Chapter 26- Pentose Phosphate Pathway

Question 1

What is the Pentose Phosphate Pathway (PPP)?

3 functions

Answer 1

1. Produces NADPH, a key product of the PPP, is source if biosynthetic reducing power for fatty acids, steroids, etc
2. Produces Ribose 5-phosphate required for the synthesis of nucleotides
3. Provides an entry point for dietary pentose sugars

Indicates there are phosphorylated pentose sugars in the pathway

Question 2

What are the 2 phases of the pentose phosphate pathway (PPP)?
What are the reactions for each phase? (Reversible)

Answer 2

First phase is oxidative generation of NADPH
3 reactions are here (irreversible)
Slide 5
Second phase is the nonoxidative interconversion of a variety of sugars
5 reactions are reversible
Slide 8
Overview slide 4

Question 3

What are the complex steps and 3 enzymes used in the 3 oxidative irreversible steps of the first phase of the pentose phosphate pathways?

Answer 3

1. Glucose 6-phosphate dehydrogenase
2. Lactonase
3. 6-phosphogluconate dehydrogenase

Slide 6

Question 4

What are the 5 nonoxidative reversible reactions?

Answer 4

Slides 7-8

Interconversion between different sugars

Question 5

How is ribulose 5-phosphate converted to ribose 5-phosphate?
What enzyme

Answer 5

Slide 9
Done by phosphopentose isomerase

Ketose to aldose

Question 6

How is ribulose 5-phosphate to xylulose 5-phosphate?

What enzyme?

Answer 6

Slide 10
Done by phosphopentose
Epimerase

Ketose to ketose

Question 7

How is xylulose 5-phosphate and ribose 5-phosphate converted to glyceraldehyde 3 phosphate and sedoheptulose 7-phosphate?
What enzyme?
Slide 11

Answer 7

Slide 11
Done by transketolase - has TPP as prosthetic group
This is interconversion between sugars
Ketose and aldose = ketose and aldose

Only place we’ll see a 7 carbon sugar!

Question 8

How is glyceraldehyde 3P and sedoheptulose 7P converted to fructose 6P and erythrose 4P?
What enzyme?
Slide 12

Answer 8

Slide 12
Done by transaldolase
Aldose and ketose = aldose and ketose
Interconversion of sugars

Question 9

How is erythrose 4P and xylulose 5P converted to fructose 6P and glyceraldehyde 3P?
What enzyme?
Slide 13

Answer 9

Slide 13
Done by transketolase
Interconversion of sugars
Aldose + ketose = ketose +Aldose

Question 10

How many carbons dose transketolase transfer compared to transaldolase?

Answer 10

Transketolase- transfers 2 carbons

Transaldolase- transfers 3 carbons

Question 11

What links the pentose phosphate pathway to glycolysis?

Answer 11

Transketolase and transaldolase

Slide 14

Question 12

What is the overall reaction of the pentose phosphate pathway?

Answer 12

3C6 (G6P) + 3H2O +6NADP —> 2C6 (F6P) + 1C3 (GAP) + 6NADPH + 6H+ + 3CO/

Question 13

Howis the pentose phosphate pathway regulated (controlled)?

What do low and high levels cause?

Answer 13

Controlled by the level of NADP+
Rate limiting step on this pathway is the dehydrogenation of glucose 6P by glucose 6P hydrogenase

Low levels of NADPH mean system needs more and there will be no effect
High levels of NADPH inhibit it’s own production on reaction 1

Question 14

What are the 4 distinct modes of the pentose phosphate pathway that result from various combinations of the oxidative phase, non oxidative, glycolysis, and gluconeogenesis?

Answer 14

1. Mainly ribose 5-phosphate is needed
2. Both NADPH and ribose 5-phosphate are needed
3. Mainly NADPH is needed
4. NADPH and ATP are required