1. Intraluminal hydrolysis 2. Membrane digestion

Carb digestion Flashcards by Briana Tillman

Carbohydrate classification

Monosaccharides-Monomers
Oligosaccharides-Short polymers
Polysaccharides-Long polymers

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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)

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Dietary Oligosaccharides

sucrose, lactose

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Dietary monosaccharides

glucose, fructose

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What can the intestine absorb in terms of carbs?

Intestine can only absorb monosaccharides

Therefore, all carbohydrates must by digested to monosaccharides

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Digestive Process

Intraluminal hydrolysis

2. Membrane digestion

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Intraluminal Hydrolysis

Enzymatic digestion within the lumen
a-Amylases
Secreted in the enzymatically active form
Salivary amylase
Initiates starch digestion
Inactivated by gastric acid

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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

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Membrane digestion

Occurs through the action of brush border oligosaccharidases

Mostly in proximal jejunum, none in large intestine

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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)

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Lactase Deficiency or Lactose Intolerance

Lactase downregulation determined hereditarily (Occurs after weaning)

Symptoms include:
cramps
diarrhea
flatus

(osmotic diarrhea, hydrogen gas)

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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)

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Three monosaccharides are readily absorbed by small intestine:

Glucose
Galactose
Fructose

synthetic sugar: lactulose not digested–> diarrhea

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2 apical membrane transporters

SGLT 1 (na/ glucose transporter)
GLUT 5

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Na/glucose transporter (SGLT1)

Responsible for glucose and galactose uptake
Secondary Active transcellular
Driven by intracellular [Na+] via Na,K-ATPase

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GLUT5

Responsible for fructose uptake
In jejunum
Facilitated diffusion

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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