Download Digestion of proteins and Carbohydrates. and more Lecture notes Biochemistry in PDF only on Docsity!
Digestion of proteins and Carbohydrates.
The source of proteolytic enzymes responsible for degrading dietary proteins: 1 - The stomach 2 - The pancreas 3 - The small intestine Protein Digestion Dietary proteins constitute : 70 - 100 g/day. Proteins are generally too large to be absorbed by the intestine. They must therefore be hydrolyzed to their constituent amino acids which can be absorbed. 1 - Digestion of proteins in Stomach:
- The digestion of proteins begins in the stomach which secrets gastric juice.
- A unique solution containing 2 components important for protein digestion: Hydrochloric acid (HCl). Pepsin. 2 - Digestion of proteins in Small intestine: A. digestion by pancreatic enzymes. B. digestion by intestinal aminopeptidase.
- The digestion in small intestine is hormonally controlled.
- Two small peptide hormones are released from cells of the upper part of small intestine:
Digestion of proteins in Small intestine: A. digestion by pancreatic enzymes. The pancreatic secretion contains a group of pancreatic proteases. Each of these enzymes has different specificity for the cleavage sites. Pancreatic enzymes are very specific. For instance, trypsin will cut only if there is arginine or lysine. These proteases are synthesized and secreted as inactive zymogens. In this step the polypeptide is broken into oligopeptides + amino acids by specific enzymes. Digestion of proteins in Small intestine: B. digestion by intestinal aminopeptidase. What is Aminopeptidase? an exopeptidase on the luminal surface of the intestine. Function \ role: cleaves Oligopeptides that result from the action of pancreatic proteases into free amino acids and smaller peptides (di and tri-peptides).
Genetic errors in amino acids transport
Cystinuria one of the most common genetic error of amino acid transport.
- It is an example of inherited disorder in the transport of certain amino acids.
- It affects the transport of Cystine and dibasic amino acids. Affected organs: the small intestine and the kidney. Urine features: Cystine and dibasic amino acids appear in the urine. Clinically: there is kidney stones formation Flank pain , blood in urine. Treatment: Oral hydration (drinking lots of water) is an important part of treatment (to prevent kidney stones formation). Enteropeptidase: An enzyme synthesized by and present on the luminal surface of intestinal mucosal cells of the brush border membrane converts trypsinogen to trypsin Trypsin then activates all the other pancreatic zymogens (including itself)
Final digestion of carbohydrates by intestinal enzymes in the small intestine
- Enzymes:
- Disaccharidases
- (1,6) Glucosidase (for branched oligosaccharides)
- Source: Secreted by & remain associated with the luminal side of the brush border membranes of the intestinal mucosal cells
- Location of their action: the mucosal lining of the jejunum.
Intestinal disaccharidases
Digestion of Carbohydrates:
Dietary cellulose cannot be digested due to the absence of enzyme that can cleave (1-4) bonds. It passes through the GIT largely intact. Despite that, it has several beneficial effects.
Absorption of Monosaccharaides by Intestinal Mucosal Cells
- Location: Duodenum & upper jejunum.
- Insulin: is NOT required for the uptake of glucose by intestinal cells
- Different monosaccharides have different mechanisms of absorption:
1. Facilitated diffusion (GLUT-mediated)
2. Active transport (Energy-dependent): Co-transport with Na+(against the conc.)
Abnormal digestion of disaccharides (e.g. of lactose)
Lactose intolerance(lactase deficiency)
Lactase (-
galactosidase)
Undigested
carbohydrate in
large intestine
osmotic
diarrhea.
Bacterial fermentation by E. coli of the undigested compounds in the large intestine
CO 2 , H 2
gas
abdominal
cramps, diarrhea
& distension
(flatulence)
