Lecture 22: Glycogen Metabolism 1

Question 1

Describe the structure of glycogen

Answer 1

homopolymers of glucose molecules that attach via alpha 1,4 glycosidic bonds with alpha 1,6 branches

The non-reducing ends contain a terminal glucose with a free hydroxyl at carbon 4 and it is where all of the reactions occur and the reducing end is attached to glycogenin

Question 2

Where is glycogen stored?

Answer 2

liver and muscle in the form of granules which aid in glycogen metabolism

liver glycogen regulates the blood glucose and the muscle glycogen is a fuel source

Question 3

Describe glycogen metabolism

Answer 3

regulated storage and release of glucose

The synthesis and degradation of glycogen occurs in independently regulated pathways

Question 4

how are the pathways of glycogen metabolism independently regulated?

Answer 4

1. allosteric control
2. covalent modification
3. hormonal control

Question 5

Describe how glucose gets trapped in the cells

Answer 5

Glucokinase and hexokinase in the cytosol helps the phosphorylation of glucose to glucose 6 phosphate, which traps the glucose molecules

Question 6

What takes G6P to G1P in glycogenesis? (second step)

Answer 6

Phosphoglucomutase

Question 7

What takes G1P to UDP-glucose in glycogenesis?

Answer 7

UDP-glucose pyrophosphorylase with the help of UTP takes the Glucose 1 phosphate, phosphate group to the UTP which makes UDP-glucose (active form of glucose)

Question 8

Describe the formation of UDP-glucose

Answer 8

catalyzed by UDP-glucose pyrophosphorylase

Question 9

Describe the second potion of glycogenesis in which the elongation of the glycogen primer occurs

Answer 9

The pre-exisiting glycogen polymer is a primer that adds glucose units via the glycogen synthase which allows for the glucose molecules to be transferred and form the alpha 1,4 bonds between the molecules

Question 10

Describe the third step of glycogenesis in which the glycogen molecule becomes branched

Answer 10

When the chains reach 11 residues, a fragment breaks off and goes to attach elsewhere with an alpha 1,6 bond with glucose transferase

Question 11

What is the purpose of branching in the glycogen molecules?

Answer 11

Branching increases solubility of glycogen and increases the number of terminal non-reducing ends

THIS increases the rate at which glycogen can be synthesized and degraded

Question 12

Describe the basics of glycogenolysis

Answer 12

glycogen is broken down to release G1P which can be converted to G6P which can go into glycolysis, the blood stream, or the pentose phosphate pathway

Question 13

What do you want the 4 enzymes in glycogenolysis to do?

Answer 13

degrade the glycogen
remodel the glycogen remnants (2)
and convert the glycogen breakdown to one product that is suitable for metabolism

Question 14

Describe chain shortening in glycogenolysis

Answer 14

uses the enzyme glycogen phosphorylase (GP) that catalyzes the cleavage of glycogen and is the rate limiting enzyme

GP goes to the non-reducing end and adds an orthophosphate which releases glucose as G1P with pyridoxal phosphate (vitamin B6) as a cofactor. GP can keep working until it gets 4 residues within the alpha 1,6 linkage

Question 15

Describe the phosphoglucomutase step in glycogenolysis

Answer 15

converts G1P to G6P, They enzyme adds a Phosphate to the G1P, which makes G16BP and then the G1 phosphate is transferred back to its enzyme to take it to its original state

Question 16

Describe the branch transfer and release of glucose from the glycogen

Answer 16

Transferase transfers 3 of the remaining 4 glucose to the non-reducing end of the main chain to make the alpha 1,4 bond

alpha 1,6 glucosidase cleaves the alpha 1,6 bond of the single remaining glucose to release the free glucose

This glucose is then phosphorylated by hexokinase.

Question 17

What are the two enzymes that take the branched glycogen molecule into the linear structure?

Answer 17

Transferase and alpha-1,6 glucosidase

Question 18

Describe the Glucose 6-phosphatase step of glycogenolysis

Answer 18

Glucose-6-phosphate cannot get out of the cell

The liver has glucose 6-phsphatase that is able to convert the G6P to glucose that can be released

The muscle DOES not have glucose 6-phosphatase so the G6P stays

Question 19

Lysosomal glycogenolysis

Answer 19

Last slide in the lecture, couldn’t think of a way to make a flashcard