L2: Protein and Amino Acid Metabolism Flashcards
What are the major nitrogen containing compounds?
Amino acids, proteins and purines and pyrimidines (DNA/RNA).
Smaller amount in creatine kinase, neurotransmitters, some hormones, prophyrins etc…
How much nitrogen is there in the body (70kg male)?
2.0kg in body protiens = 3% of body weight
What is creatinine?
Breakdown product of creatine and creatine phosphate in muscle. Good estimate of muscle mass.
How is creatinine eliminated from the body? Normal range? Why is it a useful clinical marker?
It is excreted by the kidney’s into urine.
Men 14-26 mg/kg, women 11-20 mg/kg
Usually produced at a constant rate. Elevated levels indicate muscle wasting or renal failure.
What is meant by nitrogen balance? What do we say when they are unbalanced?
The steady rate at which nitrogen is taken into the body and excreted from the body.
N equilibrium - intake = output
Positive N balance - intake > output
- growth, pregnancy, recovery from malnutrition
Negative N balance - intake < output
- never normal, trauma, infection or malnutrition
What happens during stage 1 of protein catabolism?
Broken down in GI tract by proteases and peptidases into amino acids. Amino acids used for synthesis of new proteins.
What happens during stage 2 metabolism?
Removal of NH2 group –> converted to urea (CO(NH2)2) and excreted.
Carbon skeleton–> ketogenic or glucogenic amino acids –> energy
What are ketogenic and glucogenic amino acids?
Ketogenic amino acids produce acetyl CoA which can be used in the TCA cycle or produce ketone bodies.
- e.g. leucine and lysine
Glucogenic amino acids are used to produce glucose by gluconeogenesis
- e.g. alanine, glycine, cysteine and serine
What is meant by protein turnover?
All proteins undergo continuous breakdown and synthesis, normally equal.
Rate varies on growth and ageing and type of protein.
Normal T1/2= 80 days.
What hormones have an effect on protein synthesis and breakdown?
Insulin and growth hormone increase the rate of synthesis and decrease rate of breakdown.
Glucocorticoids (cortisol) decrease rate of synthesis and increase the rate of breakdown.
What is the structure of an amino acid?
Diagram on slide 9
Amino group
R group
Carboxyl group
What type of bond forms when two amino acids join together? What is the name of the reaction? What is released?
Peptide bond
Condensation reaction
Water
What are essential amino acids? What are the 9 essential amino acids?
Essesntial amino acids are amino acids that cannot be synthesised by the body they must be obtained from the diet. If - Isoleucine Learned- Lysine This - Threonine Huge - Histidine List - Leucine May - Methionine Prove - Phenylalaine Truly - Tryptophan Valuable - Valine
What are conditionally essential amino acids? Name them?
Become essential amino acids during certain periods e.g. growth periods
Arginine, Tyroisine and Cysteine
What are high quality proteins? What are low quality proteins?
High quality proteins are from animal origin, contain all essential amino acids. Low quality proteins are form plant origin, don’t contain all essential amino acids.
How are amino acids synthesised?
Non essential amino acids.
Carbon group comes from intermediate of glycolysis (C3), pentose phosphate pathway (C4 + C5) or krebs cycle (C4 + C5).
Amino group from transamination or ammonia
What are amino acids required for?
They are needed for protein synthesis or N-containing compounds
Nitric oxide –> formed from arginine
hydrogen sulphide –> formed from cysteine
GABA –> formed from glutamate
Histamine –> formed from Histidine etc…
What are the two main pathways for removing nitrogen from amino acids?
Nitrogen needs to be removed so the carbon skeleton can be utilised in other compounds.
Removal of nitrogen by
-Transamination
-Deamination
What is transamination? What ketoacids are usually used?
Transfer of the amino group to ketoacid to form a new amino acid.
alpha-ketoglutarate –> glutamate
oxaloacetate –> aspartate
What enzyme is required for transamination?
Aminotransferase enzyme and co enzyme pyridoxal phosphate derived from vitamin B6
What are two important aminotransferase enzymes? Why are the important?
Alanine aminotransferase (ALT) --> alanine to glutamate Aspartate aminotransferase (AST) --> Glutamate to aspartate Measured in a liver function test High levels indicate extensive cellular necrosis e.g. viral hepatitis, autoimmune liver disease, toxic injury
What is deamination?
Removal of amino group to form free ammonia. It occurs mainly in the liver and kidneys.
Ammonia is very toxic and must be removed.
Converted to urea and excreted from the body.
Important for deamination of dietary D-amino acids (not good isomer)
What are the different steroisomers of amino acids?
L isomers and D isomers.
L isomers are used to make proteins
D isomers are not as they would make structurally abnormal proteins.
Converted to keto acids that are not optically active.
What enzymes can deaminate amino acids?
Amino acid oxidase
Glutaminase
Glutamate dehydrogenase
What is urea? How is it produced?
Urea is a substance that is water soluble, chemically inert, non toxic, and has high nitrogen content.
It is produced by the urea cycle and is excreted in urine.
What is the urea cycle?
Cycle that produced urea from ammonia or transminated amino acids glutamate and asparatate. Half the cycle takes place in the mitochondria, half takes place in the cytoplasm.
Takes place in the liver.
How is the urea cycle regulated?
5 enzymes involved in the urea cycle. Produced on an ‘as needed’ basis.
What is refeeding syndrome? How is it treated?
Malnourished patients given nutritional support, can often lead to hypophosphataemia (electrolyte abnormality) and ammonia toxicity, this is because the enzymes used in the urea cycle are down regulated so causes a sudden increase in ammonia.
Re-feed at 5-10 kcal/kg/day, raise gradually to full needs within a week.
What causes defects in the urea cycle? What does it lead to?
Autosomal recessive genetic disorder leads to deficiency in one enzymes in the urea cycle.
Results in hyperammonaemia and accumulation/ excretion of urea cycle intermediates.
Complete loss of enzyme is fatal.
What are the clinical symptoms of urea cycle defects? How is it managed?
Severity depends on nature of defect and amount of protein eaten. Severe defects show symptoms within one day, if untreated patient will die. Mild defects may not show symptoms until much later in childhood. Symptoms - vomiting - lethargy - irritability - mental retardation - seizures - coma Management--> low protein diet --> Replace AA with ketoacids
What is ammonia toxicity?
Build up of ammonia which is toxic to cells. Normally kept at 25-40 micromol/ litre.
At physiological pH (7.4) it is usually in the form of ammonium.
How is ammonia transported from the tissue to the liver/kidney? Explain each mechanism?
There are two mechanisms of transport.
Glutamine
- glutamate and ammonia (glutamine synthetase needed)–> glutamine
- taken to kidney or liver
- Cleaved by glutaminase to reform glutamate and ammonia
- In liver enters urea cycle
-In kidney directly excreted
Alanine
- Amine group transfer to glutamate by transamination, then transferred to pyruvate to form alanine
- Transported to liver
- Turned back into pyruvate (transamination - alanine aminotransferase)–> glutamate –> enter urea cycle
- pyruvate used to synthesis glucose
In both glutamate dehydrogenase converts glutamate to NH3 → Urea
What causes amino acid metabolism problems? What can it lead to? How is it detected?
Over 50 inherited disease in amino acid metabolism. It is rare but collectively as a group make a significant proportion of paediatric genetic diseases.
Clinical consequence is usually seen when it effects breakdown.
Usually only partial loss of function.
Can lead to intellectual impairment.
Detected by heel prick test?
How are clinical conditions caused by amino acid metabolism problems treated?
Remove specific amino acids from the diet.
What are some of the common disorders?
Phenylketonuria (PKU), Homocystinuria, Maple syrup urine disease, isovaleric acidaemia, glutaric aciduria.
What is phenylketonuria? How is it treated?
Autosomal recessive chromosome 12
Deficiency in Phenylalanine hydroxylase enzyme.
Inhibits conversion to tyrosine
Accumulation of phenylalanine.
Converted to phenylketones by transamination, excreted in urine, musty smell.
Treatment:
–> Low controlled phenylalanine diet
–> Tyrosine
–> Avoid protein rich food (eggs, milk, meat) and artificial sweeteners
What causes PKU? What are the symptoms?
Autosomal recessive disorder on chromosome 12.
Most common.
Severe intellectual disability, developmental delay, microcephaly, seizures and hypopigmentation
What is homocystinuria? What causes it? What are the symptoms?
Autosomal recessive
Excess Homocysteine or methionine
HomocystINE two homocysteine together–> oxidised form in the urINE
Deficiency in cystathionine beta-synthase–> unable to convert homocysteine to cysteine
Causes disorders of connective tissue, muscles, CNS and cardiovascular system.
What is homocystinuria often mistaken for and why?
Often mistaken as Marfans syndrome (defect in fibrillin 1 gene) because symptoms are similar.
Marfans genetic disorder of connective tissue
Neurological dysfunction seen in homcystinuria not in Marfans
How is homocystinuria treated?
Low methionine diet
Avoid milk, fish, meat, cheese, eggs, nuts and peanut butter
Cysteine, Vit B6, Betaine, B12 and folate supplement (to make remaining cystathionine B-synthase most effective)
