Glucose metabolism and gluconeogenesis

Question 1

At which end of glycogen is glc added/removed?

Answer 1

at the nonreducing end

Question 2

Why store glucose as glycogen?

Answer 2

reduce osmotic pressure

Concentration gradient

Question 3

Why glycogen instead of starch?

Answer 3

Glycogen has a high branching network which makes more glucose easier to access at one time when needed quickly.

Question 4

Phospholysis of glycogen yields:

Answer 4

(glc)n-1 and glc-1-P by glycogen phosphorylase

G1P is then converted into G6P, which undergoes glycolysis in the muscle or is converted to glc and dumped into the blood in the liver

Question 5

Can glc-1-P go directly into glycolysis?

Answer 5

Noap. Must first be converted to glc-6-P, ya ninny

Question 6

By what process is the last glucose on a glycogen branch removed? what does it yield?

Answer 6

The final glucose is removed by hydrolysis, yielding glc (not G1P)

Question 7

2 activities of debranching enzyme:

Answer 7

1) a(1–>4) transglycosylase (transferring the triglucoside)

2) a(1–>6) glucosidase (hydrolyzing off the glc)

Question 8

input and output of ATP in glycolysis when starting with G6P (product of phosphorlyisis of glycogen)

Answer 8

1 ATP input in the beginning stages of glycolysis.
4 ATPs produced
Net: 3 ATPs yielded

Question 9

5 basic steps of making glycogen

Answer 9

1. convert glc to G6P (glucokinase)
2. convert G6P to G1P (phosphoglucomutase)
3. Further activate G1P by making UDP-glc
4. Add a glc unit to the nonreducing end of a glycogen chain
5. make branches

Question 10

2 forms of phosphorylase

Answer 10

phosphorylas a (phosphorylated, more active form) 
phosphorylase b (not phosphorylated, less active)

Question 11

What enzymes add/remove phosphate from phosphorylase a and b?

Answer 11

• can be attatched or removed by phosphorylase kinase

- removed by phosphorylase phosphotase

Question 12

Hormone cascade which leads to the activation of Glycogen phosphorylase

(Phosphorylation system)

Answer 12

• [glc] low in blood = hormone GLUCAGON dumped in blood.
• glucagon binds to liver receptor, activating ADENYLATE KINASE which activates cAMP
• cAMP activates PKA
• PKA phospphorylates PHOSPHORYLASE KINASE, activating it.
• phosphorylase kinase phosphorylates phosphorylase, activating it
• phosphorylase phospholyses glycogen into G1P which becomes glc and is dumped in blood

Question 13

signal amplification

Answer 13

a small amount of hormone (glucagon) will cause the activation of a large amount of phosphorylase.

Question 14

Does muscle respond to glucagon?

Answer 14

No. That is a huger signal, therefore not the muscle’s problem

Question 15

In the muscle, does AMP or ATP activate or inhibit phosphorylase?

Answer 15

AMP will activate phosphorylase

ATP inhibits it

Question 16

Does phosphorylation of glycogen synthase turn it on or off?

Answer 16

turns it off

Question 17

2 forms of glycogen synthase

Answer 17

1) glycogen synthase a (not phosphorylated, active)
2) glycogen synthase b (phosphorylated, inactive)
(opposite of phosphorylase a and b)

Question 18

glycogen synthase is allosterically activated by…

Answer 18

glc and G6P

Question 19

What does glycogen synthase (GSK) kinase do?

Answer 19

turns off glycogen synthase by phosphorylating it

Question 20

What does insulin do to PKB?

Answer 20

insulin activates PKB, which turns off GSK, thus activating glycogen synthesis

Question 21

common gluconeogenesis precursors:

Answer 21

lac, some amino acids, glycerol

Question 22

where does the conversion of puryate to oxaloacetate take place?

Answer 22

in the mitochondria

Question 23

Can pyr, oxa, NADH or PEP cross the mitochondria?

Answer 23

only pyr and pep can. Oxa and NADH cannot

Question 24

How to overcome the problem of not having cytosolic NADH available for gluconeogenesis?

Answer 24

Depends on precursor used.
-If using lac–>glc, cytosolic NADH can be produced (no need for mitochondrial NADH):

   NAD+   NADH
        l        l lac                pyr  ----->etc.etc.

-If using ala–>pyr–>–>glc, need a transport NADH from mitochondria.

ala–>pyr (enters mito)–>oxa–>mal (can cross membrane into cytosol)–>oxa–>PEP

Question 25

Where do the following rxns take place in the cell?:

1) pyr–>oxa
2) oxa–>PEP

Answer 25

1) mitochondria

2) cytosol and mitochondria

Question 26

Which is the only place where G6P can be converted to glc?

Answer 26

liver

Question 27

How many ATP equivalents are used in glucoeogenesis? How many NADH’s consumed?

Answer 27

6 ATP equivalents used

2 NADH consumed

Question 28

What does fru-2,6-bP do to PFK-1 and FBP-1?

Answer 28

• activates PFK-1

- inhibits FBP-1

Question 29

What does high or low [fru-2,6-bP] signal?

Answer 29

high conc = signal to run glycolysis

low conc = signal to run gluconeogenesis

Question 30

Difference between PFK-2 and FBP-2

Answer 30

SAME PROTEINE. BIFUNCTIONAL ENZYME.

PFK-2 produces F6P from F-2,6-P
FBP-2 does the reverse

Question 31

low blood [glucose] leads to…

Answer 31

increased glucagon secretion–>increased [cAMP]–>Increased enzyme phosphorylation–>Activation of FBP-2 and inactivation of PFK-2–> deacreased [F-2,6-bP]–>inhibition of PFK-1 and activation of FBP-1–>increase gluconeogenesis