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Flashcards in Glycogenesis/-olysis (Ben) Deck (21)
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1

What is the 1st step of glycogen synthesis?

Phosphorylation of glucose to glucose-6-phosphate via hexokinase (muscle) or glucokinase (liver)

2

What is the 2nd step in glycogenesis, after phosphorylation of its monomers?

Substrate?

Enzyme?

Intermediate?

Product?

Isomerization of g-6-p...

Substrate: Glucose-6-phosphate

Enzyme: Phosphoglucomutase

Intermediate: Glucose 1,6-Bisphosphate

Product: Glucose-1-phosphate

3

What is the enzyme catalyzing the isomerization of glucose-6-phosphate and how does it do this?

Phosphoglucomutase

- is phosphorylated on a serine residue and gives up this phosphoryl group to G-6-P to make G-1,6-BP

- then takes it back to leave G-1-P

 

4

What inhibits phosphoglucomutase?

Diisopropylfluorophosphate 

AKA DIPF

5

What is the third step in glycogenesis (after isomerization of G6P)?

Reactants?

Enzyme?

Products?

Combination of G-1-P and UTP...

Reactants: Glucose-1-phosphate + Uridine Triphosphate

Enzyme: UDP-Glucose Pyrophosphorylase

Products: UDP-Glucose + Pyrophosphate (PPi)

6

Why must an "active" form of glucose be made by addition of a nucleoside?

To overcome energetic unfavorability of polymerization by creating high group transfer potential

7

What drives the reaction catalyzed by UDP-Glucose Pyrophosphorylase?

Pyrophosphatase hydrolyzes the pyrophosphate cleaved off of uracil-triphosphate creating two Pi

- this pulls the reaction forward by getting rid of one of its products

 

8

What characteristics of UDP-Glucose Pyrophosphorylase make its reaction not a regulatory step of glycogenesis?

- low Km for G-1-P

high concentration

9

What kind of bonds exist between glucose monomers in glycogen?

And at branching points?

between monomers: alpha 1-4 glycosidic bonds

at branching points: alpha 1-6 glycosidic bonds

10

What are the two "ends" of glycogen chains + what characterizes them?

Non-reducing End - no free glycosidic -OH

Reducing End - theoretically a free glycosidic -OH but in reality it is bound to glycogenin (which would be the "R" group in the image below)

11

Once UDP-glucose is formed, glycogen chain synthesis can begin.

Via what central molecule + how?

 

Glycogenin

autocatalyzes glucosylation via UDPGlc

- adds 7 glucose residues to itself to form glycogen primer

12

Once a glycogen primer is formed, how is it elongated?

Glycogen Synthase

- creates further alpha 1-4 bonds at the non-reducing end of the glycogen primer

13

How are glycogen branches made?

Glycogen Branching Enzyme

(Glycosyl 4,6 Transferase)

- when a glycogen branch is at least 11 glucoses long, this enzyme transfers a part of the chain to the middle of a neighboring chain via alpha 1-6 bond

14

What are 2 advantages of glycogen's branched structure?

  • higher solubility
  • faster metabolism - via more non-reducing terminals

15

How are glycogen chains (not branches) broken down?

By what?

Into what?

Glycogen phosphorylase

- cleaves 1-4 linkages via phosphorolysis to create glucose-1-phosphate

 

16

What does glycogen phosphorylase require in order to function?

How is it connected to this required co-factor?

Pyridoxal Phosphate (PLP)

- a coenzyme connected via Schiff-base linkage at the enzymes active site

 

17

How are glycogen branches broken down?

The enzyme has two activities. What are they?

Once the chains have been shortened to about 4 glucoses on either side of the branch point...

Debranching Enzyme uses...

Glucan transferase activity to move a trisaccharide from one side of the branch to the other, and...

1-6-Glycosidase activity to remove the one glucose at the branch point.

 

21

Once glycogen has been broken into monomers, how is it used?

2 steps

  1. Phosphoglucomutase changes G1P to G6P
  2. Glucose-6-Phosphatase removes phosphate to make glucose

22

In what other way can glycogen granules be broken down, especially in neonates?

What genetic disorder relates to this?

Lysosomal Degradation

- lysosomes engulf granules and maltase hydrolyzes glycogen

 

Pompe Disease

- lack of lysosomal maltase

- causes lysosomal glycogen buildup + muscle hypotonia/dystrophy

23

What genetic disorder of glycogen metabolism results from lack of a dephosphorylating enzyme in the liver?

What are its two subtypes?

Von Gierke Disease (Type I Glycogenosis)

Type Ia

  • lack of glucose-6-phosphatase
  • glycogen accumulates in liver + kidneys
  • hypoglycemia; lactic acidemia; ketosis; hyperlipemia

Type Ib

  • lack of ER G-6-P Transporter
  • same effects + impaired neutrophil function

 

 

24

What genetic disorder effects muscle glycogen metabolism?

How?

McArdle Disease (Type V Glycogenosis)

- lack of muscle-type glycogen phosphorylase

- muscle glycogen buildup + painful cramps