CHO Metabolism

Question 1

Can glucose diffuse across the hydrophobic phospholipid bilayer of the cell membrane?

Why or why not?

Answer 1

No, because it is polar

Question 2

How does glucose enter cells from the bloodstream?

Answer 2

binds to transport proteins – proteins that bind glucose on one side of the membrane and release it on the opposite side.

Question 3

What are the two types of transport proteins in intestinal epithelial cells?

Answer 3

1) Na+ dependent glucose transporter

2) Facilitative glucose transporter

Question 4

Where do Na+ Dependent Glucose Transporter reside?

Answer 4

on the lumenal side of absorptive epithelial cells

takes glucose up into cells from the GI lumen

Question 5

How does Na+ Dependent Glucose Transporter work?

Answer 5

When Na+/K+ ATPase transports sodium out the cell, it leaves a deficiency of sodium inside the cell. Sodium then flows down its concentration gradient into the cell, bringing glucose with it into the cell from the lumen (up its concentration gradient- i.e. low in the lumen to high in the cell)

It is considered a secondary active transport- symport

Question 6

T or F. Sodium dependent transporters reside on the lumenal side of absorptive epithelial cells

Answer 6

T. they bring glucose into the cell from the lumen

dependent on ATPase on basal side of intestinal epithelial cells that are keeping intracellular Na+ levels low

Question 7

How is glucose then moved out of epithelial GI cells into the bloodstream?

Answer 7

using GLUT 1-5 protein family (have 12 transmembrane spanning segments)- located on basal side of cells

these move glucose DOWN its conc. gradient (i.e. high to low)

Question 8

GLUT 1 and 3 are found where?

Answer 8

all mammalian tissue (both are particularly important in neuronal cells and BBB)

These have the LOWEST Km!- i.e. easiest/fastest binding of glucose

Question 9

What do GLUT 1 and 3 do?

Answer 9

basal glucose uptake

Question 10

GLUT 2 is found where?

Answer 10

liver and pancreatic B cells

high Km!!! (15-20mM)- only kicks in after a carb rich meal and glucose levels are elevated

Question 11

What does GLUT 2 do in both areas where it works?

Answer 11

*in pancreas- stimulates glucose uptake in pancreatic b-cells. Glucose is metabolized via glycolysis generating ATP resulting in an elevated ATP/ADP ratio in the cells. This results (eventually) in an increase in cytosolic Ca2+ which triggers the fusion of insulin-containing vesicles with the plasma membrane RELEASING INSULIN into the bloodstream. (km=~15mM- aka after meal). So the GLUT 2 are only taking up glucose when BG is high, and insulin is needed

How it works- high ATP causes closure of ATP dependent K+ channels on the membranes, which causes opening of membrane Ca2+ channels, allowing Ca2+ to enter the cell and cause insulin filled vesicles to diffuse out of the cell (uses SNARES for fusion). Insulin will u-regulate the number of GLUT 2 receptors on the cell (feedback)

in liver- removes excess glucose from the blood (high kM ~15). Excess uptake of glucose will be stored as glycogen as a spare energy source

Question 12

GLUT 4 is found where? What stimulates it?

Answer 12

muscle and fat cells

Insulin (stimulated to release in oacreatic b cells by GLUT 2) stimulates the recruitment/fusion of GLUT 4 to the membrane therefore increasing glucose transport (while the rate of re-uptake of the GLUT 4 transporters from the membrane remains the same)

Question 13

GLUT 5 is found where?

Answer 13

small intestine

Question 14

What does GLUT 5 do?

Answer 14

fructose transporter

Question 15

What are the basic 3 steps of Facilitative Glucose Transport?

Answer 15

1) glucose binds on outside of cell through its hydroxyl groups
2) Transport protein undergoes conformational change and glucose is transported without rotating
3) Transport protein closes as glucose is released

NOTE: this assumes extracellular conc of glc is high and intracellular conc is low. If it was reversed, you’d see passive movement of glc into the bloodstream through GLUT

Question 16

What happens first when glucose enters a cell?

Answer 16

it is phosphorylated (or alternatively exported back out).

Question 17

What is glucose phosphorylated by when it enters a cell?

Answer 17

hexokinase (HK) isozymes (ATP dependent process)

Question 18

What is glucose phosphorylated into when it enters a cell?

Answer 18

glucose-6-phosphate

Question 19

Phosphorylation of glucose into glucose-6-phosphate allows glucose to enter which metabolic pathways?

Answer 19

Glycolysis – source of ATP (energy) for cells

Pentose Phosphate pathway – source of NADPH

Glycogen Synthesis – storage polymer of glucose

Question 20

What are the main functions of Glc Phosphorylation? (there are 4)

Answer 20

Controls the amount of glc taken up by cells.

“Activates” glc for further metabolism.

Keeps glc within cell since Glc-6-P cannot be transported across plasma membrane.

Determine the direction of glc metabolism.

Question 21

What enzyme can convert Glucose-6-phosphate back to glucose?

Answer 21

glucose-6-phosphatase (only found in liver and kidney)

Question 22

HK IV is called what?

Answer 22

GLUCOKINASE (GK). GK and HK differ in many of their properties and tissue distribution. (has a high Km- won’t kick in until BG is high)

NOTE: HKs (hexokinases) I-III are very similar

Question 23

Does phosphorylation of glucose by hexokinsase require ATP?

Answer 23

Yes

Question 24

What kinds of tissues can HK I-III be found in?

Answer 24

all tissue

Question 25

What kinds of tissues can GK be found in?

Answer 25

ONLY liver, b-cells of pancreas

Question 26

What is the Km (for glucose) of HK I-III?

Answer 26

0.1 (i.e. wants to bind glucose)

Question 27

What is the Km (for glucose) of GK?

Answer 27

10-20mM (high!!)- under normal circumstances

Question 28

What happens to the Km of GK during fasting situations?

Answer 28

it lowers to facilitate increased uptake/processing of glucose (~5mM)

Question 29

What are HK I-III inhibited by?

Answer 29

glu-6-P. Thus if you make glu-6-p and it doesn’t enter into a metabolic pathway, it inhibits further production of glu-6-p by HK I-III

Question 30

What is GK stimulated by?

Answer 30

insulin. Bringing glucose in and phosphorylating it using GK makes glc-6-p which then goes through glycolysis. Increased ATP as a result of glycolysis results in increased insulin production (similar to how GLUT 2 works). Increased insulin production then causes more uptake of glucose, and the cycle continues until BG returns to normal and GK runs out of glucose to act on (i.e. it is never down regulated)

NOTE: This only occurs when BG is elevated (15-20mM)

Question 31

Where is glucose-6-Phosphatase found?

Answer 31

liver and kidney only. NOT muscle

Question 32

What is another way glu-6-p can be made?

Answer 32

breakdown of glu-1-p from glycogen

Question 33

Why are RBCs completely dependent on glucose for energy?

Answer 33

They don’t have mitochondria- thus, can’t oxidize fatty acids for energy and need glucose. This is why RBCs have tons of GLUT 1 transporters on their cell surface and their Km =1-7 (i.e. very low- will take up lots of glucose given the chance even if BG is low)

Question 34

Why is the brain highly dependent on glucose?

Answer 34

fatty acids cannot get over the BBB to release energy

Question 35

Why does diffusion of glucose across the BBB require GLUTs?

Answer 35

The basal membrane of BBB cells is continuous (effective tight junctions) so no passive diffusion can occur just based on concentration gradients (only other parts of the body-non-neuronal- where gap junctions are spaced further and allow for non-specific passive diffusion- needs GLUT 1

Question 36

What is GLUT 1 used for in the BBB?

Answer 36

diffusion of glucose into the cell

Question 37

What is GLUT 3 used for in the BBB?

Answer 37

utilization of glucose by neuronal cells

Question 38

Does muscle ever break down glycogen for extracellular diffusion to other tissues? Why?

Answer 38

No. Muscle only breaks down glycogen to glucose-6-p for internal use. Glu-6-p can’t be moved outside the organ because muscle does not have glucose-6-phosphatase

Question 39

Where is glu-6-phosphatase found in the cell?

Answer 39

spanning the membrane of the ER

Question 40

How many transmembrane spanning proteins does glu-6-phosphatse have? What are they?

Answer 40

3

T1, T2, and T3

Question 41

What does the T1 protein of glu-6-phosphatase do?

Answer 41

brings glu-6-p into the lumen of the ER where the catalytic portion of the enzyme makes glucose and inorganic phosphate

Question 42

What does the T2 protein of glu-6-phosphatase do?

Answer 42

transports the inorganic phosphate out of the lumen of the ER to the cytosol after breakdown of glu-6-p

Question 43

What does the T3 protein of glu-6-phosphatase do?

Answer 43

transports the glucose out of the lumen of the ER to the cytosol after breakdown of glu-6-p