L14- Biochem (digestive enzymes) Flashcards
Digestive enzymes for carbohydrates along GIT
Mouth Cavity:
- salivary α-amylase (secreted by salivary glands)
Stomach:
- none
Small Intestines:
- pancreatic α-amylase (secreted by pancreas)
- Brush border enzymes (Produced by intestinal epithelial cells, associated/ inserted in the membrane):
- Isomaltase
- Disaccharidases (maltase, sucrase, lactase)
General flow of carbohydrate digestion
Dietary carbohydrates (e.g. starch, lactose, sucrose) undergo
Sequential digestion, becoming
Monosaccharides (glucose, fructose, galactose), which are then absorpted through
Transcellular absorption
Starch will have the α1-4 glycosidic linkages cleaved by salivary and pancreatic α-amylase
Maltose will be broken down to 2 glucose by maltase
α-limit dextrin will be broken down to 2 maltose by Isomaltase
Sucrose will be broken down to 1 glucose, 1 fructose by brush border sucrase
Lactose will be broken down to 1 glucose, 1 galactose by brush border lactase
Hexagon Sugar Ring formation
In glucose, the aldehyde group linked to the electron-deficient C1 carbon atom will react and link with the oxygen atom in hydroxyl group of C5 carbon atom, giving rise to the hexagonal ring structure
Glycosidic linkages between glucose
α (1→4) glycosidic linkage
- between C1 and C4 carbon atoms
- more common glycosidic linkage
- exists in starch, maltose, maltotriose
- hydrolyzed by α amylase
α (1→6) glycosidic linkage
- between C1 and C6 carbon atoms
- less common glycosidic linkage
- exists in starch, α-limit dextrin
- hydrolyzed by isomaltase
α-limit dextrin
- Contains two α (1→4) glycosidic linkages and one α (1→6) glycosidic linkage
- a product of amylopectin digestion that retains its 1-6 linkage
- α (1→6) glycosidic linkage cleaved by isomaltase in brush border of intestinal villi cells
Isomaltase
- Cleaves α (1→6) glycosidic linkage in α-limit dextrin
- Producing 2 maltose after digestion
- Produced by intestinal epithelial cells, associated/ inserted in the membrane
Disaccharidases
- maltase, sucrase, lactase
- Produced by intestinal epithelial cells, associated/inserted in the membrane with active site facing the intestinal lumen
Deficiency in brush border enzymes
1) lactase, sucrase and isomaltase deficiency
- due to low synthesis rates of lactase, sucrase and isomaltase
- due to mutated genes
- lactase deficiency in these cases lead to lactose intolerance
2) Generalized Deficiency
- when epithelium of small intestine is damaged/ under repair
carbohydrate ingestion consequences & explanations
1) Abdominal distension and cramps
2) Copious flatus and hyperactive bowel sounds
3) Explosive diarrhea
because:
1) Partially digested carbohydrate represents a potent osmotic load, leading to water and electrolytes enter the gut lumen (diarrhea)
2) In the lower GI tract, bacteria digest and ferment the partial digest to:
- volatile short-chain organic acids (that stimulates the intestinal wall
- gases (H2, CH4, CO2), leading to flatus
Digestive enzymes of protein along GIT
Stomach:
- pepsin (from pepsinogen)
Small Intestine:
- Pancreatic proteases:
- trypsin (from trypsinogen)
- chymotrypsin (from chymotrypsinogen)
- elastase (from proelastase)
- Carboxypeptidase A (from Procarboxypeptidase A)
- Carboxypeptidase B (from Procarboxypeptidase B)
- Brush border/intracellular protease (from intestinal epithelial cells/ crypts of Lieberkuhn)
- aminopeptidase
- dipeptidase
- tripeptidase
General flow of protein digestion
Dietary proteins undergo
Lumenal digestion (first gastric phase in stomach then pancreatic phase in small intestine) and then undergo
Brush border/ Intracellular digestion
Gastric Phase of protein digestion
1) Chief cells in gastric glands release pepsinogen
2) When pH>5, catalytic domain of pepsinogen remains inhibited (by the additional amino acid chain at N-terminus)
3) When pH decreases to below 5 (due to gastric acid released by parietal cells), pepsinogen will unfold and the catalytic domain of pepsinogen will become uninhibitated (by the additional amino acid chain at N-terminus)
4) Around pH < 2, pepsinogen will undergo autolytic activation and cleave itself, removing the additional amino acid chain at N-terminus, thus becoming pepsin
5) Catalytic activation: activated pepsin can then cleave the other pepsinogen, removing the additional amino acid chain at N-terminus, thus producing more activated pepsin
6) Pepsin will then degrade food proteins into peptides (at aromatic/acidic AA e.g. Phe, Tyr, Glu, Asp)
7) Resulting peptides act as stimulants for the pancreatic phase (by stimulating secretion of cholecystokinin (cck) and secretin which promote trypsin activity)
** Thus there will be a burst of pepsin activity if pH<2 due to autolytic activation and the cascade of catalytic activation of pepsin
How does gasric phase of proteolysis stimulates pancreatic phase
Resulting peptides formed in gastric phase act as stimulants for the pancreatic phase, by stimulating secretion of cholecystokinin (cck) and secretin which promote trypsin activity
Pancreatic Phase of proteolysis
In the small intestine:
1) Pancreas release the proenzymes:
- Trypsinogen
- Chymotrypsinogen
- Proelastase
- Procarboxypeptidase A & B
2) The crypts of Lieberkühn releases:
- Dipeptidase (brush border or intracellular)
- Tripeptidase (brush border or intracellular)
- Aminopeptidase (brush border)
- Enteropeptidase aka enterokinase (brush border)
3) Enteropeptidase will cleave trypsinogen, activating trypsin through catalytic activation
4) Trypsin will then perform catalytic activation on trypsinogen, chymotrypsinogen, proelastase, procarboxycarbonase A & B, yielding more trypsin, chymotrypsin, elastase, carboxypeptidase A & B
5) Endopeptidases (i.e. trypsin, chymotrpsin, elastase) will then cleave internal peptide bonds while exopeptidase (i.e. carboxypeptidase A & B) will cleave C-terminus peptide bond
6) These processes results in 40% free amino acids and 60% oligopeptides
Intestinal epithelium digestion in proteolysis
1) The 40% free amino acid will be transported from lumen to cell via co- transporter along with Na+ influx; the absorbed AA will then be transported to the capillaries
2) The remaining 60% of oligopeptides will then, when in proximity with the microvilli of epithelial cell plasma membrane (brush border), have the N-terminus peptide bond cleaved by aminopeptidase in brush border
3) The remaining oligopeptides will either:
i) Enter the intestinal epithelial cells via co-transporter along with H+ influx; and then be broken down to amino acid by intracellular dipeptidase and tripeptidase; the resulting AA will be transported to the capillaries. OR;
ii) Broken down into amino acids by brush border dipeptidase & tripeptidase; resulting AA will then be transported from lumen to cell via co- transporter along with Na+ influx; the absorbed AA will then be transported to the capillaries
Pepsin action sites
- Internal peptide bonds (endopeptidase)
- Act on C-terminus of aromatic, acidic side chains
i) Phenylalaine
ii) Tyrosine
iii) Glutamate
iv) Asparate
Trypsin action sites
- Internal peptide bonds (endopeptidase)
- Act on C-terminus of basic amino side chains
i) Arginine
ii) Lysine
Chymotrypsin action sites
- Internal peptide bonds (endopeptidase)
- Act on C-terminus of aromatic, neutral, branched side chains
i) Phenylalaine
ii) Tyrosine
iii) Tryptophan
iv) Leucine
Elastase action sites
- Internal peptide bonds (endopeptidase)
- Act on C-terminus of neutral, branched side chains
i) Alaine
ii) Glycine
iii) Serine
Carboxypeptidase A action sites
- C-terminus terminal peptide bonds (exopeptidase)
- Act on N-terminus that displays hydrophobia
Carboxypeptidase B action sites
- C-terminus terminal peptide bonds (exopeptidase)
- Act on N-terminus of basic side chains
i) Arginine
ii) Lysine
Aminopeptidase action site
- N-terminus terminal peptide bonds (exopeptidase)
- Act on C-terminus of cleaved AA
TG digestion flow
Gall bladder releases bile salts
Pancreas releases lipase and colipase
Bile salts will emulsify lipids, forming micelles (with -OH groups facing outside, thus providing electrostatic repulsion)
Pancreatic lipase digests the TG, breaking it down into 2-Monoacylglycerol and fatty acids
Transcellular absorption of Fatty acids and 2-MG occurs.
In sER, TG reformed from absorbed FA and 2-MG
TG modified with apo B-48 and phospholipids to form chylomicron, which is exported out of the cell into the lymph
Absorption of dietary cholesterol
1) Dietary cholesterol in micelles undergo endocytosis and absorbed into the cells
2) Cholesterol enters ER, converted by ACAT-2 into cholesterol ester
3) With Microsomal triglyceride transfer protein (MTP), chylomicron assembly is completed with Apo B-48, cholesterol ester, TG and phospholipids; chylomicron undergo golgi apparatus modifications, released to the lymph
Lipid-digesting enzymes
Mouth cavity
- lingual lipase
Stomach
- gastric lipase
Pancreas
- pancreatic lipase
- lipid esterase
- phospholipase A2
