Amino Acid Metabolism

Question 1

What is the monomer of proteins ?

Answer 1

Amino acids

Question 2

How are amino acids generated ?

Answer 2

They are generated by the digestion of proteins in the intestine and degradation of cellular proteins

Question 3

Can excess amino acids be stored ?

Answer 3

Excess amino acids cannot be stored

Question 4

Describe amino acid excretion

Answer 4

They are not directly excreted, the carbon skeleton is converted to central metabolic intermediates and the amino group is excreted

Question 5

Describe the digestion and absorption of proteins

Answer 5

• 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

Question 6

Describe what happens to the amino acid pool in cells

Answer 6

• 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

Question 7

Describe events which require increased protein

Answer 7

during periods of growth :
- child
- adolescence
- pregnancy and lactation

during times of illness :
- trauma
- surgeries
- infection

Question 8

Why are proteins needed in the oral cavity ?

Answer 8

• 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

Question 9

What are the 2 types of amino acids ?

Answer 9

• essential amino acids must be supplied by the diet as they cannot be synthesised in the body
• non essential amino acids can be synthesised

Question 10

Give some examples of essential amino acids

Answer 10

• arginine
• histidine
• methionine

Question 11

Give some examples of non essential amino acids

Answer 11

• glutamate
• alanine
• asparagine
• proline

Question 12

How is the amino group separated from glutamate ?

Answer 12

Glutamate + water + NADP+ > 2-oxoglutarate + NADPH + NH4+

• glutamate is oxidised and NADP+ is reduced
• this is done by the enzyme glutamate dehydrogenase

Question 13

How is the amino group removed from other amino acids ?

Answer 13

• 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

Question 14

What happens to the free amino group ?

Answer 14

• 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

Question 15

What is urea ?

Answer 15

• 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

Question 16

Why is the urea cycle needed ?

Answer 16

• 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

Question 17

What is the evidence for the urea cycle ?

Answer 17

• 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

Question 18

Where is urea synthesised ?

Answer 18

In the liver

Question 19

Describe the urea cycle

Answer 19

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

Question 20

Describe the nitrogen routes in the urea cycle

Answer 20

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

Question 21

Describe how the urea cycle is regulated

Answer 21

• 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

Question 22

How are amino acids used as an energy source in metabolism ?

Answer 22

The amino acids undergo catabolism and then act as 7 intermediates in or around the citric acid cycle

Question 23

What are ketogenic amino acids ?

Answer 23

The carbon skeleton of ketogenic amino acids are turned into ketone bodies, Acetyl CoA or fats

Question 24

What are glucogenic amino acids ?

Answer 24

The carbon skeleton is turned into glucose

Question 25

Which 2 amino acids are only ketogenic ?

Answer 25

• Leucine
• lysine