Lecture 2 - Protein and Amino Acid Metabolism Flashcards
explain nitrogen balance
do not need numerical values
amino acids, proteins, prurines and pymramidines, creatine phosphate all contain nitrogen
in a 70kg male we have 2kg of N in proteins, 16g in amino acids and 60g in N containing compounds
we take in 16g of N every day
loose 14g of N through urine, faeces
2g lost to hair and nails
this is N equilibrium
Positive N balance - we are taking in more N than we are loosing - normal in growth, pregnancy ect
Negative N balance - loosing more N than we take in
Net loss of body protein - not normal - from trauma, infection or malnutrition
explain protein turnover
we digest proteins into amino acids
some amino acids we also synthesise
these are free amino acids
they are used to build cellular proteins like muscle
or broken down into amino acids via proteolysis
OR free amino acids enter the liver
amino groups could convert to ammonia (very toxic) so we convert to urea and expel in the urine
the carbon skeleton is used in gluconeogenisis (glucogenic aa’s) or ketogenisis (ketogenic aa’s) - these can be used for energy
explain creatinine as a clinical marker
creatine and creatine phosphate breakdown into creatinine
this is produced at a constant rate unless we are wasting away
filtered via the kidneys into urine - provides an estimate of muscle mass
and indicator of renal function
what is the role of creatine phosphate
will store Phospahte in muscles
convert to creating to make an ATP
rapid source of ATP
give an example each of the types of amino acids
glucogenic - glycine - used to synthesise things like pyrivate, can be used for gluconeogenisis - glucose synthesis
Ketogenic - lysine - feeds in at acetyl CoA - can be use to synthesise ketone bodies
Both glucogenic and ketogenic - tyrosine
they will all feed into somewhere in TCA cycle
Mobilisation of protein reserves - when and why
- Occurs during extreme stress (starvation)
- Under hormonal control -
insulin and growth hormone will increase protein synthesis and decrease protein degradation
glucocorticoids - cortisol - will decrease protein synthesis and increase protein degradation
what is Cushing’s syndrome
Excessive breakdown of protein can occur in Cushing’s syndrome (excess cortisol). Weakens skin structure leading to striae formation.
give two uses of specific amino acids outside of protein synthesis
Histidine to make Histamine
Tyrosine to make
• Catecholamines
• Thyroid hormones
why do we want to remove the nitrogen group on amino acids
what are the two methods for doing this ?
• 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
- Deamination
what is transamination ?
N containing NH2 group is transferd to a ketone, to create ie glutamate
• Most aminotransferase enzymes use α-ketoglutarate to funnel the amino group to glutamate.
Not vital • Exception to rule is aspartate aminotransferase which uses oxaloacetate to funnel amino group to aspartate
• All aminotransferases require the coenzyme pyridoxal phosphate which is a derivative of vitamin B6
Key aminotransferase enzymes you need to know
Alanine aminotransferase (ALT) 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
what is Deamination ?
- 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)
• Ammonia (and ammonium ions) very toxic and
must be removed. Ultimately converted to urea or
excreted directly in urine
an example enzyme is glutaminase, glutamine dehydrogenase
what is urea
it is a nitrogen containing compound that is non toxic, and water soluble - so can be excreted in urine
Summarise the urea 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
it is for the safe disposal of amino acids N via conversion into urea, for urine excretion
what is refeeding syndrome ?
Can occur when nutritional support given to severely malnourished patients
• Ammonia toxicity significant factor (urea
cycle down regulated) - urea cycle is down regulated so , we cant safely remove nitrogen, ammonia will build up to toxic levels if too much protein rich food is eaten quickly
• Re-feed @ 5 to 10 kcal/kg/day. Raise
gradually to full needs within a week
summarise defects in the urea cycle and what this can cause
what are some symptoms , and what is the solution ?
autosomal reccesive disorders caused by deficiency of enzymes within the cycle
leads to
hyperammoaemia - ammonia toxicity
accumulation and excretion of intermediates within the cycle
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
Management:
• Low protein diet
• Replace amino acids in diet with keto acids
Symptoms • Vomiting • Lethargy • Irritability • Mental retardation • Seizures • Coma
biochemically , why is ammonia considered toxic ?
it is readily diffusable, and very toxic to the brain
levels need to be kept low : 25-40 umol/L
may be becuase - PH effects Alters blood brain barrier interferes with amino acids transport and protein synthesis interferes with TCA cycle
how is ammonia removed from the body ?
have a go at drawing the diagram - lec 2, page 24
Two mechanisms are utilised for the safe transport of amino acid nitrogen from tissues to the liver for disposal
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
what are the clinical problems with amino acid metabolism
many inherited diseases
involve a total or partial loss of enzyme activity
left untreated they can lead to intellectual impairment
treatment will involve restricting specific amino acids within the diet
we do the heel prick test with infants - checks for sickle cell, cystic fibrosis, and errors in metabolism
the two you need to know are
Phenylketonuria (PKU)
Homocystinurias
what is Phenylketonuria (PKU)
what enzyme does it effect ?
what are the symptoms and treatments ?
what pathways does it affect ?
most common problem with amino acid metabolism
Deficiency in phenylalanine hydroxylase
Autosomal recessive.
Accumulation of phenylalanine in tissue, plasma & urine
this will be transaminated to phenylpyrivate - this will be converted to phenyl ketones, which accumulate
body cannot produce tyrosine - this effects noradrenaline and adrenaline , dopamine, melanin, thyroid hormones and protein synthesis
Phenylketones in urine
• Musty smell
Symptoms* • Severe intellectual disability • Developmental delay • Microcephaly (small head) • Seizures • Hypopigmentation
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 are Homocystinurias ?
what enzyme does it effect ?
what are the symptoms and treatments ?
• Problem breaking down methionine
• Excess homocystine (oxidised form of homocysteine) excreted in urine
• Autosomal recessive disorders.
• Defect in cystathionine β-synthase most common
• Affects connective tissue, muscles, CNS,
CVS
Treatment • Low-methionine diet • Avoid milk, meat, fish, cheese, eggs • Nuts, and peanut butter also contain methionine • Cysteine, Vit B6, Betaine, B12 & Folate supplement
Accumulation ofHomocysteine &
methionine causes disease symptoms such as cardiovascualr disease
