Biochemistry Lecture 14 - Carbohydrate Absorption

Question 1

Do people that have undergone trauma (e.g. burn victims, post-surgery, etc.) have higher calorie requirements?

Answer 1

Oh for sho.

Question 2

How many calories per gram are protein, carbs, and fat, respectively?

Answer 2

4, 4, and 9 (fat)

Question 3

Where does digestion start? What enzyme is present there?

Answer 3

Mouth - alpha amylase.

Question 4

What links can amylase break? What can it NOT break?

Answer 4

Alpha 1,4 links. Cannot break alpha 1,6 linkages.

Question 5

What are the three products of alpha amylase digestion?

Answer 5

Maltose, maltotriose, and limit dextrins.

Question 6

Does carbohydrate digestion occur in the stomach? Why or why not?

Answer 6

No, the acidity of the stomach inactivates amylase.

Question 7

Aside from salivary glands, where is amylase produced? Where does amylase from this organ function?

Answer 7

The pancreas. Amylase is spat out from the pancreas to the small intestine to continue carb digestion.

Question 8

Name four enzymes that digest carbs which are found in the intestinal mucosa of the brush border.

Answer 8

Maltase, isomaltase, sucrase, lactase.

Question 9

Which enzyme breaks the limit dextrins? Which links does it cleave?

Answer 9

Isomaltase cleaves glucose-glucose alpha 1,6 linkages.

Question 10

What linkages does sucrase cleave?

Answer 10

glucose-alpha (1,2) beta fructose linkages in sucrose.

Question 11

What linkages does lactase cleave?

Answer 11

galactose-beta (1,4) glucose linkages in lactose.

Question 12

What plant carb is not digestible by humans? Why?

Answer 12

Cellulose has beta (1,4) linkages and we do not have an enzyme to cleave those

Question 13

Why does bacterial metabolism in the lower gut make us feel like shit and give us diarrhea?

Answer 13

Bacteria metabolize undigested stuff such as lactose (in a lactase deficient person), giving off lactic and fatty acids as well as H2 and CO2 gases. The osmolites draw H2O via osmosis into the intestinal lumen. So you get watery diarrhea and gas.

Question 14

What are the beneifts of undigested polysaccharides?

Answer 14

This is dietary fiber. It affects gastric emptying, intestinal mobility/transit time, contributes to stool bulk, and may adsorb carcinogens and cholesterol.

Question 15

Does lactose intolerance go away as one ages?

Answer 15

No, the opposite. It gets worse.

Question 16

What ethnic groups commonly have lactose intolerance?

Answer 16

Africans and Asians

Question 17

Can lactose intolerance be temporary?

Answer 17

Yes, it can sometimes be caused by illnesses that injure the intestinal mucosa.

Question 18

Describe the process of glucose absorption in the gut.

Answer 18

The Na+/Gluc transporter co-transports both glucose and Na+ into the cell from the intestinal lumen. Glucose then goes through a Gluc transporter (facilitated diffusion) into the blood, and the Na+ is pumped out of the cell using the Na+/K+ pump, which uses ATP.

Question 19

Which GLUT transporter is insulin-responsive? Is this transporter found in the liver?

Answer 19

The GLUT 4 transporter is insulin responsive and it is NOT found in the liver! GLUT2 is in the liver (and in pancreatic beta cells, too)

Question 20

Where are GLUT4 transporters found?

Answer 20

Muscle, fat, and white blood cells!

Question 21

How does insulin affect GLUT4 transporters?

Answer 21

When insulin goes into a muscle, fat, or WB cell, the GLUT4 transporters are recruited to the surface to take glucose in.

Question 22

Where are the only two places where active transport is used to transport glucose?

Answer 22

Intestine and kidney.

Question 23

Where do carbs go first after being absorbed?

Answer 23

The liver via portal circulation.

Question 24

What factors affect the glycemic index?

Answer 24

1. Sugar content (eg glucose vs. fructose)
2. Type of starch (eg amylose vs amylopectin)
3. Physical barriers (eg bran)
4. Vistocity of soluble fiber
5. Fat and protein content of accompanying foods (affects gastric transport)
6. Acid content (affects gastric transport)
7. Food processing (eg rolled vs. quick oats)

Question 25

How do cells keep glucose in? What enzymes do this?

Answer 25

Cells phosphorylate glucose to keep them in. The enzymes that do this are hexokinase and glucokinase.

Question 26

Where is glucokinase found? Is it product inhibited?

Answer 26

It is found in the liver and pancreatic beta cells. It is NOT product-inhibited.

Question 27

Where are hexokinases found? Is it product inhibited?

Answer 27

It is ubiquitous (found in all cells), and it IS product inhibited.

Question 28

Of the isoenzymes glucokinase and hexokinase, which enzyme has a lower Km and Vmax? What is the benefit of this?

Answer 28

Hexokinase has a low Km and Vmax. Its Km is much lower than normal [glucose] in cells so it is constantly going full-speed. This makes it less sensitive to fluctuations in glucose concentration.

Question 29

What are the benefits of glucokinase’s kinetics?

Answer 29

Glucokinase works at high glucose concentrations in the liver and can handle high glucose concentrations to convert to g-6-P for the cell.

Question 30

What is the defect in MODY patients?

Answer 30

They have a mutation in the TFs that regulate glucokinase or a mutation in the glucokinase gene itself (more common).

Question 31

How does G-6-P raise insulin levels?

Answer 31

G-6-P is used in glycolysis to generate ATP. ATP regulates the ATP-sensitive K+ channel, which causes membrane depolarization. Ca2+ then flows into the cell, which causes secretion of insulin-containing secretory vesicles.

Question 32

What is Tauri’s disease?

Answer 32

Defect in the PFK that is used in muscle. Fructose makes these patients feel better because it bypasses this step.