GIS14 Digestive Enzymes

Question 1

Digestion of carbohydrates by enzymes

Answer 1

Sequential digestion: Saliva —> Pancreatic —> Brush border

Amylase:

• ***Salivary α-amylase: glycogen / starch —> α-dextrins
• ***Pancreatic α-amylase: α-dextrins —> tri/oligosaccharide, maltose, isomaltose
• **Disaccharidases:
• Maltase, Isomaltase: maltose, isomaltose —> glucose
• Sucrase: sucrose —> glucose + fructose
• Lactase: lactose —> glucose + galactose

—> products absorbed ***transcellular

1. Amylase: cleaves ***α1-4 glycosidic linkages (intra-branch of glycogen / starch)
2. Isomaltase: cleaves ***α1-6 glycosidic linkages in α-limit dextrin (inter-branch)
3. Disaccharidases:
   - produced by **intestinal epithelial cells
   - associated / inserted in the **membrane with active site facing intestinal lumen
   - deficiency:
   —> lactase, sucrase, isomaltase: due to low synthesis rate / mutated genes
   —> generalised: epithelium of small intestine is damaged

Question 2

Defective digestion / absorption of carbohydrate

Answer 2

Symptoms:

• abdominal distension / cramps
• copious flatus / hyperactive bowel sounds
• explosive diarrhoea

Causes:
1. partially digested carbohydrate
—> **potent osmotic load
—> water and electrolytes enter gut lumen
2. lower GI tract bacteria digest and ferment partially digested carbohydrate to
—> volatile short chain organic acids
—> **gases (hydrogen, methane, CO2)

Question 3

Digestion of protein

Answer 3

1. Luminal digestion:

• **Gastric phase:
• ***Pepsin (主角): protein —> peptide
• **Pancreatic phase:
• ***Trypsin (主角)
• Chymotrypsin
• Elastase
• Carboxypeptidase A, B
• Aminopeptidase: peptide —> di/tri-peptide + amino acids

2. Brushborder / 3. Intracellular digestion:
   - Endopeptidase
   - Aminopeptidase
   - Di/tri-peptidases (intracellular): di/tri-peptide —> amino acids

Question 4

Gastric phase

Answer 4

1. Autoactivation of Pepsinogen (burst of pepsin activity at pH<2)
   Pepsinogen (with inhibited catalytic domain)

— (pH<5) —> unfolding, ***exposing catalytic site
—> Pepsinogen (uninhibited catalytic domain)

— (pH<2) —> ***Autolytic activation (exposed catalytic site cleaves itself)
—> Pepsin
—> Catalytic activation (cleaves other pepsinogen)
—> more Pepsin formed

2. Resulting peptide from Pepsin
   —> stimulants for Pancreatic phase
   —> stimulating secretion of **Cholecystokinin (CCK) and **Secretin
   —> promote ***Trypsin activity

Question 5

Pancreatic phase

Answer 5

1. Proenzymes activated in intestinal lumen: (***neutral pH optimal for enzymes)
   * **Enteropeptidase (within duodenum): Trypsinogen —> Trypsin

Pathological condition:
autoactivation of Trypsinogen to Trypsin within pancreas
—> autodigestion of pancreas
—> pancreatitis
—> can be prevented by
- SPINK1 (serine protease inhibitor Kazan-type 1): inhibit Trypsin
- CTRC (chymotrypsin-C): breakdown Trypsin, Trypsinogen

2. ***Trypsin:
   - Convert more Trypsinogen —> Trypsin
   - Chymotrypsinogen —> Chymotrypsin
   - Proelastase —> Elastase
   - Procarboxypeptidase —> Carboxypeptidase A, B

Question 6

Endopeptidases

Answer 6

Cleaves ***internal peptide bond

1. Pepsin
   - Aromatic a.a: Phe, Tyr
   - ***Acidic a.a.: Glu, Asp
2. Trypsin
   - ***Basic a.a.: Lys, Arg
3. Chymotrypsin
   - Aromatic a.a.: Phe, Tyr
   - ***Neutral branched a.a.: Trp, Leu
4. Elastase
   - ***Neutral unbranched a.a.: Ala, Gly, Ser

Question 7

Exopeptidases

Answer 7

Cleaves ***terminal peptide bonds

1. Carboxypeptidases (cleaves C-terminus):
   - A: Hydrophobic
   - B: Basic a.a.: Arg, Lys
2. Aminopeptidases (cleaves N-terminus)

Question 8

Digestion of fat

Answer 8

90% of dietary fat is TAG
10% cholesterol-esters, phospholipids, free fatty acid

1. ***Emulsification by bile salt —> micelles
2. Digestion by ***Pancreatic lipase: TAG —> free fatty acid, mono/diacylglycerol
3. Transcellular absorption —> free fatty acid + 2MG —> recombined to TAG —> ***chylomicron in lymphatics —> bloodstream

Question 9

Absorption of dietary cholesterol

Answer 9

Not examined

Question 10

Lipid-digesting enzymes

Answer 10

1. **Acid-stable lipase
   - from glands at back of tongue, act in stomach
   - substrate: TAG
   - **limited action at water-lipid interface
2. **Pancreatic enzyme (action at **water-lipid interface)

• Lipase
  —> substrate: TAG and DAG
  —> ***inhibited by bile salt
  —> co-lipase, co-localised with lipase at the emulsified droplets, overcomes the inhibition by bile salt
• Lipid esterase
  —> substrates: cholesterol ester, monoglyceride, esters of vitamin A
  —> ***require bile salt for activity
• Phospholipase A2
  —> ***require bile salt for activity

Question 11

Digestion of protein, carbohydrate and fat involves multiple enzymes

Answer 11

1. synthesized, secreted and activated at ***different sites in the GI tract
2. act ***sequentially
3. differ in **acid-stability and **target specificity
4. synthesized by exocrine cells as ***pro-enzymes, requiring activation by enzyme-catalysed cleavage