted as urine.
How is Glutamate fed into the Urea Cycle
Glutamate can either be turned into Aspartate (which links with Citrulline to form Argininosuccinate (using ATP) which turns to Arginine which reacts with eater to form Urea)
or
Glutamate via the enzyme Glutamate dehydrogenase gets Turns to CO2
NH3 which turn
How is Glutamate fed into the Urea Cycle
Glutamate can either be turned into Aspartate which links with Citrulline to form Argininosuccinate (using ATP) which turns to Arginine which reacts with eater to form Urea)
or
Glutamate via the enzyme Glutamate dehydrogenase Turns to NH3 via Deamination
NH3 and CO2 react using 2ATP to form carbamoyl phosphate
which turns to citrulline
Citrulline links with Aspartate to form Argininosuccinate (using ATP) which turns to Arginine which reacts with eater to form Urea
Where does the Urea Cycle occur and how many enzymes does it involve
Occurs in liver and involves 5 enzymes
What affects urea cycle rate
- 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
What is re-feeding syndrome
how do you treat patient knowing this
Can occur when nutritional support is given to severely malnourished patients
Electrolyte abnormalities such as Hypophosphataemia may occur
(When you begin re-feeding, your cells demand these electrolytes (phosphate) to metabolize the food. This causes a severe shift in your body chemistry. The electrolytes you have move rapidly from your blood into your cells. But because you don’t have enough, this shift leaves low levels of them in your blood.)
Ammonia toxicity due to urea cycle down regulation
(Enzymes have been downgraded due to starvation thus they can’t metabolise amine groups efficiently)
Re-feed 5-10 kcal/kg/day more a day to full needs
What happens when there is a genetic defect in the Urea cycle
How common is it
- Autosomal recessive genetic disorders caused by deficiency of one of enzymes in the urea cycle
- Mutations cause a partial loss of enzyme function
Leads to: - hyperammonaemia
- accumulation/excretion of urea cycle intermediates
*complete loss of enzyme function can be fatal - Occur ~1 in 30,000 live births
What are the symptoms that come with a defect in the Urea Cycle
Symptoms
* Vomiting
* Lethargy
* Irritability
* Mental retardation
* Seizures
* Coma
What does the severity of the Urea cycle defect depend on
Severity depends on:
* nature of defect
* amount of protein eaten
When does severe urea cycle disorder symptoms show
Severe urea cycle disorders show symptoms within 1 day
after birth. If untreated, child will die.
When does Mild Urea cycle disorder symptoms show
Mild urea cycle enzyme deficiencies may not show symptoms until early childhood
How do you manage defects in the Urea cycle
Management:
* Low protein diet
* Replace amino acids in diet with keto acids
(to limit amount of ammonia produced)
Why is Ammonia very dangerous
What are the levels it needs to kept as
What are the potential toxic side effects of it
- 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)
Two mechanisms are utilised for the safe transport of amino acid nitrogen from tissues to the liver for disposal
Glutamine and Alanine
What is Glutamine way of transporting ammonia
Glutamine
* After ammonia is removed from the amino acid
* Ammonia combined with glutamate to form glutamine using Glutamine synthetase
* 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
What is Alanine of transporting ammonia
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. (using alanine amino transferase)
* 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
How many inherited diseases defect the amino acid metabolism
what does it do
is it rare
what happens if untreated
what is the treatment
- 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
What is the Heel prick test
Why is it used
What diseases does it pick up
Every child with in the UK when first born (below 5 days) gets the Heel Prick test
The Heel pick test is used to find diseases where if treatment occurs early you can minimise the effects of the disease
these diseases include
* Sickle cell disease
* Cystic fibrosis
* Congenital hypothyroidism
Inborn errors of metabolism -
* Phenylketonuria (PKU)
* Maple syrup urine disease
* Isovaleric acidaemia (IVA)
* Glutaric aciduria
* Homocystinuria
How rare is Phenylketonuria (PKU)
Most common inborn error of amino acid metabolism (~1 in 15,000 births)
What is Phenylketonuria (PKU)
what is its main effect
- Deficiency in phenylalanine hydroxylase thus build up of phenylalanine amino acid
- Autosomal recessive.
Affected gene is on chromosome 12 - Accumulation of phenylalanine in tissue, plasma & urine
- Phenylketones in urine
- Musty smell
PKU leads to a build-up of the amino acid phenylalanine, which is toxic to the nervous system. Without treatment, PKU can cause intellectual disabilities
What is the treatment for PKU
Treatment
* Strictly controlled low phenylalanine diet enriched with tyrosine
* Avoid artificial sweeteners (contain phenylalanine)
* Avoid high protein foods such as meat, milk, and eggs
What pathways does PKU effect and how
No phenylalanine hydroxylase enzyme
Phenylalanine can’t turn to Tyrosine
With out tyrosine many pathways are effected such as
* Noradrenaline
* Adrenaline
* Dopamine
* Melanin
* Thyroid hormone
* Protein synthesis
What are the symptoms you can get from PKU
Symptoms*
* Severe intellectual disability
* Developmental delay
* Microcephaly (small head)
* Seizures
* Hypopigmentation
What is Homocystinuria
(Autosomal recessive disorders.)
Body can’t breakdown Methionine amino acid
Defect in cystathionine β-synthase enzyme causing
Accumulation of Homocysteine
This Affects connective tissue, muscles, CNS, CVS
What is the treatment for Homocystinuria
Treatment
* Low-methionine diet
- Avoid milk, meat, fish, cheese, eggs
- Nuts, and peanut butter also contain methionine
- Cysteine, Vit B6, Betaine, B12 & Folate supplement
( transferring of Homocysteine back to Methionine is promoted by Betaine, Vit B12, and Folate)
