Chapter 20- Pentose Phosphate Pathway Flashcards
Pentose phosphate pathway
A pathway that is common to all organisms. It provides a means by which glucose can be oxidized to generate NADPH- this is the currency of readily available reducing power in cells. Occurs in the cytoplasm. Used for protection against oxidative stress
Uses of the pentose phosphate pathway (3)
- The catabolism of pentose sugars from the diet
- The synthesis of pentose sugars for nucleotide biosynthesis
- The catabolism and synthesis of less common 4 and 7 carbon sugars
2 phases of the pentose phosphate pathway
- The oxidative generation of NADPH
- The nonoxidative interconversion of sugars
NADPH vs NADH
NADH is oxidized by the respiratory chain to generate ATP, while NADPH serves as a reductant in biosynthetic processes
Pathways requiring NADPH (6)
- Fatty acid biosynthesis
- Cholesterol biosynthesis
- Neurotransmitter biosynthesis
- Nucleotide biosynthesis
- Detoxification- reduction of oxidized glutathione
- Cytochrome P450 monooxygenases
Oxidative phase of the pentose phosphate pathway
NADPH is generated when glucose 6-phosphate is oxidized to ribulose 5-phosphate. This reaction requires 2 NADP and water. It yields 2 NADPH, 2H+, and carbon dioxide
Nonoxidative phase of the pentose phosphate pathway
The pathway catalyzes the interconversion of 3, 4, 5, 6, and 7 carbon sugars in a series of nonoxidative reactions. Excess 5 carbon sugars may be converted into intermediates of the glycolytic pathway. All of these reactions take place in the cytoplasm
Glucose 6-phosphate dehydrogenase
Catalyzes the dehydrogenation of glucose 6-phosphate at carbon 1. The enzyme is highly specific for NADP+. The product is 6-phosphoglucono-δ-lactone, which is an intramolecular ester between the C-1 carboxyl group and the C-5 hydroxyl group.
Oxidative phase of the pentose phosphate pathway (3 steps)
- Glucose 6-phosphate is oxidized to 6-phosphoglucono-δ-lactone to generate one molecule of NADPH (NADP+ is reduced to NADPH)
- The lactone product is hydrolyzed to 6-phosphogluconate
- 6PG is oxidatively decarboxylated to ribulose phosphate with the generation of a second molecule of NADPH. Carbon dioxide is also released
When Glucose 6-phosphate is converted into ribulose 5-phosphate, how many NADPH are generated?
2 NADPH molecules are generated
2 enzymes that link the pentose phosphate pathway and glycolysis
- Transketolase
- Transaldolase
Phosphopentose isomerase
Isomerizes ribulose 5-phosphate to ribose 5-phosphate
In which situations would ribose 5-phosphate be converted into glycolytic intermediates?
Ribose 5-phosphate is a precursor to many biomolecules, but many cells need NADPH for reductive biosyntheses more than they need ribose 5-phosphate to make nucleotides. Certain tissues, like adipose tissue and the liver, require large amounts of NADPH for fatty acid synthesis. In these cases, ribose 5-phosphate is converted into glycolytic intermediates (glyceraldehyde 3-phosphate and fructose 6-phosphate) by transketolase and transaldolase. These enzymes create a reversible link between the pentose phosphate pathway and glycolysis by catalyzing 3 successive reactions
3 reactions of the nonoxidative phase
- C5 + C5 yields C3 and C7- transketolase
- C3 and C7 yields C6 and C4- transketolase
- C4 and C5 yields C6 and C3- transketolase
Net result of the nonoxidative phase reactions
The formation of two hexoses and one triose from 3 pentoses is the net result of this phase. 3C5 yields 2C6 and C3
Nonoxidative phase- 1st reaction
The first reaction is the formation of glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate from two pentoses. The donor of the 2 carbon unit in this reaction is xylulose 5-phosphate (an epimer of ribulose 5-phosphate)
When is a ketose a substrate of transketolase?
Only if its hydroxyl group at C-3 has the configuration of xylulose rather than ribulose. Ribulose 5-phosphate is converted into the appropriate epimer for the reaction by phosphopentose epimerase
Phosphopentose epimerase
Converts ribulose 5-phosphate into the appropriate epimer (xylulose 5-phosphate) for the transketolase reaction.
Epimers
Epimers are diastereomers that contain more than one chiral center but differ from each other in the absolute configuration at only one chiral center.
Nonoxidative phase- 2nd reaction
Synthesis of a 4 carbon sugar and a 6 carbon sugar. Glyceraldehyde 3-phosphate and sedoheptulose 7-phosphate (first reaction) react to form fructose 6-phosphate and erythrose 4-phosphate. This reaction is catalyzed by transaldolase.
Nonoxidative phase- 3rd reaction
A 4 carbon and 5 carbon sugar are converted to a 6 carbon and 3 carbon sugar. Erythrose 4-phosphate and xylulose 5-phosphate (reaction 2) are used to form fructose 6-phosphate and glyceraldehyde 3-phosphate. Transketolase catalyzes this reaction
Sum of the reactions catalyzed by the epimerase, transketolase, and transaldolase
Shows the conversion of 3 five-carbon sugars into components of the glycolytic and gluconeogenic pathways. 2 xylulose 5-phosphate and ribose 5-phosphate yields
Net reaction starting from ribose 5-phosphate
3 ribose 5-phosphate yields 2 fructose 6-phosphate and glyceraldehyde 3-phosphate. Xylulose 5-phosphate can be formed from ribose 5-phosphate by the sequential action of phosphopentose isomerase and phosphopentose epimerase
Differences between transketolase and transaldolase mechanisms
Transketolase transfers a 2 carbon unit, while transaldolase transfers a 3 carbon unit. Both enzymes stabilize carbanionic intermediates, but use different mechanisms
Transketolase reaction mechanism (5 steps)
- Thiamine pyrophosphate (TPP) forms a carbanion.
- The carbanion attacks the ketose substrate.
- A carbon–carbon bond is cleaved, releasing the aldose
product and forming TPP joined to a two-carbon fragment, the glycoaldehyde intermediate. - The intermediate attacks the aldose substrate.
- The ketose product is released, and TPP is ready for
another reaction cycle
Transketolase
Its prosthetic group is thiamine pyrophosphate. Transketolase transfers a two carbon glycoaldehyde from a ketose donor to an aldose acceptor. With the 2 carbon unit, the site of addition is the thiazole ring of TPP. The enzyme is homologous to the E1 subunit of the pyruvate dehydrogenase complex
Transaldolase reaction (7 steps)
- A Schiff base forms between the enzyme and the ketose
substrate. - Protonation of the Schiff base results in the release of the
aldose product. - The release of the aldose product generates a substituted
enzyme intermediate. - The three-carbon fragment bound to the enzyme adds to the aldose substrate.
- Protonation occurs, forming a new carbon-carbon bond
- Deprotonation occurs.
- Hydrolysis of the Schiff base releases the ketose product.
Transaldolase
Transfers a 3 carbon dihydroxyacetone unit from a ketose donor to an aldose acceptor. Transaldolase does not have a prosthetic group. Instead, a Schiff base is formed between the carbonyl group of the ketose substrate and the ε - amino group of a lysine residue at the active site of the enzyme. This enzyme is homologous to the fructose 1,6-bisphosphate aldolase enzyme found in the glycolytic pathway
Schiff bases
A vast group of compounds characterized by the presence of a double bond linking carbon and nitrogen atoms,
Carbanion intermediates
For both transketolase and transaldolase, a carbanion intermediate is stabilized by resonance. In transketolase, TPP stabilizes this intermediate. In transaldolase, a protonated Schiff base plays this role
Importance of the first reaction in the oxidative branch of the pentose phosphate pathway
The dehydrogenation of glucose 6-phosphate is essentially irreversible. This reaction is rate limiting under physiological conditions. It also serves as a control site for the oxidative branch of the pathway
Regulatory role of NADP+
The dehydrogenation of glucose 6-phosphate (and therefore the rate of the rest of the oxidative phase) is regulated by NADP+ levels. Low levels of NADP+ limit the dehydrogenation of glucose 6-phosphate because it is needed as the electron acceptor. NADP+ competes with NADPH in binding to the enzyme, intensifying the regulatory ability of NADP+. The effect of the NADP+ level on the rate of the oxidative phase ensures that NADPH is not generated unless the supply needed for reductive biosyntheses or protection against oxidative stress is low.
How is the nonoxidative phase regulated?
Primarily controlled by the availability of substrates
Interplay between glycolysis and the pentose phosphate pathway
The pentose phosphate pathway can operate in 4 distinct modes that result from various combinations of the oxidative phase, the nonoxidative phase, glycolysis, and gluconeogenesis.
4 modes of the pentose phosphate pathway
- Ribose 5-phosphate needs exceed 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.
Mode 1 of the pentose phosphate pathway
Much more ribose 5-phosphate than NADPH is required. One example- rapidly dividing cells need ribose 5-phosphate for the synthesis of nucleotide precursors of DNA. Most of the glucose 6-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate by the glycolytic pathway. Transaldolase and transketolase then convert two molecules of fructose 6-phosphate and one molecule of glyceraldehyde 3-phosphate into 3 molecules of ribose 5-phosphate
Mode 2 of the pentose phosphate pathway
The needs for NADPH and for ribose 5-phosphate are balanced. When this happens, glucose 6-phosphate is processed to one molecule of ribulose 5-phosphate while generating two molecules of NADPH. Ribulose 5-phosphate is then converted into ribose 5-phosphate
Mode 3 of the pentose phosphate pathway
Much more NADPH than ribose 5-phosphate is required. Example- adipose tissue requires a high level of NADPH for the synthesis of fatty acids. When this happens, glucose 6-phosphate is completely oxidized to carbon dioxide. 3 groups of reactions are active in this situation. Glucose 6-phosphate can be completely oxidized to carbon dioxide with the simultaneous generation of NADPH
3 groups of reactions- mode 3
- The oxidative phase of the pentose phosphate pathway forms 2 molecules of NADPH and one molecule of ribulose 5-phosphate
- Ribulose 5-phosphate is converted into fructose 6-phosphate and glyceraldehyde 3-phosphate by transketolase and transaldolase
- Glucose 6-phosphate is resynthesized from fructose 6-phosphate and glyceraldehyde 3-phosphate by the gluconeogenic pathway
Tissues with active pentose phosphate pathways (7)
- 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
- Red blood cells- maintenance of reduced glutathione
Mode 4 of the pentose phosphate pathway
Both NADPH and ATP are required. Alternatively, ribulose 5-phosphate formed from glucose 6-phosphate can be converted into pyruvate. Fructose 6-phosphate and glyceraldehyde 3-phosphate derived from ribose 5-phosphate enter the glycolytic pathway rather than reverting to glucose 6-phosphate. ATP and NADPH are simultaneously generated, and 5 of the 6 carbons of glucose 6-phosphate emerge in pyruvate
The pentose phosphate pathway and rapid cell growth
Rapidly dividing cells, like cancer cells, require ribose 5-phosphate for nucleic acid synthesis and NADPH for fatty acid synthesis- this is used to form membrane lipids. These cells must switch to aerobic glycolysis to meet their ATP needs. Glucose 6-phosphate and glycolytic intermediates are then used to generate NADPH and ribose 5-phosphate using the nonoxidative phase of the pentose phosphate pathway
Pyruvate kinase isozyme (PKM)
PKM facilitates the diversion of glycolytic intermediates into the nonoxidative phase in rapidly dividing cells. It has a low catalytic activity, which creates a bottleneck in the glycolytic pathway. Glycolytic intermediates accumulate and are then used by the pentose phosphate pathway to synthesize NADPH and ribose 5-phosphate
Reduced glutathione (GSH)
A tripeptide with a free sulfhydryl group. It protects against oxidative stress by reducing ROS to harmless forms. Once this is accomplished, glutathione is now oxidized (GSSG) and must be reduced to regenerate GSH
Reactive oxygen species
Generated in oxidative metabolism. They can inflict damage on all classes of macromolecules and can lead to cell death. They are implicated in many human diseases, like diabetes
Oxidized glutathione (GSSG)
Converted into reduced
glutathione by NADPH- GSH needs to be regenerated. The reducing power of NADPH is generated by glucose 6-phosphate dehydrogenase in the pentose phosphate pathway. Cells with reduced levels of glucose 6-phosphate dehydrogenase are especially sensitive to oxidative stress
Glucose 6-phosphate dehydrogenase deficiency
Causes hemolytic anemia. This enzymes catalyzes the first step in the oxidative branch of the pentose phosphate pathway. The deficiency results in a lack of NADPH in all cells, but it is most acute in RBCs because they lack mitochondria and have no alternative means of generating reducing power. This defect is inherited on the X chromosome
GSH and oxidative stress
GSH normally helps to control the amounts of harmful
peroxides that are released by some agents. Peroxides are a
form of ROS. In red blood cells, which lack mitochondria, the main role of NADPH is to regenerate the reduced form of GSH.
How does the absence of glutathione effect red blood cells?
Glutathione is required to maintain the normal
structure of hemoglobin because it helps to maintain the
cysteine residues in their reduced form. In the absence of glutathione, sulfhydryl bonds occur among hemoglobin molecules, forming aggregates called Heinz bodies, and the red blood cells may lyse due to deformed membranes
Favism response
Fava beans contain a purine glycoside that causes oxidative damage to red blood cells. People deficient in glucose 6-phosphate dehydrogenase
suffer hemolysis (destruction of red blood cells) from consuming fava beans or inhaling pollen of the fava flowers (called a favism response). This is because these chemicals are oxidative agents that generate peroxides- they are ROS that can damage membranes as well as other biomolecules
Vicia faba
The Mediterranean plant Vicia faba is a source of fava beans that contain the pyrimidine
glycoside vicine. Causes oxidative damage to red blood cells
Glucose 6-phosphate dehydrogenase and glutathione
In the absence of glucose 6-phosphate dehydrogenase, peroxides continue to damage membranes because no NADPH is being produced to restore reduced glutathione. Glucose 6-phosphate dehydrogenase is required to maintain reduced glutathione levels to protect against oxidative stress. In the absence of oxidative stress, the deficiency is benign
Heinz bodies
Clumps of denatured hemoglobin that adhere to the plasma membrane and stain with basic dyes. Red blood
cells in such people are highly susceptible to oxidative damage. They are caused by glucose 6-phosphate dehydrogenase deficiency- in the absence of glutathione, sulfhydryl bonds occur among hemoglobin molecules, forming aggregates
Under which circumstances does glucose 6-phosphate dehydrogenase deficiency provide an evolutionary advantage?
Glucose 6-phosphate dehydrogenase deficiency protects against malaria by depriving the parasites of NADPH that they require for growth. Because the pentose phosphate pathway is
compromised, the cell and parasite die from oxidative
damage.
Hummingbirds and the pentose
phosphate pathway
Oxygen can be readily converted to ROS. All aerobic animals that are highly active face the problem of damage by ROS, and this problem increases with increasing activity. The hummingbird is an example of one such highly active animal. Hummingbirds’ wings make up 25% of their body weight and flap up to 200 times per second. This level of activity requires muscles that are capable of maintaining a high metabolic rate for an extended period of time
Respiratory quotient
The rate of carbon dioxide produced to oxygen consumed. When organisms are using carbohydrates only as a fuel, the respiratory quotient is 1. Nectar is rich in carbohydrates and is the preferred fuel for high intensity activity with hummingbirds. The RQ of hummingbirds is actually greater than 1. This is because the oxidative phase of the pentose phosphate pathway generates NADPH, which protects against ROS. In doing this, the oxidative phase also produces carbon dioxide. This causes the RQ to be greater than 1. Therefore, hummingbirds are using carbohydrate-rich nectar to produce ATP to power muscle activity and generate NADPH to protect against ROS
Pentose phosphate pathway in humans
Human athletes who consume carbohydrates during intense, extended exercise may also use the pentose phosphate pathway for ROS protection.
