Chapter 26

Question 1

Pentose Phosphate Pathway Overview

Answer 1

Pentose phosphate pathway generates pentoses or 5-carbon sugars

Also generates NADPH = key product in PPP and key source of biosynthetic reducing power

PPP occurs in cytoplasm

PPP occurs in two phases:

1. Oxidative generation of NADPH
2. Non-oxidative interconversion of a variety of different sugars

–> Reducing power is required for a variety of different biosynthetic pathways

–> Liver and adipose tissue require large amounts of NADPH for fatty acid synthesis

Question 2

Explain NADPH Production

Answer 2

Glucose 6-phosphate dehydrogenase oxidizes glucose 6-phosphate into 6-phosphoglucono-δ-lactone ~ reducing NADP+ to NADPH

• A H2O molecule is used to add a hydroxy to set up molecule

Second round of oxidation generates:

• ribulose 5-phosphate
• a CO2
• another NADPH

Question 3

Sugar conversion of Ribulose-5-phosphate can be converted into two things. Exlpai them, and how it works

Answer 3

Ribulose-5-phosphate, generated by the oxidative phase, is isomerized into…

1. ribose-5-phosphate via phosphopentose isomerase
2. or there’s the conversion into xylulose-5-phosphate via phosphopentose epimerase

• Newly formed entities can then act as substrates for transaldolases and transketolase enzymes

Question 4

Transketolase and Transaldolase reaction overview

Answer 4

Transaldolase and transketolase enzymes transfer sugar carbons

Transketolase transfer 2-carbon sugars

Transaldolase transfers 3-carbon sugars

Net result of reaction is the formation of two hexoses (6-C) and one triose (3-C) from 3-pentoses (5-C)

Question 5

Transketolase and Transaldolase: 1. C5 + C5 –> C3 + C7

Answer 5

• Starts w/ 2-riboses (each 5-C)
• Results in generation of one 3-C and one 7-C (catalyzed by transketolase)
• The products are then used as substrates (reactants) in next reaction

Question 6

Transketolase and Transaldolase: 2. C3 + C7 –> C6 + C4

Answer 6

• Starts w/ one 3-C and one 7-C
• Results in one 6-C and one 4-C (catalyzed by transaldolase)
• Fructose 6-Phosphate leaves
• Products erythrose 4-phosphate (C-4) and xyulose 5-phosphaet (C-5) are used as substrates (reactants) in next reaction

Question 7

Transketolase and Transaldolase: 3. C4 + C5 –> C6 + C3

Answer 7

• Starts w/ one 4-C and one 5-C
• Results in one 6-C (fructose 6-phosphate) and one 3-C (glyceraldehyde 3-phosphate)

Question 8

Regulating Pentose Phosphate Pathway

Answer 8

First reaction of PPP and dehydrogenation of glucose-6-phosphate by glucose-6-phosphate dehydrogenase = rate limiting step of PPP

Rate of oxidative phase of PPP is controlled by concentration of NADP+, = most important regulatory factor of pathway

PPP can operate in 4-distinct modes that result from various combinations of oxidative phase, non-oxidative phase, glycolysis, and gluconeogenesis

4-modes occur when Ribose 5-phosphate…

 Exceeds the needs for NADPH

 The NADPH and ribose 5-phosphate needs are balanced

 More NADPH is needed than ribose 5-phosphate

 NADPH and ATP are both required

Question 9

Describe the 4 modes that PPP can operate in and what happens in each

Answer 9

 In Mode 1, ribose 5-phosphate needs exceed the needs for NADPH

 In Mode 2, the NADPH and ribose 5-phosphate needs are balanced

 In Mode 3, more NADPH is needed than ribose 5-phosphate

 In Mode 4, NADPH and ATP are both required

Question 10

List some tissues w/ active PPP

Answer 10

Adrenal gland –> steroid synthesis

Liver –> fatty acid and cholesterol synthesis

Testes –> steroid synthesis

Adipose tissue –> fatty acid synthesis

Ovary –> steroid synthesis

Mammary gland –> fatty acid synthesis

RBC –> maintenance of reduced glutathione