Pentose phosphate pathway

Question 1

Where does the pentose phosphate pathway occur?

Answer 1

In the cytoplasm

Question 2

What are the 2 phases of the pentose phosphate pathway?

Answer 2

1. oxidative generation of NADPH

2. non oxidative conversion of sugars

Question 3

Describe in detail the 3 steps of the oxidative phase of the PPP

Answer 3

1. Glucose-6-phosphate is turned into a lactone with glucose-6-phosphate dehydrogenase
   - reduces NADP+ to form NADPH
   - RATE LIMITING
2. Lactonase opens the ring by adding water
3. 6-phosphogluconate turns into ribulose-5-phosphate with 6-phosphogluconate dehydrogenase via oxidative decarboxylation
   - reduces NADP+ to form NADPH

Question 4

Why do we need NADPH?

Answer 4

NADPH is a cofactor for reductases

1. synthesis of monomers: fatty acid biosynthesis, cholesterol biosynthesis, neurotransmitter biosynthesis, nucleotide biosynthesis
2. reducing power- reduction of oxidized glutathione, cytochrome P450 monooxygenases

Question 5

Describe Glutathione

Answer 5

Glutathione is a tripeptide that is made up of glutamic acid (GLU), cysteine, and glycine: glu-cys-gly
It is a MAJOR molecule used in redox;

Takes away disulfide bonds which can either lead to refolding or protein stabilization

Can inactivate peroxides which requires glutathione peroxidase; can remove reactive oxygen species; these are able to maintain the redox of the cell but may cause damage to macromolecules

Can regenerate reduced glutathione; which needs NADPH

Question 6

Describe the relationship of GSH:GSSG in a healthy cell (GSH=reduced glutathione, GSSG=oxidized glutathione)

Answer 6

500:1

Question 7

Describe Glucose-6-phosphate dehydrogenase deficiency.

Answer 7

Deficiency in the first enzyme that is involved in the PPP, is the most common genetic disease, and it is X-linked recessive with missense mutations

There are various phenotype manifestations you should give a look over on slide 27.12

leads to an overall decrease in NADPH which causes problems with many of the cellular processes

Question 8

What are the three types of erythrocyte diseases?

Answer 8

1. hemaglobinopathies
2. membrane/cytoskeleton defects
3. metabolic errors

Question 9

Describe the pathophysiology that is related to a decreased G6PD activity

Answer 9

decreased G6PD leads to a decreased NADPH level which leads to a lower GSH level and in return an increased ROS level

This causes cell membrane damage and oxidized Hb precipitates which leads to hemolysis that can lead to hemoglobinuria, anemia, and increased bilirubin

Question 10

Describe the relationship between G6PDD and hemolytic anemia

Answer 10

RBCs are destroyed in hours and the G6PD has a shorter lifespan than normal; Fe in the heme must be reduced to bind O2; acute/induced anemias are temporary

Question 11

What are the other proteins that are able to make NADPH (aside from G6PD)

Answer 11

1. isocitrate dehydrogenase (isozymes of this)

2. malic enzyme 1

Question 12

Describe the relationship between G6PDD and neonatal jaundice

Answer 12

most severe effect; blood is normal but the liver is not able to conjugate bilirubin; so the unconjugated bilirubin aggregates and then enters the brain (kernicterus) which causes deafness, CP, or death

Question 13

Describe the structure of G6PD including the composition of each structure (monomer form, dimer form, and tetramer form), conditions that are favoring this form, and the activity of the enzyme

Answer 13

Monomer form: 514 amino acid chain, favored by high levels of NADPH, G6P, and pH (higher). INACTIVE
Dimer form: homodimer, favored by increased ionic strength and a ph that is greater than 8. ACTIVE
Tetramer form: dimer of dimers; favored by increased NADP+, and a pH that is less than 6 (decreased). ACTIVE

Question 14

Describe the relationship between G6PD and Ataxia Telangiectasia

Answer 14

AT is an autosomal recessive mutation in the ATM that leads to a progressive loss of coordination and increased sensitivity to mutagens

The ATM protein controls the rate of cell division and initiates the repair of DS breaks; also activates Hsp27

This Hsp27 interacts with G6PD and promotes dimerization

Question 15

Describe G6PD regulation (there are 3 of them)

Answer 15

1. transcription/ translation
2. location in cell
3. post-translational controls

Question 16

What are the activators of G6PD

Answer 16

Dimerization
Transcription factors for antioxidant genes
Cell cycle and synthesis activators
Insulin

Question 17

What are the inhibitors of G6PD

Answer 17

Phosphorylation

Apoptosis signaling proteins

Question 18

Describe the relationship of G6PD and diabetes

Answer 18

insulin activates G6PD which will increase the extracellular glucose, who can activate G6PD if it is in the right cell

Question 19

Describe the role of the pancreatic Beta cells

Answer 19

Produce insulin and are lost with diabetic pathology
have low levels of G6PD
High external glucose leads to decreased G6PD

Question 20

What are the overall 4 “shuffles” that take place in the non oxidative phase of the PPP

Answer 20

1. Ribulose-5 phosphate isomerization
2. Transketolase transfers 2C from the xylulose 5-phosphate, leaving behind glyceraldehyde 3 phosphate
3. Transaldolase transfers 3C units
4. Regeneration of Glucose 6-phosphate

Question 21

Describe the FIRST shuffle in the non oxidative phase

Answer 21

Ribose 5 phosphate isomerase converts ribose 5-phosphate

Ribulose 5-phosphate wpimerase converts to xylulose 5-phosphate

starts as Ribulose 5- phosphate and ends as xylulose-5-phosphate

Question 22

Describe the SECOND shuffle that occurs in the non oxidative phase (the end product of the First shuffle is xylulose-5-phosphate)

Answer 22

Transketolase transfers a 2C from xylulose 5 phosphate and leaves glyceraldehyde 3-phosphate behind

This 2C is added to ribose-5-phosphate to make sedoheptulose-7-phosphate
OR
added to the erythrose 4-phosphate to make fructose 6- phosphate

Question 23

What is the transition state in the second shuffling reaction of the nonoxidative phase

Answer 23

TPP coenzyme

Question 24

Describe the THIRD shuffle that takes place in the nonoxidative phase of the PPP (the end products of the second shuffle include sedoheptulose 7-phosphate or fructose 6-phosphate)

Answer 24

Transaldolase Transfers 3C units from sedoheptulose leaving behind erythrose-4-phosphate and takes the 3C to the glyceraldehyde 3-phosphate to make fructose-6-phosphate

*** makes the same product as the second shuffle

Question 25

What is the transition state of the third shuffle of the non oxidative phase stabilized by?

Answer 25

Lys side chain

Question 26

Describe the FOURTH shuffle that takes place in the non oxidative phase

Answer 26

regeneration of the glucose 6-phosphate and utilizes the gluconeogenesis pathway

Question 27

Describe the first mode of the PPP: nucleotide synthesis needs

Answer 27

The body needs more ribose 5P than NADPH (because ribose 5-P is needed for nucleotide synthesis)

Glucose is taken through glycolysis so that the intermediates (fructose-6-phosphate and glyceraldehyde 3-phosphate) can be used to make more ribose-5-phosphate with the help of transketolase and transaldolase

Question 28

Describe the second mode of the PPP: balancing nucleotides and NADPH

Answer 28

glucose 6-phosphate is taken to ribulose 5-phosphate with a reaction that yields NADPH for the body. Then the ribulose 5-phosphate is taken to the ribose-5-phosphate

So basically there are “equal amounts” so to speak

Question 29

Describe the third mode of the PPP: anabolic needs

Answer 29

The body is needing more NADPH,

G6P is taken into ribulose-5-phosphate (gives off NADPH) which is then able to be taken to ribose-5 phosphate and basically go through a pathway that is similar to the last half of gluconeogensis (from G3P/DAG to glucose 6-phosphate) in order to make more G6P which can then go through another reaction of forming ribulose-5-phosphate which will release NADPH

Question 30

Describe the 4th mode of the PPP: more power!

Answer 30

the same reaction of the G6P being taken to ribulose phosphate is used (as in mode 3) to generate NADPH and a similar pathway is taken in order to generate more G6P to lead to more NADPH molecules However in this reaction, glycolysis is favored from the G3P to pyruvate because ATP is made (2)