Chapter 14: Carbohydrate Metabolism Flashcards
What is the role of the pentose phosphate pathway?
- Produces NADPH, Ribose-5-Phosphate, and glycolytic intermediates
- Reduces oxidative stress via NADPH production
- NADPH can be used as energy to detoxify reactive oxygen species.
Explain the outcomes of the oxidative and non-oxidative phases of PPP.
Oxidative phase:
- Turns 6 Glucose-6-Phosphate into 6 Ribulose-5-Phosphate and reduces two molecules of NADP+ —> NADPH
- The enzyme G6P dehydrogenase catalyzes the irreversible reaction and is the rate limiting step
Non-oxidative phase:
- Carbon Shuffle
- 6 Ribulose-5-P are converted into 4 Fructose-6-P and 2 GAP (G3P)
- End of Carbon Shuffle
- 4 Fructose-6-P + 2 GAP are converted to 5 Glucose-6-P molecules
Draw the reactions in the oxidative phase of PPP.
Which enzyme of the pathway is regulated? Why and how?
- Glucose-6-phosphotase dehydrogenase is regulated (rate limiting step)
- It is regulated by the concentration of NADPH and NADP+
If NADP+ is high:
- G6PDH works and accelerates PPP generating NADPH
If NADPH is high:
- G6PDH is negatively regulated and glycolysis is favored to make ATP
Describe the mechanism of G6PDH deficiency?
- A lack of the enzyme Glucose-6-P dehydrogenase results in the inability to maintain high enough GSH which detoxifies harmful oxygen radicals
Where does the PPP occur? What happens? and what is the electron acceptor?
- Occurs in the cytosol
- Oxidation of Glucose-6-P occurs
- NADP+ (electron acceptor) —-> NADPH
What are the two phases in the PPP?
Oxidative and Non oxidative
How is flux controlled?
- If NADPH concentration is low
- If nucleotides need to be replenished
- If ATP levels are low
Where is Ribose-5-phosphate used?
- Precusor of nucleotides needed in
- DNA and RNA synthesis
- Synthesis of ATP, NAD+, FAD, Coenzyme A, ect.
- Mainly used by rapidly dividing cells like Bone marrow, skin, intestinal mucosa, and tumors
Where is NADPH used?
- Tissues with active reductive biosynthesis
- Liver, adipose, lactating mammary glands (fatty acid synth.)
- Tissues that need to counter damaging effects of oxygen radicals
- Erythrocytes, cells of the lens and cornea: directly exposed to O2
What are the steps and outcomes of the Oxidative phase?
Input:
- 6 Glucose-6-P
- 2 NADP+
- H2O (reversible)
Output:
- 6 Rubulose-5-P
- 2 NADPH + H+
- H+ (reversible)
Transformations:
Glucose-6-P —(G6P dehydrogenase)—> 6-Phosphogluconolactone —(Lactonase)—> 6-Phosphogluconate —(6-Phosphogluconate dehydrogenase)—> Ribulose-5-P
What happens in the carbon shuffle reaction?
- 6 Ribulose-5-P are converted into 4 F6P and 2 GAP
What do Transketolases do?
Catalyze the transfer of a 2C group from a donor to an acceptor
What do Transaldolases do?
Catalyze the transfer of a 3C group from a donor to an acceptor
What are the outcomes of the initial steps in the non-oxidative phase?
- 3(2 Ribulose-5-P) from oxidative phase
2a. 2 Ribulose-5-P —(Ribose-5-P isomerase)—> 2 Ribose-5-P
2b. 4 Ribulose-5-P —(Ribulose-5-P isomerase)—> 4 Xylulose-5-P
- Transkelotase uses 2 Ribose-5-P + 2 Xylulose-5-P to make 2 Sedoheptulose-7-P + 2 Glyceraldehyde-3-P
- Transaldolase converts 2 Sedoheptulose-7-P + 2 Glyceraldehyde-3-P to 2 erythrose-4-P + 2 Fructose-6-P
- Transkelotolase converts 2 Erythrose-4-P + 2 Xylulose-5-P to 2 Glyceraldehyde-3-P + 2 Fructose-6-P
Net: 6 Ru-5-P –> 4 F6P + 2 G3P
What are the steps and outcomes of the final steps in the non-oxidative phase?
- 4 Fructose-6-P —(Phosphoglucoisomerase)—> 4 Glucose-6-P
1a. G3P —(Triose-P isomerase)—> Dihydroxyacetone phosphate
- Dihydroxyacetone phosphate + G3P —(Aldose)—> Fructose 1,6 BP
- Fructose 1,6 BP —(FBPase-1)—> Fructose -6-P
- Repeat step 1 to make 1 Glucose-6-P
Net: 4 F6P + 2G3P —> 5 Glucose-6-P
What enzymes are components of both glycolytic pathway and gluconeogenic pathway? Which Enzyme is unique to gluconeogenesis?
Both:
Phosphoglucoisomerase, Triose Phosphate isomerase, and Aldolase
Fructose-1,6-Bisphosphate-1 (FBPase-1) is unique to the Gluconeogenic pathway
Describe the ΔG of the oxidative and non oxidative part of the PPP. What is the reate limiting step?
Oxidative part:
ΔG’ < 0 (favorable)
Non-oxidative part:
ΔG’ = 0
G6PDH is the rate limiting step
What is allosteric regulation?
- When the binding of a molecule causes a change in the shape of the enzyme that can either inhibit or enhance the enzyme’s function
What allosterically regulates G6PDH activity?
- The [NADP+] to [NADPH] ratio
- High NADP+ causes activation of G6P dehydrogenase which activates the PPP which makes NADPH
- High NADPH is an inhibitor for G6P dehydrogenase which activates glycolysis to make ATP
What is the function of NADPH?
- A biosynthetic as well as detoxification pathway
What role does NADPH play in detoxification?
- NADPH is required as a coenzyme in glutathione reductase to conver Glutathione into its reduced form (GSSG –> GSH)
- Build up of harmful oxygen radicals is prevented by glutathione and the enzyme glutathione peroxidase (H2O2–> 2H2O)
- O2- will bind to 2H+ and electrons to form H2O2
- G6PDH rxns in PPP are needed to make enough NADPH in RBCs to maintain high levels of GSH
What happens if someone is G6PDH deficient?
- Lipid peroxidation leads to breakdown of cell membrane
- Oxidation of proteins and DNA
- Most asymptomatic
- Can be fatal in cases of high oxidative stress (some drugs, herbicides, and foods Fava)
What is Favism?
- Vicine, toxic ingredient of fava beans generates reactive O2 species as product
- People who are G6PDH deficient can develop Favism
- Leads to RBCs lyse in 24-48H releasing Hb
- Jaundice and kidney failure
- Can be fatal
What natural resistance do G6PDH deficient people express?
Resistance to malaria as the parasite is killed by the oxidative stress that G6PDH deficient individuals can tolerate
Under what conditions do liver cells promote glycolysis or gluconeogeneis?
- Glycolysis is promoted if blood glucose is low
-Gluconeogenesis is promoted if blood glucose is high
Draw the irreversible reaction of gluconeogenesis.
- Oxaloacetate + GTP —(Phosphoenolpyruvate carboxykinase)—> Phosphoenolpyruvate + CO2 + GDP
- Fructose-1, 6-BP + H2O —(Fructose-1, 6-BPase-1)—> Fructose-6-P + Pi
- Glucose-6-P + H2O —(Glucose-6-Phosphatase)—> Glucose + Pi
Which enzymes are shared between glycolysis and gluconeogenesis? Which are unique? Why?
Shared:
- Enolase
- Phosphoglycerate mutase
- Phosphoglycerate kinase
- Glyceraldehyde-3-Phosphate dehydrogenase
- Triose Phosphate isomerase
- Aldolase
- Phosphoglucose isomerase
Unique to Gluconeogenesis:
- Pyruvate Carboxylase
- Phosphoenolpyruvate Caboxykinase
- Fructose-1, 6-bisphosphotase-1
- Glucose-6-phosphotase
- These ezymes are unique to gluconeogenesis because they are highly exergonic and are not reversible. Glycolysis also has its own enzymes that perform irreversible reactions.
Why and how does OAA convert to PEP in the cytosol under aerobic metabolism instead of in anaerobic conditions in the mitochondria?
- Pyruvate —(pyruvate carboxylase)—> OAA
- OAA—(Mitochondrial malate dehydrogenase)—> Malate
- Malate is transported out of the mitochondria where cytosolic malate dehydrogenase converts it back to OAA
- OAA —(Cytosolic Phosphoenolpyruvate carboxykinase)—> PEP
Cytosol:
- By making PEP in the cytosol, gluconeogensis can proceed and glucose can be made and transported to different tissue
Mitochondria:
- By making mitochondrial PEP, anapluerosis of citric cycle intermediates can be replenished which help with energy production
How are glycolysis and gluconeogenesis differentially regulated?
Low blood sugar:
1. Glucagon (make glycogen)is released by liver cells
2. Protein kinase A uses ATP to phosphorylate PFK-2/FBPase-2
3. This activates the inhibited FBPase-1 and turns Fructose-2,6-BP into Fructose-6-phosphate
4. Fructose-6-phosphate is used in gluconeogenesis
High blood sugar:
1. Insulin (breaks down glycogen)is released by liver cells
2. H2O replaces the phosphorylated serine via Protein phosphotase 1
3. PFK-1 is activated as a result and turns Fructose-6-P into Fructose-2,6-BP
4. Fructose-2, 6-BP is used in glycolysis
Describe the role of the Cori Pathway.
- Transports Lactate from skeletal muscle to blood stream to the liver where it is converted to pyruvate then glucose and sent back to the skeletal muscle
- Cost 4 ATP
What are the carbon sources for glucose synthesis?
- Glycerol, Amino acids, Lactate, and CO2 fixation in plants
- Triglycerides are converted to glycerol which can be turned to dihydroxyacetone phosphate
- CO2 fixes as G3P
- Nutrient limataion increases conversion of amino acids into pyruvate or OAA
- Anaerobic respiration leads to formation of Lactate used to make pyruvate
How are triglycerides part of the carbon source for glucose synthesis?
Triglycerides are converted to glycerol which can be converted to Dihydroxyacetone phosphate.
What happens in nutrient limitation?
Amino acids are converted to pyruvate or OAA for the citrate cycle
How is Oxaloacetate synthesized?
- Pyruvate is converted using the enzyme Pyruvate carboxylase
- Biotin is an important cofactor for Pyruvate carboxylase
- Biotinylated Lys carries carboxyl group from one region to another
How does oxaloacetate transform into Phosphoenolpyruvate?
- Uses the enzyme Phosphoenolpyruvate carboxykinase
- Decarboxylation leads to rearrangement of Electrons
- Facilitates the attack of carbonyl O of the OAA on phosphate of GTP
- Can occur in the Mitochondrial matrix or cytosol depending on aerobic or anaerobic metabolism
What happens to OAA in Aerobic metabolism?
- OAA —(Mitochondrial malate dehydrogenase)—> Malate in mitochondrial matrix
- Malate transported to cytosol
- Malate —(cytosolic malate dehydrogenase)—> OAA in cytosol
- Produces NADH in cytosol
- Needed for GAPDH
- OAA —(Cytosolic Phosphoenolpyruvate carboxykinase)—> Phosphoenolpyruvate in cytosol
What happens in Anaerobic metabolism to OAA?
- During exercise, lactate produced in muscles is transported to liver
- Lactate —(Lactate dehydrogenase)—> Pyruvate
- Cytosolic NADH levels are maintained by lactate dehydrogenase reaction
- Pyruvate —(Pyruvate carboxylase)—> OAA
- OAA —(Mitochondrial phosphoenolpyruvate carboxykinase)—> Phosphoenolpyruvate
- PEP is then exported to cytosol
What two hydrolytic reactions bypass irreversible glycolytic reactions?
- Fructose-1,6-BP + H2O —(Fructose-1, 6-Bisphosphotase-1)—> F6-P + Pi
- Glucose-6-Phosphate + H2O —(Glucose-6-phosphotase)—> Glucose + Pi
Where is G6P localized and how does it get to the extracellular space?
- Found in Hepatocytes, Renal cells, Epithelial cells of small intestine
- Glucose-6-P goes from cytosol to ER lumen via T1
- Glucose-6-P turns into Glucose and Pi (Glucose-6-Phosphotase) which are transported by T3 and T2 respectively to the cytosol
- Glucose exits to extracellular space via GLUT2 transporter
Describe the relation between PFK-1 and FBPase-1
- They are recipriocally regulated in response to energy the cell needs
Glycolysis:
- Via PFK-1
- Upregulated by AMP and F-2,6-BP
- Downregulated by Citrate
Gluconeogenesis:
- Via FBPase-1
- Upregulated by citrate
- Downregulated by AMP and F-2,6-BP
What does Fructose-2, 6-Bisphosphate do?
- It is an activator for PFK-1 and negative regulator of FBPase1
- F6P –(PFK2)–> F2,6BP
- F2,6BP –(FBPase-2) F6P
What does Phosphofructokinase-2-Fructose-2,6-bisphosphatase do?
- It is a dual function enzyme that has a ATP analog site and a Phosphate site
- Has a Phosphofructokinase-2 domain
- Fructose-2,6-BPase domain
How do hormones regulate glycolysis?
- High Blood Sugar = Insulin
- Dephosphorylation leads to activation of Glycolysis
How do hormones regulate gluconeogenesis?
- Low Blood Sugar = Glucagon
- Phosphorylation leads to activation of Gluconeogenesis
What is the Cori cycle?
- The conversion of Lactate produced by anaerobic glycolysis in the muscle cells to glucose through gluconeogenesis in liver cells
- Cost 4 ATP
Describe a glycogen particle.
- 20-40 Glycogen core complexes make a glycogen particle
- Core complexes consist of glycogenin protein
- 50,000 Glc molecules that are α-1,4 linked. and α-1,6 branches creating 2000 non-reducing ends
How does Glycogen Phosphorylase attach?
Catalyzes phosphorolysis of α-1,4 bonds to make glucose-1-phosphate from the non-reducing ends
What does Phosphoglucomutase do?
Converts G1P to G6P
What does Glycogen debranching enzyme do
Removes α-1,6 branches
What important coenzyme attaches to glycogen phosphorylase?
Pyridocal-5’-phosphate (PLP) a vitamin B6 derivative (FISH)
How does glycogen phosphorylase catalyze phosphorolysis to generate Glc-1-P
- Glycogen Phosphorylase + PLP coenzyme
- Inorganic phosphate attacks the α 1,4 glycosidic bond
What does Glucose-1-P look like?
What happens if glycogen phosphorylase cleaves towards the branch point?
It will eventually stop as it gets too close to the α-1,6 branch point
What does the first part of glycogen debranching enzyme activity look like?
What does the second part of glycogen debranching enzyme activity do?
- Cleaves the α-1,6 glycosidic bond which results in the glycogen becoming a linear molecule
What does phosphoglucomutase do?
- Glucose-1-P takes Pi from phosphoserine
- Glucose-1,6-BP is formed and then Pi 1 attaches back to serine
- Glucose-6-P is used in liver cells by dephosphorylating and exporting or is used by muscles to metabolize glycolysis
What does Glucose-6-Phosphatase do?
- Turns G6P into Glucose and Pi to be exported to other tissue
How is Glycogen phosphorylase hormonally regulated?
- Is a homodimer with two serine residues
- Phosphorylation causes Ser –> Phosphoserine causes the activation (R state)
- Signals Glucagon (+ Epinephrine)or glycogenolysis
- Hydration results in Phosphoserine –> Ser (inactive T state)
- Signals insulin or Glycogenesis
How is Glycogen phosphorylase allosterically regulated?
- AMP activates —> Glycogenolysis
- G6P/ATP inactivates —> Glycogenesis
What happens to phosphorylagted glycogen phosphorylase when blood glucose levels are high?
- Glycogen phosphorylase is rapidly inhibited by high blood glucose
What does glycogen synthesis require the synthesis of?
Glucose-1-P + UTP —(UDP glucose pyrophosphorylase)—> Uridine diphosphate glucose (UDP)
How does Glycogen synthase add glucose to glycogen?
- Glycogen sythnase uses UDP-glucose to add glucose residues one at a time to the non-reducing ends of glycogen
How is UTP regenerated?
- Enzyme Nucleoside diphosphate kinase uses ATP as a phosphoryl donor
What does Glycogen branching enzyme work?
- Creates a new α-1,6 branch point and a new non-reducing end
- It transfers 7 Glc residues from the end of one chain to an internal point
- Cannot cut chain closer than 4 residues from another branch point
What is glycogenin?
- An anchor protein for the glycogen core complex
- Catalyzes the glycosyltransferase and synthase reactions to make initial glycogen chain
How is glucose added to glycogenin?
- Glc from UDP-Glc attaches to a Tyr residue through O linked glycosidic bonding via glycosyltransferase activty
- The second Glc is attached via glycogen synthase which forms an α-1,4 glycosidic bond
What is a glycogenin primer and what does it do?
- Glycogen synthase is repeated to make a 7 glucose residue primer
- It will then be used by glycogen synthase and glycogen branching enzyme
How is glycogen synthase regulated?
- Phosphorylation by a kinase will make glycogen synthase enter the T state (glucagone/epinephrine)
- Dephosphorylation by a phosphotase will result in glycogen synthase entering the R state (insulin)
How are glycogen phosphorylase and glycogen synthase related?
- They are reciprocally regulated. If glucose concentration is high, G.phosphorylase activity will decrease to not make more glucose-1-P. Meanwhile G.synthase activity will increase to make glycogen
What happens when Blood glucose is High? What about when it is Low?
What is Pompe?
A glycogen storage disease that affects the muscles of infants resulting in excess lysosomal glycogen which cannot be broken down. It can be dealt with by weekly infusions of the recombinant enzyme
