Nitrogen

Question 1

State the main nitrogen containing molecules in the body

Answer 1

Mainly found in amino acids, ammonia and urea within the body

Question 2

Protein digestion can be split into what three stages?

Answer 2

• gastric
• pancreatic
• intestinal

Question 3

What happens in the gastric stage of protein digestion?

Answer 3

Denaturation of proteins by HCl leaves them more open to the actions of pepsin

Question 4

What happens in the pancreatic stage of protein digestion?

Answer 4

Pancreatic enzymes create a mix of free amino acids and short peptides around 2-8 units in length

Question 5

What happens in the intestinal stage of protein digestion?

Answer 5

• free amino acids are absorbed into portal system

- di/tri peptides are also absorbed and broken down to free amino acids in the enterocytes of the brush border

Question 6

What is the only amino acid that can obtain its nitrogen from ammonia?

Answer 6

Glutamate

Question 7

Where do all other amino acids obtain their nitrogen?

Answer 7

from pre-existing amino acids through transaminase reactions, involving the interconversion of a pair of amino acids and a pair of keto acids

Question 8

Each transaminase enzyme is specific for

Answer 8

only one pair of amino acids and keto acids

Question 9

Once broken down into individual amino acids, what can happen to the remaining carbon skeletons?

Answer 9

Remaining carbon skeletons can be further catabolised into intermediaries for the citric acid cycle and glycolysis e.g. alanine -> pyruvate, aspartate -> oxaloacetate

Question 10

These intermediates can be converted into ___ via ___

Answer 10

glucose

gluconeogenesis

Question 11

What two fates are there for ketogenic amino acids?

Answer 11

• catabolised for energy in the CAC

- used to form ketone bodies

Question 12

How is nitrogen transferred in the body?

Answer 12

Using glutamine and alanine

Question 13

Nitrogen created from catabolised protein in muscles can be transported

Answer 13

back to the liver as alanine, where it is built up into glucose once more

Question 14

The process of alanine -> glucose in the liver creates

Answer 14

ammonia and urea in the liver and also requires the use of the transferase enzymes to convert alanine into pyruvate or vice versa

Question 15

Give three reasons why nitrogen is transported as glutamine and alanine instead of glutamate

Answer 15

• glutamate has a net negative charge so transporting it would require an accompanying cation
• this charge also means it does not pass readily through membranes
• alanine and glutamine bear no net charge

Question 16

How is urea formed?

Answer 16

From the ammonia that comes from several of the reactions involving amino acids

Question 17

How is ammonia formed?

Answer 17

Through oxidative deamination whereby glutamate loses its nitrogen as ammonia, this is then fed into the urea cycle where it is converted to urea