3. Protein & Amino acid metabolism Flashcards
What is creatinine?
The breakdown product of creatine and creatine phosphate
What do creatinine levels provide an estimate of?
Muscle mass
Indicator of renal function - plasma levels raised if nephron damage
Who would you expect to have a positive N balance?
Pregnant women, growing children
What can cause patients to have a negative N balance?
Trauma, infection, malnutrition
What 3 things can cause an increase in free amino acid conc?
- Dietary protein
- Muscle breakdown (proteolysis)
- De novo synthesis
Where are free amino acids broken down?
Liver
What are amino acids broken down into?
Carbon skeleton and amino group
What happens to the carbon skeleton of amino acids?
If glucogenic then it enters gluconeogenesis
If ketogenic then produces ketone bodies
What determines if an amino acid is glucogenic or ketogenic?
Side chains it possesses.
Give examples of a ketogenic and a glycogenic amino acid.
Glucogenic - Alanine
Ketogenic - Leucine, Lysine
Which amino acid is both ketogenic and glycogenic?
Threonine
Under what conditions are protein reserves mobilised for energy?
Starvation
Which hormones stimulate increased protein degradation?
Glucocorticoids
Which hormones inhibit protein degradation?
GH
Insulin
What 3 processes produce carbon skeletons which can be used to AA synthesis?
- Glycolysis - 3C
- TCA cycle - 4C and 5C
- Pentose phosphate pathway - C4 and C5
Other than protein synthesis, what other important compounds is tyrosine needed for?
Melanin
Catecholamines
Thyroid hormones
In AA synthesis, where can the amino group be taken from?
Transamination from other amino acids or from ammonia.
What is transamination?
When an amino group is transferred from an amino acid to a keto acid, producing a new amino acid and kept acid.
What co-enzyme do all aminotransferase enzymes require?
Pyridoxal phosphate - derivative of vit B6
Which 2 aminotransferases are assayed in LFT?
ALT (alanine aminotransferase)
AST (aspartate aminotransferase)
When might ALT and AST levels rise?
Conditions that cause hepatocellular necrosis:
- Vital hepatitis, toxic injury, autoimmune liver diseases
How does deamination differ from transamination?
Amino group is removed and released as free ammonia
Where does deamination occur, why only these places?
Kidney and liver, where the urea cycle is present to detoxify the ammonia and excrete in urine.
What form will ammonia be present in at physiological pH?
Ammonium ion (NH4+)
What process leads to striae formation in Cushing’s disease?
Excess protein breakdown due to excess cortisol (glucocorticoid), which weakens skin structure.
Which 3 enzymes can deaminate amino acids?
Amino acid oxidases
Glutaminase
Glutamate dehydrogenase
How are the enzymes needed for the urea cycle controlled?
Induced by a high protein diet as increased disposal of ammonia is needed.
Starvation or low protein suppresses levels.
INDUCIBLE but not regulated.
When is refeeding syndrome likely to occur and how can it be prevented?
When nutritional support given to malnourished patients. Ammonia toxicity as low levels of urea cycle enzymes lead to ammonia toxicity. Gradually raising protein levels will avoid this and allow enzyme induction to occur.
What symptoms are associated with deficiency in urea cycle enzymes?
Vomiting, lethargy, irritability, mental retardation, coma, seizures due to hyperammonaemia.
Why are raised ammonia levels so dangerous?
Extremely toxic and readily diffusible so affect the brain.
- disrupt cerebral blood flow
- pH effects, alkaline
- alter BBB
- interfere with TCA cycle, AA transport and protein synthesis
What 2 amino acids are used in the removal of ammonia from tissues?
Glutamine and alanine
Explain how glutamine is used to remove ammonia from tissues.
Ammonia is combined with glutamate to form glutamine. Glutamine into blood to the liver/kidneys where it is cleaved by glutaminase back to glutamate and ammonia.
Explain how alanine is used to remove ammonia from tissues?
Ammonia is combined with pyruvate to form alanine. Alanine to liver, converted to pyruvate by transamination. Amino group fed via glutamate to urea cycle.
How is the pyruvate product from alanine transport used in the liver?
Gluconeogenesis back to glucose and fed back into tissues.
What does the heel prick test in babies check for?
Cystic fibrosis
Sickle cell disease
Congenital hypothyroidism
Inborn errors in metabolism
Which 2 inborn errors in metabolism are related to defects in amino acid metabolism, which is most common?
Phenylketonuria (PKU) = most common
Homocystinuria
If untreated then intellectual impairment.
What enzyme is deficient in phenylketonuria?
Phenylalanine hydroxylase
How are PKU and homocystinuria inherited?
Autosomal recessive
What reaction does phenylalanine hydroxylase catalyse?
Phenylalanine -> Tyrosine
What happens in PKU?
Accumulation of phenylalanine leads to the transamination to form phenylpyruvate which forms phenylacetate. Accumulation of phenylketones in urine.
What symptoms are associated with PKU?
Severe intellectual delay Developmental delay Microcephaly Seizures Hypopigmentation
What is the treatment for PKU?
Strict low phenylalanine diet
Avoid artificial sweeteners and high protein foods - milk,eggs and meat
Which amino acid breakdown is impaired in homocystinuria?
Methionine - build up of methionine and homocysteine
Which enzyme is deficient in homocystinuria?
Cystathione beta-synthase
What tissues does homocystinuria usually affect?
CT, muscles, CNS, CVS
Elevated homocysteine is shown to be associated with CVD
What is the treatment for homocystinuria?
Low-methionine diet
Betaine, Vitamin B12, Folate, Vit B6
How is methionine metabolised usually?
Methionine -> homocysteine -> Cystathionine -> Cysteine
Why are Vit B6 supplements advised in patients with homocystinuria?
Vit B6 is a co-factor for cystathione beta-synthase
How are betaine, Vit B12 and folate involved in methionine metabolism?
Promote conversion of homocysteine to methionine
What would you expect to see in the urine/plasma of a patient with homocystinuria?
Raised methionine and homocysteine
Low Cysteine
What are ketogenic amino acids converted to?
acetyl Co-A which can be used to synthesise ketone bodies.
How are some amino acids both ketogenic and glycogenic?
Larger amino acids can give rise to both acetyl-CoA and one of the other organic precursor molecules.
Why will a patient with PKU require dietary tyrosine?
Tyrosin synthesis pathway is compromised, as it is converted from phenylalanine by phenylalanine hydroxylase.
Why do the signs and symptoms of homocystinuria initially resemble Marfan syndrome?
High levels of homocysteine affect CT and bone. Damage to collagen and elastic fibres in CT by binding to lysine residues.
What is an important distinguishing feature between Marfan syndrome and homocystinuria?
Neurological symptoms present in homocystinuria but not in Marfan’s.
Why is homocystinuria associated with inquired risk of early onset CVD?
Increased homocysteine levels is prothrombotic.
