Protein digestion

Question 1

Name the proenzyme secreted by zymogen (chief) cells in the stomach, and the enzyme it is activated to.

Answer 1

Pepsinogen, activated by hydrochloric acid to for the proteolytic enzyme pepsin.

Question 2

What is the function of gastric acid?

Answer 2

To kill any potentially pathogenic microorganisms, and to denature proteins.

Question 3

What is the pH above which healing can occur?

Answer 3

pH 3.

Question 4

What does pepsin break protein down into?

Answer 4

Large polypeptides.

Question 5

Name three proenzymes secreted by the pancreas.

Answer 5

Trypsinogen, chymotrypsinogen, procarboxypeptidase.

Question 6

Name the integral enzyme on the enterocytes which begins the activation of trypsinogen to trypsin.

Answer 6

Enteropeptidase.

Question 7

What converts chymotrypsinogen, trypsin, and procarboxypeptidase to their active proteolytic enzyme form?

Answer 7

Trypsin.

Question 8

Which terminal does carboxypeptidase hydrolyse the polypeptide from?

Answer 8

The carboxyl terminal.

Question 9

How are the large polypeptides absorbed into the enterocyte?

Answer 9

They’re hydrolysed to dipeptides and free amino acids by peptidases on the enterocyte brush border.

Question 10

How are the dipeptides absorbed?

Answer 10

They move into the enterocyte through a cotransporter with hydrogen ions, then they are hydrolysed to free amino acids by cytoplasmic peptidases, then absorbed into the blood.

Question 11

How are free amino acids absorbed?

Answer 11

They move into the enterocytes through a cotransporter with sodium ions. The influx of sodium ions also causes water to be absorbed, and the sodium ions move out of the enterocyte through a sodium-potassium ATPase pump.

Question 12

What does COX 1 enzyme do?

Answer 12

Produces prostaglandins which stimulate secretion of mucus and bicarbonate and inhibit secretion of hydrochloric acid to protect the stomach lining.

Question 13

Why were H2 antagonists superseded by PPIs in the treatment of gastric acid hypersecretion?

Answer 13

PPIs have a longer lasting effect due to them irreversibly binding to and blocking the K+/H+ ATPase, so can be taken once daily instead of more frequently.
PPIs have fewer side effects.
PPIs have no tachyphylaxis, so don’t require a higher dose over time.

Question 14

Where are lipid micelles absorbed?

Answer 14

The jejunum.

Question 15

Where is water absorbed?

Answer 15

The duodenum, the jejuno-ileum, and the caecum.

Question 16

Where is the hormone cholecystokinin (CCK) secreted and what is its function?

Answer 16

CCK is secreted in the crypts of Lieberkühn in the duodenum, in response to chyme containing amino acids, triglycerides and fatty acids entering the duodenum.
CCK triggers relaxation of the sphincter of Oddi, and ejection of bile and secretion of pancreatic juice, and a feeling of satiety.

Question 17

Where is pepsinogen secreted, and what is its function?

Answer 17

Pepsinogen is secreted by chief (zymogen) cells in the stomach, and its secretion can be triggered by gastrin and by the vagus nerve. It is activated by HCl to pepsin which digests proteins.

Question 18

Where is secretin secreted and what is its function?

Answer 18

Enteroendocrine S cells in the crypts of Lieberkühn in the duodenum. Secreted in response to acid containing bile, and triggers the secretion of pancreatic juice and bile which is rich in bicarbonate ions to neutralise the acid. Secretin and CCK work together to enhance each other’s effect.

Question 19

Where is somatostatin secreted, and what are its functions?

Answer 19

Somatostatin is secreted by D cells in the pancreatic Islets of Langerhans, and by the stomach and intestines.
Somatostatin:
1) Suppresses the release of gastrin, CCK, secretin, motilin, GIP and other hormones.
2) Reduces gastric emptying.
3) Suppresses the release of pancreatic hormones.
4) Suppresses the exocrine secretory action of the pancreas.

Question 20

Where is trypsinogen secreted and what are its functions?

Answer 20

Trypsinogen is secreted by the acini of the pancreas. In the duodenum and jejunum a part of the molecule is cleaved off by enteropeptidase to form trypsin. Trypsin digests polypeptides and also activates the peptidase precursors (trypsinogen, chymotrypsinogen, procarboxypeptidase).

Question 21

How is gastric acid produced by parietal cells?

Answer 21

1) carbon dioxide and water form carbonic acid in the cytosol of parietal cells in a reaction catalysed by carbonic anhydrase
2) the carbonic acid dissociates into H+ and HCO3-
3) HCO3- leaves the parietal cell through HCO3-/Cl- antiporters in the basolateral membrane, and Cl- is brought into the cell
4) HCO3- diffuses into submucosal blood vessels and the pH of blood is increased after a meal (alkaline tide)
5) Cl- diffuses into the stomach lumen through Cl- channels in the apical membrane
6) H+/K+ ATPase pumps H+ into the stomach lumen, and K+ leaves the parietal cell again through K+ channels on the apical surface

Question 22

What is it called when the blood pH increases after a meal due to HCO3- diffusion into submucosal blood capillaries in exchange for Cl-?

Answer 22

Alkaline tide.

Question 23

Which 3 things stimulate gastric acid production?

Answer 23

Acetylcholine secreted by parasympathetic neurones
Histamine secreted by enterochromaffin-like cells
Gastrin secreted by G cells in the antrum

Question 24

What binds to CCK2 receptors on enterochromaffin-like cells to stimulate histamine release?

Answer 24

Gastrin.

Question 25

What are the 4 functions of gastric acid?

Answer 25

Kills any pathogens
Stimulates secretion of hormones that promote flow of bile and pancreatic juice
Activates pepsin from pepsinogen
Denatures and unfolds proteins

Question 26

Which receptor does gastrin bind to?

Answer 26

CCK2

Question 27

Name three types of drugs used to treat too much gastric acid secretion.

Answer 27

Histamine H2 receptor antagonists (end in -idine)
Proton pump inhibitors (end in -prazole)
Antacids

Question 28

Which of the cyclo-oxygenase isozymes constitutively produces protective prostaglandins in the stomach?

Answer 28

COX-1