Biochemistry of the GI Tract

Question 1

mouth - purpose and methods?

Answer 1

• mastication increases surface area
• saliva hydrates food stuffs
• alpha-amylase begins breakdown of starch

Question 2

Stomach - purpose and methods?

Answer 2

• acidic conditions denature proteins - proteins are difficult to breakdown! Need to get proteins ready for the enzymes!
• secretion and activation of pepsin

Question 3

Pancreas - purpose and methods?

Answer 3

• secretes sodium bicarb to neutralize chyme

- secretes amylase, lipases, and peptidases

Question 4

Liver and gallbladder - purpose and methods?

Answer 4

-synthesize and store bile acids

Question 5

Small intestine - purpose and methods?

Answer 5

-produces disaccharidases and peptidases to complete proteins and carb digestion

Question 6

oral phase of carb digestion

Answer 6

-salivary alpha amylase hydrolyzes alpha 1,4-glycosidic bonds in starch and glycogen

Question 7

intestinal phase of carb digestion:

Answer 7

• pancreatic alpha-amylase continues hydrolysis of alpha1,4-glycosidic bonds into into glucose (mono), maltose (di) and maltotriose (trisaccharides)
• oligosaccharidase, isomaltase and alpha-glucosidase complete breakdown into glucose

Question 8

maltose linkage?

Answer 8

alpha 1,4

Question 9

maltose breakdown yields?

Answer 9

2 glucose

Question 10

lactose linkage?

Answer 10

beta 1,4

Question 11

lactose breakdown yields?

Answer 11

glucose and galactose

Question 12

sucrose linkage?

Answer 12

alpha1-beta2

Question 13

sucrose breakdown yields?

Answer 13

glucose and fructose

Question 14

How is glucose taken up by intestinal epithelium?

What other nutrient does this method work for?

Answer 14

• Na ATPase builds high sodium concentration in intestinal lumen
• Na + glucose enter intestinal epithelium via SGLT1 transporter following Na gradient

-also works for Gal bc it is very similar structurally to Glu

Question 15

How do we bring glucose from the intestinal epithelial cells into circulation?

Answer 15

-passive transport down gradient using GLUT2 transporter

Question 16

How do cells take up glucose?

How do sells make sure that they don’t loose the glucose that they take up? How do cells keep the glucose concentration steep?

Answer 16

• steep concentration of Glucose used –> facilitated diffusion or other GLUT transporters
• phosphorylate glucose as soon as it comes in — phosphorylation of glucose also allows the concentration gradient of glucose to remain high outside

Question 17

How do intestinal epithelial cells take up fructose?

Answer 17

Use GLUT5 transporter - facilitated diffusion

NO Na GRADIENT USED

Question 18

How does fructose get into the circulation from intestinal epithelial cells?

Answer 18

uses GLUT5 transporter - facilitated diffusion

Question 19

Where does fructose go after getting into circulation?

What happens to it next? WHy?

Answer 19

• fructose goes to the LIVER
• hepatocytes phosphorylate Fructose = keeps it from leaving cell and keep the concentration of fructose higher in circulation

Question 20

What is lactose intolerance?

What happens?

Answer 20

• reduced ability to break down lactose
• lactose is broken down by intestinal bacteria –> the 2 to 3 carbon metabolites draw water into intestines (diarrhea)
• H2 and CO2 cause gas pain

Question 21

Diagnostic for lactose intolerance?

Answer 21

hydrogen breath test

administer lactose before the h breath test

Question 22

Protein digestion process:

Answer 22

1) Gastric enzymes - pepsin = denature protein
2) Pancreatic enzymes - trypsin, chymotrypsin, Elastase, carboxypeptidase = breakdown denatured protein into amino acids and oligopeptides
3a) (40%) amino acids taken up
3b) (60%) Brush border enzymes - endopeptidase, aminopeptidase, dipeptidiase = breakdown oligos into amino acids, dipeptides, and tripeptides

Question 23

zymogens are?

Answer 23

enzymes that need acidic conditions and/or proteolytic processing to activate an inactive precursor (zymogen)

Question 24

stomach zymogens?

Answer 24

-pepsinogen—>pepsin

Question 25

pancreas zymogens?

Answer 25

- chymotrypsingen-->chymotrypsin - trypsinogen-->trypsin - procarboxypeptidases--> carboxypeptidases - proelastase-->elastase

Question 26

intestine zymogens?

Answer 26

- ->aminopeptidase - ->dipeptidase - ->endopeptidase

Question 27

activation of zymogens starts with what signal on what cells?? What happens next?

Answer 27

cholecystokinin (CKK) acts on pancreatic acinar cells and intestinal mucosal cells (relevant to this example- it does more) the mucosal cell secretes enteropeptidases and the acinar cell secretes trypsinogen trypsiogen activated to tyrpsin by the enteropeptidase Trypsin then helps with more activation of trypsin. also activates chymotrypsin, proelastase, procarboxypeptidase

Question 28

master protease of zymogens?

Answer 28

trypsin

Question 29

Gastric phase- protein digestion:

Answer 29

- HCl denatures proteins | - pepsinogen digests proteins into large fragments

Question 30

Intestinal phase - protein digestion:

Answer 30

- pancreas synthesizes chymotrypsin, trypsin, carboxypeptidases A/B and elastase to break proteins into amino acids and oligopeptides - intestinal epithelium mades aminopeptidase, dipeptidase, and endopeptidase

Question 31

Pancreatic autodigestion - what happens and why?

Answer 31

- protein digestion disorder - trypsinogen inhibitor (TI) prevents premature activation of trypsin in the pancreas - blockage of pancreatic duct or deficiency of TI results in autodigestion of the pancreas = pancreatitis

Question 32

trypsinogen inhibitor (TI) purpose?

Answer 32

prevents premature activation of trypsin in the pancreas

Question 33

Cystic fibrosis - what happens and why?

Answer 33

- CFTR deficiency=thickened pancreatic mucous --> occluded ducts - secretion of pancreatic enzymes impaired in 80% - 10% develop pancreatitis - CF patients need oral enzyme supplements to help w/ malabsorption

Question 34

Celiac disease- what happens and why?

Answer 34

- unidentified protease defect prevents complete degredation of gluten - small gluten fragments=toxic --> inflammatory reaction

Question 35

Amino acid uptake process- 1) which transporters? 2) locations for these transporters?

Answer 35

1) many amino acid transporters (SLC) that transport more than one amino acid - lots of back-up 2) intestines and also used in kidney for reuptake of amino acids

Question 36

Amino uptake disorders?

Answer 36

so much redundancy among SLC transporters that a mutation in one wont really hurt the system -in the kidney this may affect kidney function

Question 37

Cystinuria: | -what is it? what happens?

Answer 37

-inefficient re-uptake of cystin (Cys-Cys) in the kidney cystin crystals (poooor solubility) form in GU tract - does not sounds fun bc of obstruction and stones

Question 38

Hartnup disease- what is it? what happens?

Answer 38

- defective uptake of neutral amino acids from intestine and kidney filtrate - can get deficiency of tryptophan --> needed for NAD synthesis = pellagra-like symptoms

Question 39

Problem with high protein intake?

Answer 39

liver ends up making a lot of urea!!

Question 40

Lipid digestion and uptake:

Answer 40

- bile salts solubilize large lipid droplets - pancreatic lipase separates 2 FAs from triacylglycerol (TAG becomes MAC) - enterocytes take up lipid myceles and remake TAG - enterocytes pack lipids into ApoB48 chylomicrons

Question 41

Bile salts made where and derived from what precursor? secreted how?

Answer 41

- made in liver from cholesterol | - secreted via the gall duct

Question 42

Purpose of bile salts?

Answer 42

solubilize lipids

Question 43

How much of secreted bile salts are recycles?

Answer 43

80% via intrahepatic circulation

Question 44

what breaks down the TAGs?

Answer 44

pancreatic lipase breaks it into 2 FAs and a monoacyl glycerol