Lecture 9 : Digestion and Absorption of Nutrients (Carbs and Proteins) Flashcards
(16 cards)
Carbs and Proteins brief
Both polymers
Similar pathways for digestion and absorption
Digestion carried out in 2 steps :
- Intraluminal hydrolysis (before small intestine)
- Membrane digestion (in small intestine)
Digested molecules then absorbed by enterocytes (in small intestine)
Dietary Carbs examples
Polysaccharides
- Starch (plants, contains α-1,4 and α-1,6
linkages)
- Glycogen (animals, more branching, contains α-1,4 and α-1,6 linkages)
- Cellulose (plants, β-1, 4 glycosidic bonds)
Oligosaccharides
- Simple sugars, multisaccharide but not quite poly
Disaccharides
- 30-40% of dietary carbs
- Sucrose
- Maltose
- Lactose
Monosaccharides
- 5-10% of dietary carbs
- Glucose
- Fructose
- Galactose
Digestion of dietary Carbs
Small intestine cannot absorb Disaccharides, oligosaccharides or polysaccharides, ONLY MONOSACCHARIDES.
2 Step process.
- Luminal hydrolysis
- Starch -> Oligosaccharides
- Done by salivary / pancreatic AMYLASE - Membrane digestion
- Oligosaccharides -> Monosaccharides
- By brush-border DISACCHARIDASES (enterocytes)
Despite being called disaccharidases, they can break down oligosaccharides
Enzymes which digest carbs
Luminal = inside alimentary canel via enzymes
Membrane = at microvilli in small intestine
Pancreatic amylase digests more carbs than salivary amylase.
Amylase cleaves α-1,4 bonds! So ONLY DIGESTS STARCH AND GLYCOGEN, NOT Cellulose, disaccharides, proteins or fats. Cannot act on terminal α-1,4 links, or α-1,4 links which are adjacent to α-1,6 links.
Salivary and pancreatic α-amylase are present in serum (in blood), so rising levels of pancreatic amylase can lead to a diagnosis of acute pancreatitis.
Luminal Digestion (Hydrolysis)
- Salivary amylase starts process of digesting carbs but is inactivated by acidic conditions of stomach when food is swallowed
- Pancreatic amylase is induced by CCK, completing starch digestion
Quick thing on amylase
Amylase cleaves α-1,4 bonds! So ONLY DIGESTS STARCH AND GLYCOGEN, NOT Cellulose, disaccharides, proteins or fats. Cannot act on terminal α-1,4 links, or α-1,4 links which are adjacent to α-1,6 links.
Disaccharidases in small intestine can cleave the α links which amylase cannot.
Membrane Digestion (Microvilli)
Done by Brush border DISACCHARIDASES, such as :
- Maltase
- Sucrase
- Isomaltase
All cleave internal AND terminal α-1,4 links.
Sucrase splits sucrose into Glucose + Fructose.
Isomaltase cleaves α-1,6 bonds.
Maltase splits maltose into 2 glucose molecules
Uptake and Absorption of carbs into blood by TRANSPORT PROTEINS in membrane of microvilli
GLUT2 : Transports all monosaccharides by FACILITATED diffusion
GLUT5 : Fructose transporter, facilitated diffusion
SGLT1 : Na / Glucose transporter, ACTIVE Transport
What happens if lactase isnt available?
Lactose isnt broken down into glucose and galactose, causes diarrhoea or cramps or bloating.
After breastfeeding finishes, lactase reduces, could cause lactose intolerance.
Proteins
50% obtained from diet, other 50% synthesised, endogenous sources.
Broken down into oligopeptides and amino acids, and uptaken into blood stream by ENTEROCYTES.
Except in neonates (babies under 6 months), where gut can absorb intact proteins by endocytosis. This adaptation is so that antibodies in breast milk are absorbed and implicated in the baby’s immune system
Luminal digestion of proteins IN STOMACH
Gastric / Pancreatic proteases do this, secreted as zymogens (or pro-enzymes).
- Chief cells, in STOMACH, secrete pepsinogen. Pepsionogenm in low pH, ie. In HCL, converts to pepsin, the active enzyme which digests proteins luminally
Endopeptidases too.
In stomach, 10-15% of proteins are digested by protease and endopeptidases, 85-90% done by pancreatic enzymes from duodenum onwards.
The 5 pancreatic proteases responsible for digestion of proteins in the duodenum / small intestine
Produces by acinar cells
Secreted as zymogens.
Main site of digestion is duodenum
Luminal digestion of proteins from duodenum onwards. Endopeptidases and exopeptidases
Pancreatic enzymes are responsible for this now. Either exopeptidases or endopeptidases.
Endopeptidases :
- high affinity for peptide bonds or specific amino acids
- Digests 70% of polypeptides
- Into oligopeptide with 2-6 amino acids
Exopeptidases :
- Hydrolyse peptide bonds, adjacent to C-terminus (COOH group)
- Can hydrolyse the oligopeptides which endopeptide broke the polypeptides into
- hydrolysis 30% of polypeptides)
- Into amino acids
Membrane digestion of proteins in small intestine
Brush Border refers to the microvilli-covered surface of the small intestine’s epithelial cells.
Enterocytes are the epithelial cells which are covered in microvilli, and the brush border is made of all of these enterocytes.
Enterocytes contain brush border enzymes containing lots of exo/endo peptidases
Eg.
Dipeptidases
Aminopeptidases (hydrolise from amino end of peptide)
Tripeptidyl peptidase
Dipeptidyl peptidase
Peptidyl dipeptidase
Absorption of proteins into the blood at the enterocytes
- PepT1 (Peptide Transporter 1)
- Transports dipeptides and tripeptides into enterocytes
- MechH⁺-dependent cotransporter (uses proton gradient)
- Once inside, intracellular peptidases break these peptides down into amino acids
- Amino Acid Transporters
- Transport free amino acids into enterocytes
- Usually Na⁺-dependent cotransporters (use sodium gradient)
- Different transporters exist for different groups of amino acids (neutral, acidic, basic)
- Additional Notes
- After peptides are broken down inside enterocytes, amino acids exit the cell into the bloodstream via facilitated diffusion or other transporters
- Most protein absorption happens as small peptides (di- and tripeptides) via PepT1, then hydrolyzed inside the cell.
Cystinuria
Autosomnal recessive heriditary disorder
Reabsorption of cysteine in kidneys impaired due to mutation in genes coding for the proximal tubule amino acid transporter.
