Post-absorption processing of proteins Flashcards
What does the human body do with amino acids?
Protein synthesis, glucose/glycogen, energy (ATP), fatty acids, ketone bodies, nitrogen-containing metabolites.
What can histidine be synthesised to?
Histamine
Which amino acid is used in the synthesis of serotonin?
Tryptophan
Which amino acid is used in the synthesis of dopamine, noradrenaline and adrenaline?
Tyrosine
For what compound are both glycine and arginine required?
Creatinine
What is Kwashiorkor?
A form of severe protein–energy malnutrition characterized by edema, irritability, anorexia, ulcerating dermatoses, and an enlarged liver with fatty infiltrates.
What is the aetiology of kwashiorkor?
Extreme lack of protein causes an osmotic imbalance in the gastro-intestinal system causing ascites and fatty liver. Exchange between lymphatic system and bloodstream is stunted due to inability of the body to effectively overcome the hydrostatic pressure gradient. Proteins are responsible for creating the colloid osmotic pressure (COP) observed in the blood and tissue fluids. The difference in the COP of the blood and tissue is called the oncotic pressure. The oncotic pressure is in direct opposition with the hydrostatic pressure and tends to draw water back into the capillary by osmosis. However, due to the lack of proteins, no substantial pressure gradient can be established to draw fluids from the tissue back into the blood stream. This results in the pooling of fluids, causing the swelling and distention of the abdomen.
What waste metabolite is produced from energy reactions involving proteins?
Ammonia
What are the symptoms of hyperammonaemia?
Tremor, vomiting, cerebral oedema, coma and death.
What can cause hyperammonaemia?
Genetic defects, liver disease etc..
Where does disposal of ammonia mainly occur?
Liver
What are the three steps in the ammonia disposal pathway?
Transamination, deamination and urea synthesis
What is transamination?
Transfer of amino group from AA to ketoglutarate forming glutamate (in most tissues).
What is deamination?
Release of ammonia from glutamate (mainly in the liver).
How is urea synthesised?
Via the urea cycle in the liver.
How is ammonia transported safely to the liver?
As glutamate or glutamine.
What happens once glutamate/glutamine reaches the liver?
It is deaminated and ammonia released.
What other product is released in the transaminase reaction?
Keto acid
Aside from ammonia, what other products are released from the deamination reaction?
NADPH (NADP required) and alpha-ketoglutarate
What is pyridoxal phosphate the active form of?
Vit B6
Aside from transamination, what else is Vit B6 required for?
Decarboxylation reactions required for neurotransmitter synthesis, haem synthesis, some aspects of energy metabolism and lipid synthesis.
What does lack of B6 lead to?
Anaemia (lack of haem), neurological symptoms (lack of neurotransmitters), poor growth, skin lesions, poor immune response (lack of protein synthesis).
Where does the urea cycle take place?
Inside the mitochondrion and the cytosol.
What is the control step in the urea cycle?
Carbamoyl group transferred to ornithine to form citrulline (inside the mitochondria). This the entry point to the urea cycle.
What urea cycle reactions occur in the cytosol?
Second amino group added from aspartate - arginine formed and urea released.
How is carbamoyl phosphate synthetase activated?
Allosterically - by N-acetylglutamate, which is formed when glutamate levels are high (ie signalling high levels of protein catabolism).
What activates the urea cycle?
Protein catabolism
From what is uric acid derived?
Purine nucleotides
What can excess uric acid in the blood (hyperuricaemia) lead to?
Deposition of sodium urate crystals in kidneys. Gout.
What is the aetiology of hyperuricaemia?
Either overproduction of urate or poor excretion via kidneys.
Given examples of high purine foods.
Red meat, seafood, yeast-containing foods inc. beer.
What might the carbon skeletons of AAs be used for?
Kreb’s cycle, fatty acid and ketone synthesis, gluconeogenesis.
Which amino acids cannot be made into glucose?
Lysine and leucine (ketogenic only).
What is a glucogenic amino acid?
Can be degraded to glucose precursors.
What is a ketogenic amino acid?
Can be degraded to precursors of fatty acids and ketone bodies.
What is gluconeogenesis and where does it mainly take place?
Making glucose from non-carbohydrate intermediates during fasting or exercise. Main site is the liver.
What are the possible substrates for gluconeogenesis?
Keto acids (derived from AAs), lactate (from anaerobic glycolysis) and glycerol (from triacylglycerol breakdown).
How long do glycogen stores last?
18 hours
What are the three main reactions in gluconeogenesis?
Pyruvate -> phosphoenolpyruvate (via several steps)
Fructose-1,6-bisphosphate -> fructose-6-phosphate
Glucose-6-phosphate -> glucose
How many ATP molecules are required for gluconeogenesis?
6
How is gluconeogenesis controlled?
Low [glucose] and/or low energy = gluconeogenesis
What is the Cori cycle?
Converts lactate from rapidly respiring muscle back to glucose.
2lactate -> 2pyruvate -> glucose
Requires 6 ATP
What is the function of the glucose - alanine cycle?
Sustains transamination in peripheral tissue.
How does alcohol inhibit gluconeogenesis?
Ethanol metabolism produces large amounts of NADH. Tends to cause conversion of pyruvate and alanine to lactate by shifting the position of equilibrium. This prevents the conversion of gyceraldehyde-3-phosphate to glucose.
