Biochemistry: Digestion

Question 1

What bonds do:

Glycosidase
Esterase
and
Amidase

break?

Answer 1

Glycosidase = glycosidic

Esterase = ester

amidase = amide

Question 2

What does pancreatic lipase do?

Answer 2

Breaks down lipids

Question 3

What is the structure of amylopectin?

Answer 3

Open, branched structure of alpha-1,4 and alpha-1,6 linkages

(Broken by alpha-amylase)

Question 4

What are some indigestible molecules and how are they broken down or removed in the body?

Answer 4

Cellulose (not broken down in the body; excreted with fecal matter)

Raffinose (found in beans, hydrolyzed by flora of the colon)

Question 5

What is the source of symptoms in lactase deficiency?

Answer 5

Undigested lactose makes its way to the colon.

• Bacterial fermentation makes it into lactic acid
  and releases gas (flatulence)
• Lactic acid in the colon creates an osmotic effect, pulling water into the lumen (diarrhea)
• Increased fluid load cuases distention of gut walls, increased peristalsis, and malabsorption of fats, proteins, and drugs

Question 6

What are examples of zymogens released from the pancreas?

Answer 6

trypsinogen

Chymotripsinogen

proelastase

procarboxypetidases A and B

Question 7

How are proteins digested?

Answer 7

• Pepsin and HCl in the stomach break down the protein into peptides
• aminopeptidases (along with other zymogens of the small intestine) break down the peptides into di- and tri-peptides and amino acids that aare able to be drawn into the intestinal epithelial cells and into the blood stream

Question 8

What activates trypsinogen?

Answer 8

Enteropeptidase (released from intestinal mucosal epithelial cells) converts trypsinogen –> trypsin

Question 9

What enzymes does trypsin activate?

Answer 9

Chymotrypsinogen –> chymotrypsin

proelastase –> elastase

procarboxypeptidase –> peptidase

Question 10

How do enzymes like pepsin, trypsin, aminopeptidase, elastase, etc, break proteins into peptides and amino acids?

Answer 10

Each enzyme acts at a specific set of amino acid chain sites.

–> for instance, Pepsin cleaves any amino-bond between:

N - [Phe, Tyr, Glu, Asp] and [Arg, Lys] - C

Question 11

What transporters are used to move fructose, glucose, and galactose from the lumen of the small intestine into the intestinal epithelium?

Answer 11

Fructose – passive Fructose transporter (GLUT 5)

Glucose – Facilitated glucose transporter
and
Na-Glucose cotransporters (SGLT 1)

Galactose – Na-Glucose cotransporters (SGLT 1)

All are pumped through the basal surface of the cell with GLUT 2 –> passive glucose (and fructose and galactose) transporter

Question 12

How are bile salts used in digestion?

Answer 12

They are used in the emulsification of ingested triglycerides

– Pancreatic lipase and Colipase act on bile salt-emulsified TGs and break TGs into 2-Monoacylglycerol so that TGs can pass through the lipid membrane of intestinal cells

Question 13

What happens to 2-Monoacylglycerol as it enters intestinal epithelial cells?

Answer 13

It is resynthesized into TG via the monoglyceride pathway, then made into chylomicrons to be transported to the liver

Question 14

What is the “protein sparing” effect of ketone bodies?

Answer 14

Rise in blood ketones (~2 days into fast) –> more ketonse taken up by brain –> less need for glucose –> less need for gluconeogenesis –> less need for alanine –> less need for muscle proteolysis –> less muscle wasting

Question 15

What are precursors for gluconeogenesis?

Answer 15

**Fat ** (12%) –> Glycerol –> Glycerol-3-P –> DHAP –> gluconeo.

Gylcolysis (in RBC and muscle) (28%) –> Lactate –> Pyruvate –> gluconeo.

Muscle Protein (60%) –> Alanine –> pyruvate —> gluconeo