Chapter 20: Pentose Phosphate Pathway

Question 1

Pentose phosphate pathway

Answer 1

Generates two NADPH and synthesizes five-carbon sugars

• NADPH is a key product of the pentose phosphate pathway, source of biosynthetic reducing power

The first phase of the pentose phosphate pathway is the oxidative generation of NADPH via glucose 6-phosphate dehydrogenase

The second phase is the nonoxidative interconversion of a variety of phosphorylated sugars

The reactions take place in the cytoplasm.

Question 2

Functions of NADPH

Answer 2

Assists cellular bleach activity; acts as an antibacterial

Maintains reduced glutathione to protect against reactive oxygen species (ROS)

Question 3

PPP regulation

Answer 3

The rate of the oxidative phase of the pentose phosphate pathway is controlled by the level of NADP⁺

The first reaction, dehydrogenation of glucose 6-phosphate by glucose 6-phosphate dehydrogenase is the rate-limiting step of PPP

The most important regulatory factor is the concentration of NADP⁺

Question 4

PPP modes

HIGH need for ribose-5-phosphate

Answer 4

Fructose-6-phosphate AND glyceraldehyde-3-phosphate are shuttled into the production of ribose-5-phosphate

Question 5

PPP modes

Balanced need for NADPH and ribose-5-phosphate

Answer 5

Glucose-6-phosphate is oxidized to ribulose-5-phosphate and then into ribose-5-phosphate

Question 6

PPP modes

HIGH need for NADPH

Answer 6

Ribose-5-phosphate is shuttled back into glyceraldehyde-3-phosphate and fructose-6-phosphate to be converted back into glucose-6-phosphate and subsequently oxidized to produce NADPH

Question 7

PPP modes

Balanced need for NADPH and ATP

Answer 7

Glucose-6-phosphate is oxidized to ribulose-5-phosphate to produce NADPH

Ribose-5-phosphate is then fed back into glycolysis as fructose-6-phosphate and glyceraldehyde-3-phosphate

Question 8

NADPH-glutathione pathway

Answer 8

Question 9

Oxidative pathway

PPP

Answer 9

Generates two NADPH molecules

1. G6P dehydrogenase converts G6P to 6-phosphgucono-δ-lactone via the reduction of NADP⁺ to NADHP
2. Lactonase converts 6-phosphgucono-δ-lactone to 6-phosphogluconate via a ring-opening reaction
3. 6-phosphogluconate dehydrogenase converts 6-phosphogluconate to ribulose-6-phosphate via the reduction of NADP⁺ to NADHP and the loss of one carbon as CO₂

Question 10

Non-oxidative pathway

PPP

Answer 10