4.2 Protein Metabolism

Question 1

Describe how proteins are digested in the mouth, stomach, and small intestine

Answer 1

Mouth: chewing (mechanical)

Stomach: acidic environment (HCl), pepsin (from pepsinogen, which is secreted by gastric chief cells and activated by acid)

Small intestine: other proteases such as trypsin and chymostripsin

Question 2

Where are proteins absorbed in the GI tract? Do we absorb full polypeptides?

Answer 2

• Absorbed in the small intestine
• Absorbed as amino acids

Question 3

What is nitrogen balance? How do we measure it? What might cause a positive or negative balance?

Answer 3

• Nitrogen balance is a measure of the total losses and intake of protein from the body
• Measured by quantifying oral protein intake, and measuring loss of urea in urine
• Positive balance might indicate growth (net synthesis of proteins), and negative balance might indicate malnutrition/starvation

Question 4

What are the two processes by which the body can remove the amine group from amino acids to obtain the carb skeleton? Which amino acid(s) use which processes?

Answer 4

• Deamination: remove the amine from the molecule as ammonia (only with glutamate; excited)
• Transamination; transport the amine group onto another molecule (all other amino acids use this; don’t want to waste it)

Question 5

What’s the difference between ketogenic and glucogenic amino acids?

Answer 5

• Ketogenic amino acids have carbohydrate structures that are converted into Acetyl-CoA during deamination
• Glucogenic AAs have carb skeletons that are converted into pyruvate, or other citric acid cycle intermediaries, which can be used int gluconeogenesis to produce glucose (hence glucogenic)

Question 6

How does the body deal with ammonium from the muscle vs from other tissues?

Answer 6

Other tissues: ammonium combined with glutamate to form glutamine, which is carried to liver

Muscle: ammonium converted to alanine, carried to liver

At the liver: amine group is removed again, and ammonium enters urea cycle