Loading in 2 Seconds...
Loading in 2 Seconds...
Carbohydrate Metabolism 1: Pentose Phosphate Pathway, Gluconeogenesis, Reciprocal Regulation. Bioc 460 Spring 2008 - Lecture 33 (Miesfeld). primaquine. The dual function enzyme PFK-2/FBPase-2 controls flux through gluconeogenesis and glycolysis by controlling levels of F-2,6-BP in the cell.
Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author.While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server.
Carbohydrate Metabolism 1:Pentose Phosphate Pathway, Gluconeogenesis, Reciprocal Regulation
Bioc 460 Spring 2008 - Lecture 33 (Miesfeld)
The dual function enzyme PFK-2/FBPase-2 controls flux through gluconeogenesis and glycolysis by controlling levels of F-2,6-BP in the cell
Athletes like Jenna Gresdal rely on the Cori Cycle to maintain glucose levels
Deficiencies in the enzyme glucose-6P dehydrogenase affects 400 million people
Metabolism of ribose sugars in the pentose phosphate pathway is used to generate NADPH and to provide the carbohydrate component of nucleotides.
The major sources of carbon in gluconeogenesis are amino acids and glycerol in animals, and glyceraldehyde-3-phosphate (GAP) in plants.
1. What does the pentose phosphate pathway accomplish for the cell?
2. What is the overall net reaction of the pentose phosphate pathway when it is utilized to generate the maximum amount of NADPH?
6 Glucose-6P + 12 NADP+ + 12 H2O → 5 Glucose-6P + 12 NADPH + 12 H+ + 6 CO2
3. What are the key enzymes in the pentose phosphate pathway?
Glucose-6P dehydrogenase (G6PD)–This reaction is the commitment step in the pathway and is feedback-inhibited by NADPH.
Transketolase and Transaldolase - together these two enzyme catalyze the reversible "carbon shuffle" reactions of the nonoxidative phase of the pathway.
4. What are examples of the pentose phosphate pathway in real life?
Glucose-6P dehydrogenase G6PD) deficiency is the most common enzyme deficiency in the world and affects over 400 million people. Red blood cells do not produce enough NADPH to protect against reactive oxygen species that are generated by anti-malarial drugs (primaquine) and by compounds in fava beans (vicine).
mosquito biting a human
A big bowl of fava beans
The pentose phosphate pathway can be divided into two phases, the oxidative phase, which generates NADPH, and the nonoxidative phase, which regenerates G-6P.
G6PD is the commitment step in the Pentose Phosphate Pathway because 6-Phosphogluconon-d-lactone has no other metabolic fate except to be converted to 6-phosphogluconate.
3Three enzymatic reactions in the oxidative phase
The non-oxidative phase of the PPP
The carbon shuffle reactions of the nonoxidative phase are used to regenerate glucose-6P using the same transketolase and transaldolase enzyme reactions as the Calvin Cycle.
Six glucose-6P (36 carbons) are metabolized to regenerate five glucose-6P (30 carbons).
What happened to 6 carbons?
Metabolic flux through the Pentose Phosphate Pathway is tightly-regulated
• If increased NADPH is required for biosynthetic pathways.
• If cells need to replenish nucleotide pools.
• If ATP levels in the cell are low.
The pentose phosphate pathway is responsible for maintaining high levels of NADPH in red blood cells (erythrocytes) for use as a reductant in the glutathione reductase reaction. Glutathione is a tripeptide that has a free sulfhydryl group which functions as an electron donor in a variety of coupled redox reactions in the cell.
When erythrocytes are exposed to chemicals that generate high levels of superoxide radicals, GSH is required to reduce these damaging compounds.
The pentose phosphate pathway in erythrocytes normally provides sufficient levels of NADPH to maintain the GSH:GSSG ratio at about 500:1.
Primaquine inhibits growth of the malarial parasite in red blood cells by creating a hostile environment. The biochemical basis for this drug-induced illness was found to be lower than normal levels of NADPH due to a G6PD deficiency.
The acute hemolytic anemia seen in individuals with G6PD who are treated with primaquine explains the symptoms of favism. One of the compounds in fava beans is vicine, a toxic glycoside that induces oxidative stress in erythrocytes.
What might explain the observation that cultures with high amounts of fava beans in the diet were associated (in ancient times) with low malaria rates?
1. What does gluconeogenesis accomplish for the organism?
The liver and kidney generate glucose from noncarbohydrate sources.
Plants use the gluconeogenic pathway to convert GAP into glucose which is used to make sucrose and starch.
2. What is the overall net reaction of gluconeogenesis?
2 pyruvate + 2NADH + 4ATP + 2GTP + 6H2O →
Glucose + 2NAD+ + 2H+ + 4ADP + 2GDP + 6Pi
3. What are the key enzymes in gluconeogenesis?
Pyruvate carboxylaseconverts pyruvate to oxaloacetate.Phosphoenolpyruvate carboxykinase (PEPCK) converts oxaloacetate to phosphoenolpyruvate (PEP).Fructose-1,6-bisphosphatase-1 (FBPase-1) catalyzes the dephosphorylation of fructose-1,6BP to form fructose-6P.
Glucose-6-phosphatase catalyzes the dephosphorylation of glucose-6P to form glucose.
Glucose monitoring devices are based on an assay using the enzyme glucose oxidase which produces gluconate and hydrogen peroxide (H2O2) from glucose.
The level of H2O2 in the sample is detected by an indicator dye that is oxidized in a reaction catalyzed by peroxidase.
Two of the bypass enzymes in gluconeogenesis,fructose-1,6-bisphosphatase-1 (FBPase-1) and glucose-6-phosphatase, simply reverse the reaction
However, 4 extraATP/GTP, and pyruvate carboxylase and phosphoenolypyruvate carboxykinase (PEPCK), are required to catalyze the bypass reaction that converts pyruvate to PEP.
How do you come up with the 4 extra ATP/GTP for gluconeogenesis compared
The activities of PFK-1 and FBPase-1 are regulated by the allosteric effectors AMP, citrate, and fructose-2,6-bisphosphate (F-2,6-BP), but in a reciprocal manner.
What is the metabolic logic of reciprocal regulation by citrate?
In the presence of F-2,6-BP, the affinity of PFK-1 for its substrate fructose-6P is 25 times higher than it is in the absence of F2,6BP.
Looking at the activity curves for FBPase-1 in the presence and absence of F-2,6-BP it can be seen that the affinity of FBPase-1 for its substrate fructose-1,6BP is 15 times lower in the presence of F-2,6-BP.
The amount of F-2,6-BP in the cell is controlled by the activity of a dual function enzyme containing two catalytic activities, 1) a kinase activity called phosphofructokinase-2 (PFK-2) that phosphorylates fructose-6P to form F-2,6-BP, and 2) a phosphatase activity called fructose-2,6-bisphosphatase (FBPase-2) that dephosphorylates F-2,6-BP to form fructose-6P.
More F-2,6-BP and glycolysisLevels of F-2,6-BP in the cell are controlled by a dual function enzyme called PFK-2/FBPase-2
When the PFK-2/FBPase-2 dual function enzyme is unphosphorylated, then the PFK-2 activity in the enzyme is stimulated resulting in the net phosphorylation of fructose-6P to produce more F-2,6-BP which stimulates glycolytic flux.
In contrast, when PFK-2/FBPase-2 is phosphorylated, the activity of FBPase-2 is stimulated, leading to less F-2,6-BP, and reduced flux through glycolysis, with a concomitant increase in flux through gluconeogenesis.
Activation of the glucagon receptor in liver cells results in stimulation of protein kinase A signaling, leading to an increase in gluconeogenesis, whereas, insulin signaling stimulates protein phosphatase-1, leading to an increase in glycolysis .
The Cori cycle provides a mechanism to convert lactate produced by anaerobic glycolysis in muscle cells to glucose using the gluconeogenic pathway in liver cells. The Cori cycle has an energy cost (4 ATP equivalents), but it is worth it.
These 4 extra ATP/GTP come from the fact that glycolysis yields a net of 2 ATP per glucose, but gluconeogenesis costs a total of 4 ATP and 2 GTP per glucose.
Athletes "warm down" after exercise to enhance circulation so that lactate will be cleared from the muscle and be used in the liver for glucose synthesis via the Cori cycle.