The Pentose Phosphate Pathway

Question 1

*Understand that the pentose phosphate pathway consists of two phases:

Answer 1

The oxidative generation of NADPH - used in biosynthetic pathways
NADPH is generated when glucose 6-phosphate is oxidized to ribulose 5-
phosphate
The non-oxidative interconversion of sugars
ribulose 5-phosphate is then converted into ribose 5-phosphate which is a
precursor to RNA, DNA, and ATP, NADH, FAD, coenzyme A

Question 2

What are the four modes of the pentose phosphate pathway (figure 26.3) ?

Answer 2

1. Ribose 5-phosphate needs exceed need for NADPH:
   Rapidly dividing cells require precursors for DNA and RNA.
2. The NADPH and ribose-5-phosphate needs are BALANCED: Biosynthetic reducing power and nucleotide precursors needed
3. More NADPH is needed than ribose 5-phosphate: NADPH is need for fatty acid and cholesterol synthesis in the liver.
4. NADPH and ATP are both required:
   REDUCING POWER and ATP (through PYRUVATE) are both needed.

PPP can operate on 4 modes using different combinations of oxidative, non-oxidative, glycolysis and gluconeogenesis.

Question 3

What is the major substrate for the pentose phosphate pathway?

Answer 3

Glucose 6-phosphate

Question 4

What is the purpose of pentose phosphate pathway?

Answer 4

The purpose of the pentose phosphate pathway-
- it is an important source of NADPH: which is a biosynthetic reducing power (can synthesize amino acids, cholesterol, nucleotides and neurotransmitters)
- Pathway also catalyzes the interconversion of 3 and 6 carbon intermediates of glycolysis with 5-carbon carbohydrates. These pentose sugars are used in synthesis of RNA and DNA.
PP1 is parallel to glycolysis.

Question 5

What is the first phase of pentose phosphate pathway?

Answer 5

First phase- OXIDATIVE generation of NADPH.

Glucose 6 phosphate and 2 NADP+ and H2O will react to form , Ribulose , 2 NADPH, 2H+ and 2 CO2.

Question 6

What is the second phase of pentose phosphate pathway?

Answer 6

Second phase- NON-Oxidative interconversion of variety of sugars.

Question 7

What is the rate limiting step in the Pentose phosphate pathway? What enzyme catalyzes the reaction. What is the main regulatory factor in this reaction?

Answer 7

The rate limiting step of PPP- the first oxidative reaction.
The dehydrogenation of glucose 6-phosphate to ribulose 5 phosphate.
This reaction is catalyzed by Glucose 6-phosphate DEHYDROGENASE.
The main regulatory factor in reaction is CONCENTRATION OF NADP+

Question 8

What are the three main things, pentose phosphate is made for?

Answer 8

1. produce NADPH (oxidative)- biosynthetic reducing power
2. produce ribose 5-phosphate (make DNA and RNA)
3. To feed glycolytic pathway