Cardio - Biochemistry - Pentose Phosphate Pathway Flashcards
True/False.
Glucose molecules that just entered the liver are likely to both be phosphorylated to glucose 6-phosphate and enter both glycolysis and the pentose phosphate shunt.
True.
Where does the pentose phosphate pathway take place?
What types of tissue needs are answered by increased pentose phosphate pathway activity?
In the cytosol;
NADPH needs (e.g. for fatty acid synthesis, cholesterol/steroid synthesis, glutathione reduction)
What substance is very important as a reducing agent in fatty acid/steroid/cholesterol synthesis?
Via what pathway is it produced?
NADPH;
the pentose phosphate pathway
What are the two phases of the pentose phosphate pathway?
Oxidative phase (irreversible)
Nonoxidative phase (reversible)
Describe the general reaction that occurs in the oxidative phase of the pentose phosphate pathway.
Glucose-6-phosphate is oxidized to:
- 2 NADPH
- Ribulose-5-phosphate
- CO2
(irreversible)
Describe the general reaction that occurs in the nonoxidative phase of the pentose phosphate pathway.
Ribose-5-phosphate is converted to:
- Nucleic acids
- GAP (glyceraldehyde 3-phosphate)
- F6P
(reversible)
What is the first enzyme of the oxidative phase of the pentose phosphate pathway?
This enzyme is especially important because it is the:
Glucose-6-phosphate dehydrogenase;
rate-determining enzyme
What pathway uses ribose-5-phosphate as a substrate?
Nucleotide biosynthesis (DNA synthesis)
NADPH is used as a reducing agent for which biosynthetic pathways?
Fatty acid synthesis
Steroid/cholesterol synthesis
Glutathione reduction
What is the main difference between NADPH and NADH in their function/use?
NADPH - used for biosynthesis
NADH - used for energy production
For each individual glucose molecule that enters the pentose phosphate pathway, how many NADPH are produced?
2 NADPH per G6P
The pentose phosphate pathway committed step is performed by which enzyme?
Glucose-6-phosphate dehydrogenase
Differentiate between the structure of ribulose-5-phosphate and ribose-5-phosphate sugars in the pentose phosphate pathway.
RibULOSE-5-phosphate = keto sugar
RibOSE-5-phosphate = aldose sugar
(Note: image is of trioses, not pentoses)
The isomerization of ribulose-5-phosphate to ribose-5-phosphate occurs as part of what reaction in the oxidative phase of the pentose phosphate pathway?
The 4th (and final) reaction
(beginning of nonoxidative steps)
Transketolase is the enzyme used in the 2nd step of the nonoxidative phase.
What cofactor is required by this enzyme?
Thiamine pyrophosphate
(derivative of vitamin B1)
What are the three enzymes of the nonoxidative phase of the pentose phosphate shunt?
Epimerase, transketolase, transaldolase
Describe the ‘shell game’ that the nonoxidative phase of the pentose phosphate shunt uses to move carbons around and get from ribose 5-phosphate to any of the following:
glyceraldehyde 3-phosphate, fructose 6-phosphate, glucose 6-phosphate
How does the nonoxidative phase of the pentose phosphate shunt get from a pentose (ribose 5-phosphate) back to trioses (glyceraldehyde 3-phosphate) and hexoses (fructose 6-phosphate, glucose 6-phosphate)?
2-carbon (taken from X5P) added to 5-carbon (R5P)
–> 7-carbon (S7P)
1-carbon taken from S7P (and added to 3-carbon (G3P) –> 4-carbon (E4P))
–> F6P
–> G6P
What three glycolytic intermediates can be made from the pentose phosphate shunt?
From which phase?
Glyceraldehyde 3-phosphate, fructose 6-phosphate, glucose 6-phosphate;
nonoxidative
The nonoxidative phase of the pentose phosphate pathway requires 3 enzymes for its four reactions
- epimerase, transketolase & transaldolase -
which one(s) require thiamine pyrophosphate as a cofactor?
Only transketolase
Discuss the role of the epimerase used in the nonoxidative phase of the pentose phosphate pathway.
It converts ribulose-5-phosphate to its epimer xylulose-5-phosphate
What two glycolytic intermediates are formed by the reactions of xylulose-5-phosphate (from the nonoxidative phase of the pentose phosphate pathway)?
Glyceraldehyde-3-phosphate;
fructose-6-phosphate
As concentrations of NADPH increase in the cell, how will this regulate enzymatic function?
Strongly inhibits glucose-6-phosphate dehydrogenase
(initial enzyme in PPP)
True/False.
If a cell needs (1) only NADPH, or (2) only ribose 5-phosphate, or (3) both, it can tailor the pentose phosphate shunt to meet its needs.
True.
Only NADPH - Ribulose 5-P turned back into G6P (goes back into oxidative phase)
Both - Ribulose 5P turned into ribose 5-P
Only ribose 5-phosphate - Oxidative portion shut down; nonoxidative runs in reverse (F6P and GAP –> R5P)
How would a cell that requires a lot of nucleotide synthesis and no NADPH regulate the pentose phosphate pathway?
Only the nonoxidative portion matters, but in reverse –> F6P and GAP are converted to R5P
(Oxidative phase will be shut down to prevent ANY NADPH production (which would inhibit glucose-6-phosphate dehydrogenase and prevent ribose-5-phosphate production).)
If the oxidative phase of the pentose phosphate pathway is effectively shut down, what would a cell use to create a steady stream of ribose-5-phosphate?
Glycolytic intermediates:
glyceraldehyde-3-phosphate and fructose-6-phosphate
What is the major controlling factor for the pentose phosphate pathway?
What other factor plays an important role?
NADPH concentrations;
xylulose 5-phosphate levels
Discuss the effect of rising xylulose-5-phosphate levels in the cell following a large meal.
(Note: Xylulose 5 -phosphate is produced in the pentose phosphate pathway.)
Xylulose-5-phosphate activates phosphoprotein phosphatase (PP2A);
fed state –> dephosphorylates PFK-2/FBPase-2 enzyme –> increases glycolysis / decreases gluconeogenesis
What is the importance of creating xylulose-5-phosphate from ribulose-5-phosphate?
Xylulose-5-phosphate is a regulatory molecule for carbohydrate and lipid metabolism.
*X5P structure is also used to form glycolytic intermediates
What is the role of phosphoprotein phosphatase (PP2A)?
Dephosphorylates PFK-2/FBPase-2 enzyme
–>
Increases glycolysis / decreases gluconeogenesis
When the PFK-2/FBPase-2 enzyme is dephosphorylated, what results?
PFK-2 is activated
FBPase-2 is inhibited
(increases glycolysis / decreases gluconeogenesis)
When PFK-2 is activated by the regulatory activity of xylulose-5-phosphate, what are the downstream effects?
PFK-2 increases fructose-2,6-bisphosphate levels,
activating glycolysis and inhibiting gluconeogenesis
When xylulose-5-phosphate levels rise and induce their regulatory actions on phosphoprotein phosphatase and the PFK-2/FBPase-2 enzyme, what is the end result of this regulatory pathway?
Increased fructose-2,6-bisphosphate levels –> increased levels of acetyl-CoA (through glycolysis)
How does the tripeptide (3 amino acids) glutathione help protect the cell membrane from reactive oxygen species (ROS)?
Glutathione is an antioxidant (the SH group is easily oxidizable);
also, it and glutathione peroxidase decrease H2O2 levels
What three amino acids make up a glutathione molecule?
Glutamine - cysteine (SH side chain here) - glycine
How many molecules of glutathione are needed to reduce H2O2 to H2O? What redox state do the glutathione molecule(s) need to be in?
2;
reduced state (GSH, not GS-SG)
What are the steps of the respiratory (oxidative) burst (from O2 to hypochlorite)?
What cells perform this function to create reactive oxygen species?
NADPH + 2 O2 – (NADPH oxidase) –> NADP+ + O2-
2 O2- – (superoxide dismutase) –> H2O2
H2O2 – (myeloperoxidase) –> HOCl
monocytes, macrophages, neutrophils
What three enzymes are part of the respiratory (oxidative) burst?
What NADPH-dependent enzyme helps limit its damaging effects?
NADPH oxidase, superoxide dismutase, myeloperoxidase;
glutathione peroxidase
