Nutrition: Digestion 2 - Protein Digestion

Question 1

How much protein does the gut receive per day from the diet?

Answer 1

70-100g protein per day

Question 2

In addition, how much endogenous protein is secreted by the gut and from epithelial cell turnover per day?

Answer 2

35-200g per day

Question 3

How much protein is lost per day?

What does this mean.

Answer 3

Only 6-12g per day

This means that digestion and absorption of proteins is very efficient.

Question 4

What are proteins hydrolysed by?

Answer 4

Enzymes called peptidases.

Question 5

What are the two different types of peptidases and what are their functions?

Answer 5

Endopeptidase: cleaves internal peptide bonds. It does this by breaking down large polypeptides into smaller oligopeptides.

Exopeptidase: cleaves from the ends of the protein or peptide. It does this by breaking down oligopeptides into di- and tripeptides and amino acids.

Question 6

What is the name of the exopeptidase that cleaves the carboxyl (-COOH) group?

Answer 6

Carboxypeptide

Question 7

What is the name of the exopeptidase that cleaves the amino (-NH2) group?

Answer 7

Aminopeptide.

Question 8

Draw the sequence of protein digestion.

Answer 8

protein > peptide > amino acid
dipeptide
tripeptide

Question 9

Where does protein digestion begin?

Answer 9

In the stomach.

Question 10

What are the first 4 stages of protein digestion?

Answer 10

1. HCl is secreted. This reduces the pH to 1-2
2. The reduced pH causes protein denaturation.
3. This unfolding makes the protein more accessible to the digestive enzymes.
4. Chief cells of gastric mucosa secrete pepsinogen (the inactive precursor of pepsin).

Question 11

In %, how responsible is pepsin in the digestion of dietary protein.

Answer 11

Pepsin is responsible for the digestion of only 15% of dietary protein.

Question 12

Pepsinogen is already partially activated even before they are released by the chief cells of the gastric mucosa. What causes this initial partial activation?

Answer 12

Pepsinogen is initially (partially) activated by low pH (H+)

Question 13

What can active pepsin do?

Answer 13

It can autocatalyse the conversion of further molecules of pepsinogen to pepsin.

Question 14

What happens to the target protein?

Answer 14

The target protein is digested by pepsin into large peptide fragments and amino acids

Question 15

What is stimulated as a result of the target protein being digested by pepsin into large peptide fragments and amino acids?

Answer 15

it stimulates Cholecystokinin (CCK) release in the duodenum.

Question 16

What stimulates the secretion of the peptide hormone SECRETIN in the duodenum?

Answer 16

The acidity of the stomach contents entering the duodenum stimulates the secretion of secretin.

Question 17

What does secretin stimulate?

Answer 17

Secretin stimulates secretion of bicarbonate (HCO3-) and the secretion of alkaline bile and alkaline intestinal juice.

Question 18

What is bicarbonate?

Answer 18

A rich pancreatic fluid that neutralises and dilutes the acid in the duodenum

Question 19

What does Cholecystokinin stimulate?

And what does it cause?

Answer 19

Cholecystokinin stimulates release of the main digestive enzymes by the pancreas (as an enzyme rich pancreatic juice).

CCK causes contraction of the gall bladder and relaxation of the sphincter of Oddi, promoting entry of bile and pancreatic juices into the duodenum.