Digestion and absorption of nutrients - part 1 Flashcards
Carbohydrates and proteins
Carbohydrates and proteins are both polymers
The pathways for the digestion and absorption of carbohydrates and proteins are similar.
Digestion of both polymers is carried out in 2 steps
1. Intraluminal hydrolysis (before the small intestine)
2. Membrane digestion (in the small intestine)
The digested molecules are then absorbed by the enterocytes
Dietary carbohydrates
Polysaccharides
Starch- from plants -contains α-1,4 and α-1,6 linkages
Glycogen- from animals (more highly branched) -contains α-1,4 and α-1,6 linkages
Cellulose – from plants – contains β-1, 4 glycosidic bonds
Oligosaccharides
Disaccharides (30% to 40% of dietary carbohydrates)
-Sucrose
-Lactose
-Maltose
Monosaccharides (5% to 10% of dietary carbohydrates )
Carbohydrates
There is no evidence of any intestinal absorption of either starches or disaccharides
Small intestine can absorb only monosaccharides
All dietary carbohydrate must be digested before absorption
glucose + galactose -> lactose
glucose + glucose -> maltose
glucose + fructose -> sucrose
Digestion
Digestive process for dietary carbohydrates is a two-step process:
1. Intraluminal hydrolysis
Starch to oligosaccharides
Salivary and pancreatic amylases
- Membrane digestion
Oligosaccharides to monosaccharides
Brush-border disaccharidases
Enzymes for digestion
enzyme
location
substrate
type of digestion
α-amylase
Saliva
Pancreas
Polysaccharides
Luminal
Disaccharidases
Intestine
Disaccharides
Membrane
α-1,6-glucosidase
Isomaltase
Intestine
Branch points of oligo- and disaccharides
Membrane
Salivary and pancreatic α-amylase are present in serum.
Salivary amylase «_space;pancreatic amylase
Rising levels of pancreatic amylase -> diagnosis of acute pancreatitis
Why does pancreatic α-amylase target polysaccharides?
No carbohydrate digestion occurs in the stomach
Because the pH of the stomach inactivates the salivary α-amylase
1 - luminal digestion
Amylase is an endoenzyme that cleaves α-1,4 bonds
Salivary Amylase
Starts the process > Inactivated by acid
Pancreatic Amylase
Induced by CCK
Completes starch digestion
Amylase cannot act on the following
Terminal α-1,4 links or α-1,6 links
α-1,4 links adjacent to α-1,6 links
2- Membrane Digestion
Disaccharidases
Brush border / Membrane associated
Maltase, Sucrase & Isomaltase
Cleaves internal α-1,4 links
Cleaves terminal α-1,4 links
Sucrase
Splits sucrose > Glucose + Fructose
Isomaltase
Cleaves α-1,6 links
α-Limit dextrin
Uptake and absorption
Transport
SGLT1: Na/glucose transporter (secondary active transport)
GLUT5: fructose transporter (facilitated diffusion)
GLUT2: all monosaccharide transporter (facilitated diffusion)
Clinical Relevance
Lactase deficiency
Post weaning > Lactase reduces
Cattle farming ???
Symptoms
Cramps / bloating
Osmotic diarrhoea
Primary lactase deficiency is extremely common; 3% White Caucasians, 55% Asian, 82% Afro-Caribbean
Diagnosis: measuring disaccharidase activity in homogenates of small bowel biopsies or by breath-testing.
Proteins
50% of protein is obtained from the diet
- 50% is from endogenous sources
- Enzymes / hormones
- Desquamated cells (4/5 days)
Digestion / Absorption
- Broken down into oligopeptides & AA’s
- Taken up by enterocytes
Exception
- Neonates (6 months)
- Intact proteins > Immunoglobulins > breast milk
- Endocytosis
Digestion
Process for luminal digestion involves gastric and pancreatic proteases
- Secreted as pro-enzymes
Chief cells
- Pepsinogen
- Low pH (HCl) > Pepsin
Pepsin
- Endopeptidase (aromatic AA’s – TYR & PHE)
- Partially digests 10-15% of protein
Pepsin is an endopeptidase with primary specificity for peptide linkages of aromatic and larger neutral amino acids.
pepsinogen –> pepsin (HCL usedd)
dietary proteins –> large polypeptides (pepsin used)
Five pancreatic proteases
-Acinar cells
-Secreted as pro-enzymes
-Duodenum (main site of protein digestion)
proenzyme
activating agent
active form
Trypsinogen
Enterokinase
Trypsin
Chymotrypsinogen
Trypsin
Chymotrpysin
Proelastase
Trypsin
Elastase
Procarboxypeptidase A
Trypsin
Carboxypeptidase A
Procarboxypeptidase B
Trypsin
Carboxypeptidase B
CCK-PZ is CCK and Enteropeptidase is enterokinase
Pancreatic proteolytic enzymes
either exopeptidases or endopeptidases.
Endopeptidases
Have an affinity for peptide bonds adjacent to specific amino acids
70% polypeptides
Oligopeptides with two to six amino acids
Exopeptidases
Hydrolyse peptide bonds adjacent to the C-terminus
30% polypeptides
Individual amino acids
active form
action
product
Trypsin
Endopeptidase
Oligopeptides
Chymotrpysin
Endopeptidase
Oligopeptides
Elastase
Endopeptidase
Oligopeptides
Carboxypeptidase A
Exopeptidase
Amino acids
Carboxypeptidase B
Exopeptidase
Amino acids
Brush border
-Small peptides undergo further hydrolysis to AA’s
-Multiple exo/endopeptidases are present on both the brush border and in the cytoplasm
-Dipeptidase
-Aminopeptidase (hydrolyse peptides from amino end)
-Tripeptidyl peptidase
-Dipeptidyl peptidase
-Peptidyl dipeptidase
Absorption
H+ dependent co-transporter (PepT1)
Tripeptides are absorbed
Cellular peptidases further hydrolyse the peptide into amino acids
H+-Na+ co-transporter
Dipeptides are absorbed
Cellular peptidases further hydrolyse the peptide into amino acids
Amino acid Na+
Na+ dependant co-transporter
AA’s > inside the enterocytes
Clinical Relevance - cystinuria
Cystinuria is an autosomal recessive hereditary disorder.
The re-absorption of cysteine from the kidneys is impaired due to mutation in the genes encoding proximal tubule dibasic amino acid transporter.
