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GI test 2 > Digestion of Carbohydrates and Proteins > Flashcards

Digestion of Carbohydrates and Proteins Flashcards

1
Q

carb. classification?

A

Monosaccharides- Monomers
Oligosaccharides - Short polymers
Polysaccharides- Long polymers

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2
Q

Non-digestible polysaccharides

A

Fiber: Pectin or cellulose from plants

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3
Q

Digestible polysacchs.

A

Starch: digestible polysach. from plants including amylose and amylopectin.
Glycogen: primarily from animal foods

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4
Q

dietary oligosaccharides

A

Sucrose: contains glucose and fructose
Lactose: contains glucose and galactose

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5
Q

Dietary monosaccharides

A

glucose
fructose
- intestine can only absorb monosaccharides

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6
Q

Step 1. intraluminal hydrolysis.

A
  • occurs in the lumen
  • due to alpha-Amylases: Secreted in the enzymatically active form - (cannot break a terminal alpha 1,4 or alph 1.6 linkage branch point):
    1. Salivary amylase: Initiates starch digestion immediately in mouth, Inactivated by gastric acid
    2. Pancreatic amylase: completes inralumenal digestion in small intestine. secretion is stimilated by CCK. cannot digest linkages and resultant products are oligosaccharides

results in sucrose, lactose, glucose, fructose remaining in small intestine after process is over

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7
Q

Step 2: Membrane digestion

A
  • occurs through brush border (integral membrane proteins) called “oligosacchardiases”
  • mostly found in proximal jejunum, none are located in large intestine
  • occurs through three enzymes: lactase, glucoamylase, sucrase-somaltase
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8
Q

three Oligosaccharidases

A
  1. Lactase: digests lactose into glucose and galactose. expression decreases with age, but is absolutely required in breast feeding
  2. glucoamylase (maltase): cleaves terminal alpha 1,4 linkages (breaks down maltotriose residues to glucose residues)
  3. sucrase-somaltase: the somaltase can break terminal alpha 1,4 linkage. the sucrase portion splits sucrose. THIS ENZYME IS NECESSARY TO DIGEST ALL POLYSACCHARIDES BESIDES AMYLASE.
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9
Q

Absorption of carbohydrates: what are readily absorbed by small intestine?

A

glucose, galactose, and fructose

- can be accomplished by two transporters: SGLT1 and GLUT5

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10
Q

Na/glucose transporter SGLT1

A
  • Responsible for glucose and galactose uptake
  • Secondary Active transcellular
  • Driven by intracellular [Na+] via Na,K-ATPase
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11
Q

GLUT5

A
  • Responsible for fructose uptake
  • In jejunum
  • Facilitated diffusion: thus is concentration dependent on fructose
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12
Q

basolateral membrane transport of monosaccharides?

A

GLUT2

  • Responsible for the transport of all three monosaccharides into interstitium from cell
  • At basolateral membrane
  • Facilitated diffusion
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13
Q

Lactase deficiency/Lactose intolerance

A
  • mostly affects non-white
  • lactase downregulated after weaning: downregulation can be affected by genetics and epigenetic modifications
  • symptoms include: cramps, diarrhea, flatus
  • symptoms determined by rate of peristalsis and gastric emptying

Colonic bacteria metabolize undigested lactose into

  1. Short chain fatty acids - Induce osmotic diarrhea
  2. CO2- Contributes to flatulence
  3. H2 - Released in breathe (Hydrogen breath test)
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14
Q

Glucose-Galactose Malabsorption

A
  • could be due to single AA substitutions in SGLT1: Inhibits uptake of glucose and galactose via SGLT1
  • Results in diarrhea as a consequence of reduced Na+ absorption via SGLT1–> water flows into lumen –> exasberated osmotic diarrhea (more severe than lactose intolerance)

-Treatment
Eliminate glucose, galactose and lactose from diet. must go onto fructose based diet.

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15
Q

Path 1: Luminal proteases

A
  • secreted by stomach and pancreas

- hydrolyze peptides to amino acids

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16
Q

Path 2: Brush border proteases

A
  • hydrolyze peptides to AA’s
17
Q

Path 3: Luminal proteases

A
  • hydrolyze peptide into oligopeptides, which will be taken up by enterocyte and hydrolyzed intracellularly to AA’s
18
Q

Path 4: Luminal proteases

A
  • Hydrolyze peptide into oligopeptides
  • Oligopeptides taken up by enterocyte
  • Oligopeptides moved directly to interstitium then into circulation
19
Q

Endogenous proteins

A

enzymes, hormones, secretions

  • sloughed cells
  • account for ~50% of protein entering small intestine
20
Q

Endopeptidase

A

cleave internal sequence
Affinity for bonds adjacent to specific amino acids
Products are oligopeptides

21
Q

Exopeptidase

A

cleave external sequence - just release signle AA’s
Hydrolyze bonds adjacent to amino or carboxy terminus
Releases single amino acids

22
Q

Pepsinogen

A
  • Pepsin is secreted as the proenzyme/zymogen pepsinogen by chief cells
  • pH dependent activation of 1.8-3.5
  • at pH 7 in SI it is inactivated
  • endopeptidase
  • only gastric protease
23
Q

Trypsinogen

A
  • trypsinogen is proactive form
  • pancreatic proenzyme
  • activated by jejunal brush border enzyme
  • enterokinase cleaves trypsinogen to trypsin
  • also activates the other pancreatic proteases
  • produces oligopeptides 2-6 AA’s
24
Q

Chymotrypsinogen

A
  • activated by trypsin to chymotrypsin
  • pancreatic endoepeptidase
  • produces oligopeptides 2-6 AA’s
25
Q

Proelastase

A
  • activated by trypsin to elstase
  • pancreatic endopeptidase
  • produces oligopeptides 2-6 AA’s
26
Q

Procarboxypeptidase A and B

A
  • activated by trypsin to Carboxypeptidase A and B
  • pancreatic exopeptidase
  • produces single AA’s via cleaving carboxyterminis
27
Q

Brush border peptidases

A
  • located in SI
  • large numbers of them because each peptidase recognizes only some peptide bonds
  • proteins are ingested in gastric lumen encouter gastrin, proteins then reach intestine where they meet pancreatic peptides, and then almost all of the peptides are oligopeptides. this prevents whole protein absorption. once they are digested to di and tri and single AA’s they can be transported into the cell.

brush border peptidases: large oligopeptides
cytoplasmic peptidases: less numberous, bring in smaller dipeptides and tripeptides

28
Q

PepT1 transporter

A

transports oligopeptides

29
Q

Whole protein absorption

A
  • can be accomplished by enterocytes and M cells (located in Peyer’s patches, have fewer lysosomes), these whole proteins are able to pass into lumen and function in Ab protection
  • during neonate’s first six months, the child absorbs antibodies into the lumen. eventually this mechanism will close due to hormones such as corticosteroid.
  • in adults, only finite amounts of whole proteins are reabsorbed, mostly just related to immunity. In enterocytes whole proteins are endocytosed and degraded in lysosomes. M cells are specific for this, they expose whole proteins to underlying Peyer’s patches. They uptake proteins and package them in clathrin coated vesicles and present them to immunocompotent cells at basolateral membrane for mucosal immunity.
30
Q

Oligopeptide absorption

A
  • occurs a lot : ~90%
  • enter through PepTi-H+/oligopeptide cotransporter: has kinetic advantage over single AA transporters in that substrate enters blood faster and is drive by a H+ and Na+ gradient. Low proton concetration outside helps move oligopeptides into the cell.
31
Q

Single AA absorption

A
  • some are dependent on Na+/ATPase, but some are independent of it.
  • some transporters are bidirectional and require sodium
32
Q

Hartnup Disease

A
  • Autosomal recessive hereditary disorder (thus half of the product is fine)
  • Defect in apical transport of neutral aa
    Symptoms:
  • Pellagra, Cerebellar ataxia, Psychiatric abnormalities
  • No treatment because normal oligopeptide absorption: avoid Leu
  • but still can use PepT1 transporter, because it creates enough
33
Q

Cystinuria

A
  • Autosomal recessive hereditary disorder
  • Defect in apical transport of basic aa and cysteine
    Symptoms:
  • Kidney stones
  • No treatment because normal oligopeptide absorption through PepT1 transporter
34
Q

Glucose/Galactose Malabsorption

A
  • can be confirmed by Hydrogen breath test (there are carbohydrates in the colon that are being metabolized by bacteria, giving off a high H+)
  • treatment: Fructose based diet

GI test 2 (14 decks)

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