Non-ruminant protein metabolism

Question 1

What type of digestive reaction takes place in the stomach?

Answer 1

Hydrolysis

Question 2

Describe the digestive processes occurring in the stomach

Answer 2

1. Ingesta arrives in stomach
2. Parietal cells secrete HCl. H+ pumped through parietal membrane to gland lumen by H+-K+ ATPase
3. Stomach pH lowered to 2-2.5
4. dietary protein denatured by low pH, making it more accessible to protease activity
5. Chief cells secrete pepsinogen
6. On contact with acidic stomach content, pepsinogen is autoactivated and cleaved to pepsin
7. Pepsin degrades protein to peptides by braking peptide bonds
8. Free peptides stimulate HCl secretion

Question 3

Describe the digestive process that occurs in the duodenum

Answer 3

1. Conc. of peptides and AA in duodenum increases stimualting duodenum to secrete cholecystokinin
2. Cholecystokinin stimulates pancreas to increase its enzyme secretion of inactive zymogens
3. Zymogens become activated and degrade peptides further
4. Pancrease also secretes bicarbonate which neutralises acidic chyme entering duodenum
5. H2CO3, free AA, di- and tripeptides are produced

Question 4

Describe the digestive process that occurs in the small intestine

Answer 4

1. Membrane bound enzymes in villi digest di-, tri- and oligopeptides
2. End products are free AA, di- and tripeptides
3. These are absorbed by enterocytes by secondary active transport
4. Na+ dependant membrane symporters mediate peptide uptake at brush border
5. For every peptide pumped across, 1 Na+ is pumped out of lumen as well
6. AA diffuse into portal circulation down their concentration gradient and are uptaken by hepatocytes

Question 5

What process occurs in hepatocytes?

Answer 5

Deamination

Question 6

Name the enzymes that are involved in deamination

Answer 6

1. Transaminase
2. Glutamate dehdrogenase and glutamate synthetase
3. Glutaminase

Question 7

Describe the processes that occur in the urea cycle

Answer 7

1. Carbomyl phosphate synthesised from ammonium
2. Carbomyl phosphate reacts with ornithine to form citrulline
3. Citrulline reacts with aspartate to form arginosuccinate
4. Aginosuccinate cleaved to form arginine and fumarate
5. Arginine cleaved to form ornithine and urea
6. Ornithine trasnported back to mitochondria to continue cycle, urea diffuses into blood and travels to the kidneys