Carb digestion Flashcards
Carbohydrate classification
Monosaccharides-Monomers
Oligosaccharides-Short polymers
Polysaccharides-Long polymers
Polysaccharides- 2 classifications and some examples
Non-digestible polysaccharides
(Fiber, Pectin and cellulose)
Digestible polysaccharides
- Starch (45-60% of dietary carbohydrates in western society, Primarily from plants)
- Glycogen (Primarily from animals)
- Amylose (Straight polymer)
- Amylopectin (Branched polymer)
Dietary Oligosaccharides
sucrose, lactose
Dietary monosaccharides
glucose, fructose
What can the intestine absorb in terms of carbs?
Intestine can only absorb monosaccharides
Therefore, all carbohydrates must by digested to monosaccharides
Digestive Process
- Intraluminal hydrolysis
2. Membrane digestion
Intraluminal Hydrolysis
Enzymatic digestion within the lumen a-Amylases Secreted in the enzymatically active form Salivary amylase Initiates starch digestion Inactivated by gastric acid
pancreatic alpha amylase
Completes starch digestion in lumen of small intestine
Secretion stimulated by Cholecystokinin (CCK)
Cannot digest all linkages and the resultant products are oligosaccharides
Cannot digest terminal linkages and branch points
Membrane digestion
Occurs through the action of brush border oligosaccharidases
Mostly in proximal jejunum, none in large intestine
Integral membrane proteins
Lactase
Digests lactose into glucose and galactose
Expression decreases after weaning in the infant
Glucoamylase (Maltase)
Sucrase-isomaltase (Two enzymes, Cleaves sucrose and splits branch points)
Lactase Deficiency or Lactose Intolerance
Lactase downregulation determined hereditarily (Occurs after weaning)
Symptoms include:
cramps
diarrhea
flatus
(osmotic diarrhea, hydrogen gas)
Lactase deficiency symptoms are determined by…
Rate of peristalsis and gastric emptying
Colonic bacteria (Metabolize undigested lactose into:
- Short chain fatty acids: Induce osmotic diarrhea
- CO2: Contributes to flatulence
- H2: Released in breathe (Hydrogen breath test, Similar to glucose tolerance test)
Three monosaccharides are readily absorbed by small intestine:
Glucose
Galactose
Fructose
synthetic sugar: lactulose not digested–> diarrhea
2 apical membrane transporters
SGLT 1 (na/ glucose transporter) GLUT 5
Na/glucose transporter (SGLT1)
Responsible for glucose and galactose uptake
Secondary Active transcellular
Driven by intracellular [Na+] via Na,K-ATPase
GLUT5
Responsible for fructose uptake
In jejunum
Facilitated diffusion
basolateral membrane transport
GLUT2
Responsible for the transport of all three monosaccharides into interstitium
At basolateral membrane
Facilitated diffusion
Glucose-Galactose Malabsorption
Single aa substitutions in SGLT1
Inhibits uptake of glucose and galactose via SGLT1
Results in diarrhea
Consequence of reduced Na+ absorption via SGLT1
Subsequent osmotic diarrhea
Treatment
Eliminate glucose, galactose and lactose from diet
Protein Digestion (brief overview)
Proteins must first be digested into oligopeptides and amino acids to be absorbed
Four pathways of protein digestion
Luminal proteases
brush border proteases
luminal proteases- 2 types
Protein digestion Path 1: Luminal proteases
Secreted by stomach and pancreas
Hydrolyze into peptides and amino acids
Protein digestion Path 2: Brush Border proteases
Hydrolyze peptides to amino acids
Protein digestion Path 3: Luminal proteases
Hydrolyze peptide into oligopeptides
Oligopeptides taken up by enterocyte
Hydrolyzed intracellularly to amino acids
Protein digestion Path 4: Luminal proteases
oligopeptides
Oligopeptides taken up by enterocyte
Oligopeptides moved directly to interstitium
Proteins: endogenous
enzymes, hormones, secretions
sloughed cells
About 50% of protein entering small intestine
Proteins: digestion
Endopeptidase
- Affinity for bonds adjacent to specific amino acids (sequences)
- Products are oligopeptides (cutting in the middle)
Exopeptidase
- Hydrolyze bonds adjacent to amino or carboxy terminus
- Releases single amino acids (cutting from the end)
Gastric proteases
Secreted as proenzymes
Pepsinogen
- Secreted by chief cells
- pH-dependent activation at 1.8-3.5 to
- Pepsin (irreversible, inactivated at pH7, endopeptidase)Pancreatic proteases: Trypsinogen
Trypsinogen
– Activated by jejunal brush border enzyme to Trypsin (Autoactivates trypsinogen and other pancreatic proteases)
Pancreatic proteases: Chymotrypsinogen
Activated by trypsin to
Chymotrypsin
Endopeptidase
Pancreatic proteases: Proelastase
Activated by trypsin to
Elastase
Endopeptidase
Pancreatic proteases: Procarboxypeptidase A and B
Activated by trypsin to
Carboxypeptidase A and B
Exopeptidase
Brush Border peptidases
Small intestine
Large numbers of them because each peptidase recognizes only some peptide bonds
Larger oligopeptides
Cytoplasmic peptidases
Smaller dipeptides and tripeptides
Less numerous
PepT1 Transporter transports what?
Oligopeptides
Protein Absorption: Whole protein absorption
In neonate
First 6 months
Mechanism of passive immunity from mother to child
Closure hormonally mediated
Corticosteroid administration induces closure
Whole protein absorption in adults
Finite amounts of whole protein absorbed
Mechanism not well defined
Enterocytes:
- Whole protein endocytosed is degraded in lysosome
- Small amount transferred to interstitium
M cells:
- Found in Peyers patches (lymphoid tissue in lamina propria)
- Specialized for protein uptake
limited lysosomes
- Package proteins as antigens in clathrin coated vesicles
- Present to immunocompotent cells at basolateral membrane (Processed and transferred to lymphocytes, Important for mucosal immunity)
Oligopeptide absorption
- Greater than 90% of absorbed protein enters blood as single aa
- Some enter the enterocyte as di-, tri-, or tetrapeptides
- PepT1- H+/oligopeptide cotransporter
(Active, driven by a H+ and Na+ gradient)
Single AA Absorption: apical membrane
Multiple identified,
at least 7 with overlapping affinities apically
Divided into those driven by Na/K pump and Na+ independent
- Na+ dependent neutral aa
- Na+ independent basic aa and cysteine
Single AA absorption: basolateral membrane
At least 5 present
- Bidirectional
3 for exit (Na+ independent)
2 for entrance (Na+ dependent, For enterocyte nutrition and growth)
Hartnup Disease
Autosomal recessive hereditary disorder
Defect in apical transport of neutral aa
Symptoms: - Pellagra - Cerebellar ataxia - Psychiatric abnormalities No treatment because normal oligopeptide absorption
Cystinuria
Autosomal recessive hereditary disorder
Defect in apical transport of basic aa and cysteine
Symptoms
Kidney stones
No treatment because normal oligopeptide absorption
