Amino Acid Metabolism Flashcards
What is the monomer of proteins ?
Amino acids
How are amino acids generated ?
They are generated by the digestion of proteins in the intestine and degradation of cellular proteins
Can excess amino acids be stored ?
Excess amino acids cannot be stored
Describe amino acid excretion
They are not directly excreted, the carbon skeleton is converted to central metabolic intermediates and the amino group is excreted
Describe the digestion and absorption of proteins
- protein digestion begins in the stomach as the acidic environment denatures proteins and pepsin degrades proteins
- further degradation takes place in the lumen of the intestine by pancreatic proteases and peptidases e.g. trypsin and aminopeptidases
Describe what happens to the amino acid pool in cells
- some amino acids are converted into proteins
- some amino acids are converted into nitrogen compounds
- surplus amino acids are split into the nitrogen amino group which is excreted and the carbon skeleton which is a fuel for energy
Describe events which require increased protein
during periods of growth :
- child
- adolescence
- pregnancy and lactation
during times of illness :
- trauma
- surgeries
- infection
Why are proteins needed in the oral cavity ?
- collagen : basis of tooth enamel, bone, dentine and cementum
- immunoglobulin A in saliva : resistance to infection
- mucin : prevent the formation of caries
- lysozymes : breakdown of food
What are the 2 types of amino acids ?
- essential amino acids must be supplied by the diet as they cannot be synthesised in the body
- non essential amino acids can be synthesised
Give some examples of essential amino acids
- arginine
- histidine
- methionine
Give some examples of non essential amino acids
- glutamate
- alanine
- asparagine
- proline
How is the amino group separated from glutamate ?
Glutamate + water + NADP+ > 2-oxoglutarate + NADPH + NH4+
- glutamate is oxidised and NADP+ is reduced
- this is done by the enzyme glutamate dehydrogenase
How is the amino group removed from other amino acids ?
- the amino group is transferred to 2-oxoglutarate by a transaminate enzyme causing it to turn into glutamate
- the amino group is then removed from glutamate by glutamate dehydrogenase to give 2-oxoglutarate
- the amino acid turns into a keto acid
- there is a free NH4+ group but it is toxic
What happens to the free amino group ?
- aquatic carnivores e.g. sharks will secrete excess nitrogen as free NH4+ because there is abundant water to disperse the NH4+ fast : ammoniotelic
- birds and terrestrial reptiles have a limited water intake and so they want to minimise water loss and therefore secrete excess nitrogen as a paste of uric acid crystals : uricotelic
- most terrestrial vertebrates secrete excess nitrogen as soluble urea : ureotelic
What is urea ?
- a small molecule
- rich in nitrogen which allows for little waste of other materials
- non toxic so may be accumulated before excretion
- highly soluble so can be excreted via urine
- no ionic charge so can accumulate without ionic perturbation
- it is produced in the urea cycle
Why is the urea cycle needed ?
- urea is non toxic and neutral
- access ammonium is toxic : it shifts glutamate dehydrogenase in the favour if glutamate and this lowers the production of 2-oxoglutarate and so there is less ATP produced leading to a coma
What is the evidence for the urea cycle ?
- removing the liver stopped the appearance of urea in the urine
- a liver perfumed with nutrients produces urea
- the liver contains a urea producing enzyme : arginase which cleaves arginine into ornithine and urea
Where is urea synthesised ?
In the liver
Describe the urea cycle
1) carbomoyl phosphate is produced in the mitochondrial matrix and this requires 1 ATP
2) Citrulline formation also takes place in the mitochondrial matrix
3) condensation of citrulline with aspartate takes place in the cytosol and this requires 1 ATP
4) cleavage of arginosuccinate will preserve the carbon skeleton of aspartate
5) arginine is hydrolysed to produce urea and ornithine
Describe the nitrogen routes in the urea cycle
Nitrogen can enter the urea cycle via 2 routes :
1) ammonia is generated from glutamate via glutamate dehydrogenase
2) a transaminase enzyme will transfer the amino group from glutamate to oxaloacetate and this is converted to aspartate
Describe how the urea cycle is regulated
- short term regulation occurs by allosteric regulation of the first enzyme in the pathway : carbamoyl phosphate synthase and this is done by N-acetylglutamate
- long term regulation depends on levels of synthesis of urea cycle enzymes : a high protein diet means it is unregulated and a low protein diet means it is downregulated
How are amino acids used as an energy source in metabolism ?
The amino acids undergo catabolism and then act as 7 intermediates in or around the citric acid cycle
What are ketogenic amino acids ?
The carbon skeleton of ketogenic amino acids are turned into ketone bodies, Acetyl CoA or fats
What are glucogenic amino acids ?
The carbon skeleton is turned into glucose
Which 2 amino acids are only ketogenic ?
- Leucine
- lysine
