Lecture 28: Glucose Catabolism (Pentose Phosphate Pathway) Flashcards
Pentose Phosphate Pathway Overview
What is the Pentose Phosphate Pathway, generalized?
alternate pathway for oxidation of glucose to generate NADPH and metabolize 5-carbon sugars
Pentose Phosphate Pathway Overview
What are the main products of Pentose Phosphate Pathway?
NADPH and Ribose-5-phosphate
Pentose Phosphate Pathway Overview
Why is NADPH important?
It is an electron Donor
* Used in reductive biosynthesis of fatty acids and steroids
* repair of oxidative damage
Pentose Phosphate Pathway Overview
Why is different betwen NADH and NADPH?
NADPH has an additional phosphate
Pentose Phosphate Pathway Overview
Why is Ribose-5-Phosphate important?
It is a biosynthetic precursor of nucleotides
* used in DNA and RNA synthesis
* synthesis of some coenzymes
Pentose Phosphate Pathway Overview
Where does the Pentose Phosphate Pathway occur?
the cytosol
Pentose Phosphate Pathway Overview
What are the two phases of Pentose Phosphate Pathway?
- Oxidative phase
- Nonoxidative phase
Pentose Phosphate Pathway Overview
What are some important aspects about the oxidative phase?
produces NADPH
irreversible
Pentose Phosphate Pathway Overview
What are some important aspects about the nonoxidative phase?
produced ribose-5-phosphate
interconcerts phosphorylated sugars
reversible reactions to feed glycolysis
Oxidative Phase
What is the overall equation for this phase?
Glucose-6-phosphate + 2 NADP + H2O
–>
ribulose-5-phosphate + 2 NADOH + 2 H + CO2
Oxidative Phase
What does this phase produce?
NADPH and ribulose-5-phosphate
ribULOSE not ribOSE
Oxidative Phase
What enam catalyzes the first reaction and what is the result?
Enam: glucose-6-P dehydrogenase (rate limiting step)
Product: sigma-Phosphoglucono-b-lactone
Releases NADPH
Oxidative Phase
What enam catalyzed the second reaction and what is the result?
enam: lactonase
product: 6-phosphogluconate
Oxidative Phase
What enam catalyzed the third reaction and what is the result?
enam: 6-phosphogluconate
product: ribulose-5-phosphate
Releases NADPH
Oxidative Phase
Why is NADP used as the hydrogen acceptor instead of NAD?
The Km for glucose-6-phosphate dehydrogenase is 1000x lower using NADP than it is using NAD
Non-Oxidative Phase
What does this phase do?
isomerizes Ribulose-5-phosphate into ribose-5-phosphate
Non-Oxidative Phase
What is it that catalyzes this isomerization?
Phosphopentose isomerase
Non-Oxidative Phase
What are the two enams in this reaction?
Transketolase
Transaldolase
Non-Oxidative Phase
What will transKetolase transfer?
2-C units
Non-Oxidative Phase
What will transAldolase transfer?
3-C units
Non-Oxidative Phase
What are the 3 reactions, their enam, and the net reaction?
- C5 + C5 <-> C3 + C7 (Transketolase)
- C3 + C7 <-> C6 + C4 (Transaldolase)
- C4 + C5 <-> C6 + C3 (Transketolase)
- 3 C5 <-> 2 C6 + C3 (net reaction)
Non-Oxidative Phase
What can Glucose-6-phosphate be regenerated from?
3-C glyceraldehyse-3-phosphate or the 6-C fuctose-6-phosphate via enzymes of Gluconeogenesis
Non-Oxidative Phase
How many six-carbon sugars can be created from 6 five-carbon sugars?
5
(6 ribose can be converted into 5 glucose)
Non-Oxidative Phase
Why would glucose be regenerated during this phase?
When tissues, like liver and adipose, require more NADPH than ribose-5-phosphate
