MCB Lecture 13 Glycogenolysis and Glycogenesis, PDH, Citric acid cycle

Question 0

Describe the steps involved in glycogenolysis

Answer 0

1. Glycogen phosphorylase enzyme breaks off glucose-1-phosphate molecules from the non-reducing end

NB phosphate attached, phosphate cleaved the a-1,4 bond

2. Debranching enzyme has two functions:
   a. transferring strings of glucose to the linear chain to isolate on glucose on the side branch
   b. cleaving the a-1,6 bond of the glucose on the side chain

This creates a linear chain for further glycogen phosphorylase activity

3. Phosphoglucomutase enzyme move the phosphate from 1C to 6C

The serine residue on the phosphoglucomutase enzyme is phosphorylated. This phosphate group move to the 6-carbon position, then the phosphate on the 1-carbon is cleaved and ends up on the serine residue.

Question 1

Differentiate between Glycogenesis, glycogenolysis, gluconeogenesis and glycolysis.

Answer 1

Glycogenesis: making glycogen from glucose monomers
Glycogenolysis: breaking down glycogen into glucose monomers
Gluconeogenesis : converting pyruvate back to glucose
Glycolysis: breaking glucose down into pyruvate

Question 2

Outline the steps in Glycogenesis

Answer 2

1. UDP-glucose phosphorylase catalyses Glucose-1-phosphate molecules being added to UTP, making UDP-glucose, releasing pyrophosphate (PiPi)
2. Glycogen synthase transfers the glucose from the UDP-glucose onto the end of a glycogen chain
3. Glycogen branching enzyme transfers a group of glucose monomers in a linear chain onto the 6-carbon of another monomer, creating a branch.
4. Glycogenin is the primer for a glycogen molecule.

Glycogen synthase isn’t able to start the molecule itself, a primer is required.
Glycogen in is both the primer and the enzyme making the primer

Question 3

Describe the structure of glycogen

Answer 3

Branched chains of a-D-glucose monomers.
a-1,4 glycosidic bonds between the monomers in the linear chain
a-1,6 glycosidic bonds between the molecules at the branch points

It has a reducing and a non reducing end.

Question 4

How is Citrate formed?

Anything else?

Answer 4

Acetyl-CoA + Oxaloacetate -> Citrate
Enzyme: citrate synthase

This is inhibited by ATP

Question 5

Citrate -> ?
Enzymes?
Anything else?

Answer 5

Isocitrate

Aconitase

Question 6

Isocitrate -> ?
Enzymes?
Anything else?

Answer 6

Alpha-ketogluterate
Isocitrate dehydrogenase
NADH produced

Question 7

Alpha-ketogluterate -> ?
Enzymes?
Anything else?

Answer 7

Succinyl-CoA
Alpha-ketogluterate dehydrogenase complex
NADH produced
CO2 evolved

Question 8

Succinyl CoA -> ?
Enzymes?
Anything else?

Answer 8

Succinate
Succinyl CoA synthetase
GTP evolved

Question 9

Succinate -> ?
Enzymes?
Anything else?

Answer 9

Fumarate
Succinate dehydrogenase
FADH2 produced

Question 10

Fumarate -> ?
Enzymes?
Anything else?

Answer 10

Malate

Fumarase

Question 11

Malate -> ?
Enzymes?
Anything else?

Answer 11

Oxaloacetate
Malate dehydrogenase
NADH produced

Question 12

At which points is GTP formed?

Answer 12

Succinyl-CoA -> Succinate

Question 13

In which steps is NADH produced?

Answer 13

Isocitrate -> alpha-ketogluterate

Alpha-ketogluterate -> succinyl CoA

Malate -> Oxaloacetate

Question 14

In which steps is FADH2 produced?

Answer 14

Succinate -> fumarate

Question 15

Describe the function of the PDH complex

Answer 15

Converts pyruvate -> acetyl-CoA

3,4,5 reaction : 3 subunits of PDH
4-step reaction
5 cofactors

TPP, lipoate, NAD, FAD, CoA-SH

Question 16

Which vitamins are especially important for metabolism?

Give some examples

Answer 16

These vitamins are cofactors for many important steps in cellular respiration

Niacin: NADH and NADPH
Riboflavin: FAD and FMN
Thiamin: TPP
Pantothenic acid: coenzyme A
Biotin: pyruvate carboxylase

Question 17

Where is glycogen stored?

Answer 17

Mostly in muscle cells and liver cells as granules

Question 18

What percentage of the total energy stored in a 70kg person is in the form of glycogen

Answer 18

Not much 1%

Question 19

Which bonds are present in glycogen?

Answer 19

Alpha-1,4

Alpha-1,6

Question 20

Where is the PDH complex?

Answer 20

In the mitochondria

Question 21

What are the three irreversible steps of the citric acid cycle?

What is also important about these steps?

Answer 21

1. Pyruvate -> acetyl CoA
2. Isocitrate -> alpha ketogluterate
3. Alpha ketogluterate -> succinyl co-A
   They are the control steps of the citric acid cycle

Question 22

How many vitamin Bs are there?

Answer 22

8

Question 23

What is the first point of control of the citric acid cycle?

Answer 23

The PDH complex

Pyruvate -> acetyl CoA

Question 24

Describe the first control point of the citric acid cycle

Answer 24

1. The PDH complex

(+) ADP, pyruvate
(-) Acetyl-CoA, NADH, ATP

Question 25

Describe the second control point of the citric acid cycle

Answer 25

2. Isocitrate -> alpha ketogluterate
   Isocitrate dehydrogenase

(+) ADP
(-) NADH, ARP

Question 26

Describe the third control point of the citric acid cycle

Answer 26

3. Alphaketogluterate dehydrogenase complex

(+) none
(-) NADH, ATP, succinyl-CoA

Question 27

Which steps of glycolysis produce NADH?

Answer 27

i. Isocitrate -> alpha ketogluterate
ii. Alpha ketogluterate -> succinyl-CoA
iii. Malate -> oxaloacetate

Question 28

Which steps of the citric acid cycle produce FADH2?

Answer 28

Succinate -> fumarate

Question 29

Which steps of the citric acid cycle produce GTP?

Answer 29

Succinyl-CoA -> succinate

Question 30

Which steps of the citric acid cycle evolve CO2?

Answer 30

Isocitrate -> alpha ketogluterate

Alpha ketogluterate -> succinyl-CoA