Protein Metabolism Flashcards
Explain nitrogen balance
When nitrogen intake = outtake
• Positive nitrogen balance - intake > output
○ Increase in total body protein
○ Normal state in growth and pregnancy or in adult recovering from malnutrition
• Negative nitrogen balance - intake < output
○ Net loss of body protein
○ Never normal - occurs in trauma, infection, malnutrition
How does nitrogen enter and exit the body
Enters mainly as protein
Leaves mainly as urea, but also creatinine, ammonia and uric acid
Some through protein loss in hair, skin, nail loss
Describe how amino acids are metabolised
• Stage 1 - in the GI tract, proteases and peptidases hydrolyse peptide bonds to released free amino acids
○ Amino acids absorbed into circulation and are used by tissue for protein synthesis and synthesis of various nitrogen-containing compounds (haem, purines, creatine)
○ Under hormonal control
§ Insulin and growth hormone increases protein synthesis and decreases protein degradation
□ Increase uptake into skeletal muscles, adipose tissue and liver
§ Glucocorticoids (cortisol) decrease protein synthesis and increase protein degradation
• Stage 2 - each amino acid in the protein has own pathway of catabolism
○ Most amino acids broken down in liver
○ Removal of amino group - converted to urea and excreted from body as urine
○ C-skeletons converted to pyruvate, acetyl CoA, succinate, a-ketoglutarate, fumarate, oxaloacetate
○ These products enter TCA cycle
What are the functions of amino acids
Protein synthesis and synthesis of N-compounds
Excess amino acid are stored or broken down
What are the process of amine group removal
Transamination and deamination
Outline transamination
○ Transamination - swap amine group with oxygen of keto acid
§ Amino acid 1 + keto acid 2 amino acid 2 + keto acid 1
§ Certain amino groups are able to deal with the amino groups and remove from body
§ Most use α-ketogluterate (keto acid 2) to funnel amino group to glutamate (amino acid 2)
§ Aspartate aminotransferase (AST) uses oxaloacetate to convert glutamate to aspartate
§ Alanine aminotransferase (ALT) uses α-ketogluterate to convert alanine to glutamate
§ Plasma ALT and AST levels measured routinely as part of liver function test
□ Levels high in conditions that cause cellular necrosis - viral hepatitis, autoimmune liver disease
§ Cortisol stimulates transaminase synthesis
§ All aminotransferases require coenzyme pyridoxal phosphate, which is a derivative of vitamin B6
Outline deamination
○ Deamination - liberates amino group as free ammonia
§ Mainly occurs in liver and kidney
□ Important in the liver as both disposal of amino acids and synthesis occurs - can control direction
§ L and D-amino acid oxidases convert amino acids to keto acids and ammonia
□ Keto acids can be utilised for energy
□ Deamination of dietary D-amino acids needed as only functional in plants and bacteria
§ Ammonia very toxic and must be removed, converted to urea
What is the common symptom, treatment and test for amino acid metabolism disorders
○ Frequently lead to intellectual impairment (mental disability)
○ Treatment involves restricting specific amino acids in diet
○ Heel prick test - sample of blood screened for genetic diseases including sickle cell, cystic fibrosis, congenital hypothyroidism
§ Important as after eaten in diet over time, problems irreversible
Outline the principles of phenylketonuria (PKU), including symptoms, test and treatment
○ Most common inborn error of amino acid metabolism
○ Deficiency in phenylalanine hydroxylase - important in tyrosine synthesis from phenylalanine
○ Autosomal recessive
○ Accumulation of phenylalanine in tissue, plasma and urine
○ Phenylketonuria in urine - musty smell
○ Test by measuring phenylalanine content in blood (heel prick test) and urine
○ Affected pathways involving tyrosine - noradrenaline, adrenaline, dopamine, melanin, thyroid, protein synthesis
○ Symptoms - severe intellectual disability, developmental delay, seizures, hypopigmentation
§ Phenylpyruvate inhibits pyruvate uptake into mitochondria and interferes with energy metabolism in brain
○ Treatment - strictly controlled phenylalanine diet, avoid artificial sweeteners, avoid high protein food
Outline the principles of homocystinuria
○ Problem breaking down methionine
○ Type 1 - defect in cystathionine ß-synthase (CBS) enzyme, which converts homocysteine to cystathionine and then to cysteine
§ Excess homocysteine excreted in urine
§ Some homocysteine converted to methionine
○ Autosomal recessive disorders
○ Test - high methionine and homocysteine in plasma and homocystine in urine
○ Affects connective tissue, muscles, CNS, CVS - similar to Marfan’s syndrome
○ Elevated plasma homocysteine associated with cardiovascular disease
○ Treatment - low methionine diet, avoid high protein food
Outline the effects of ammonia toxicity
○ Readily diffusion and extremely toxic to brain
○ Interfere with amino acid transport and protein synthesis
○ Disruption of cerebral blood flow
○ pH effects (alkaline)
§ Interfere with neurotransmitter release and synthesis
○ Alteration of blood-brain barrier
○ Interference with TCA cycle
§ Ammonia reacts with a-ketoglutarate to form glutamate via glutamate dehydrogenase
§ Removes a-ketoglutarate from TCA cycle - slows and disrupts energy supply to brain
○ Symptoms - blurred vision, tremors, slurred speech, coma, death
How is ammonia removed
Detoxified through synthesis of glutamine or alanine, conversion to urea or directly secreted in urine
Outline how glutamine synthesis is used in ammonia detoxification
○ Glutamine - ammonia combined with glutamate to form glutamine via 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, kidney secreted as urine
Outline how alanine synthesis is used in ammonia detoxification
○ Alanine - ammonia combined with pyruvate 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
§ Pyruvate used in synthesis of glucose
Outline the urea cycle and how it links with refeeding syndrome
○ Glutamate and aspartate regularly feed into cycle
○ Glutamate dehydrogenase converts glutamate using CO2 from Krebs cycle to ammonia
○ Allow amino groups of amino acids to be safely disposed of as urea
§ Non-toxic, metabolically inert, high nitrogen-content
○ Urea is synthesised in the liver by the urea cycle and transported via the blood to kidneys for excretion
○ Has 5 enzymes - amount of enzymes relate to need to dispose of ammonia
§ High protein diet upregulates enzymes, low protein diet or starvation represses levels
§ Not subject to end product inhibition - need to dispose of all ammonia as urea
○ For severely malnourished people, when they eat lots of protein the enzymes have been downregulated so much that it cannot process the ammonia
§ Refeeding syndrome
