Gluconeogenesis (Carbohydrate Anabolism) Flashcards
The brain requires how many grams of glucose
120 grams/day out of the entire bodies 160g
What provides the glucose supply needed
- Glycogen reserves (190g)
- Bodily fluids (20 g)
T or F: When glucose is depleted (fasting or prolonged exercise), glucose must be synthesized from other sources
True (gluconeogenesis)
Gluconeogenesis
Gluconeogenesis is the biosynthesis of glucose from non-carbohydrate precursors (Like a reverse glycolysis , but the three “irreversible” reactions must be bypassed)
Gluconeogenesis takes place in the
- LIVER
- Kidney (kidney uses up a lot of glucose so not so much here)
T or F: Glucose levels in the blood are not relatively constant
False, Concentration of glucose levels maintains relatively stable all the time due to glycolysis and gluconeogenesis
Glycolysis and gluconeogenesis mainly occurs in
Glycolysis: Mainly muscles and brain
Gluconeogenesis: Mainly liver
Name the 5 gluconeogensis precursors and their origin
Gluconeogenesis substrates photo
Anything form the Kreb Cycle (TCA cycle) can become glucose
Problem with gluconeogenesis
Must bypass the irreversible steps of glycolysis, so needs high energy investment
Compare and contrast reaction 1,3, 10 of glycolysis with gluconegensis (enzymes involved)
2 reactions of gluconeogenesis reaction 1 are necessary to bypass glycolysis reaction 10
Reaction 1: Carboxylation of pyruvate into oxaloacetate
Reaction 2: Phosphorylation and decarboxylation of oxaloacetate in phosphoenolpyruvate
Reaction 1 of gluconeogenesis
Carboxylation of pyruvate into oxaloacetate (catalyzed by pyruvate carboxylase)
**Pyruvate carboxylase is also used to convert pyruvate into oxaloacetate to replenish the intermediates of the TCA cycle, this cycle take places in the mitochondrial matrix (anaploresis)
**Biotin ( not a cofactor) = prosthetic arm and adds COO group to molecule
Reaction 1 of gluconeogenesis is catalyzed by [ blank] and occurs in the [blank]
pyruvate carboxylase (can only be found in the mitochondrial matrix) and the mitochondrial matirx
Mitochondria has 2 purposes
- Send pyruvate to TC cycle (Krebs cycle)
- Convert pyruvate to oxaloacetate (move away from TC/Krebs cycle
Pyruvate enters the mitochondria by
Mitochondrial Pyruvate Carrier (MPC).
Mitochondrial pyruvate carrier is made of
Heterodimer of MPC1 and MPC2
What is another function of the mitochondrail pyruvate carrier transporter
This transport mechanism is also used for the conversion of pyruvate into Acetyl-CoA (TCA cycle substrate).
Reaction 1 gluconeogenesis (malate shuffle) 2 steps:
- To exit the mitochondria, oxaloacetate must be reduced to malate by the mitochondrial malate dehydrogenase (MDH2)
- Once in the cytosol, malate is reoxidized to oxaloacetate by the cytosolic malate dehydrogenenase (MDH1)
Malate shuffle step 1
To exit the mitochondria, oxaloacetate is reduced to malate using NADH (provides electron (oxidation) to convert to L-malate) by MDH2 (mitochondrial malate dehydrogenase)
Malate Shuttle step 1 transporter
Use the Malate-Aspartate shuttle to export malate to the cytoplasm
**Mainly, the malate α-ketoglutarate transporter located in the mitochondrial membrane
Reaction 1 gluconeogenesis (malate shuffle) step 2
In the cytosol, malate is reoxidized into oxaloacetate and NADH
4 Characteristics of gluconeogenesis reaction 1
- reducing equivalents (stored on NAD+) are transported from the mitochondria to the cytosol
- These reducing equivalents (NADH) will be used later by the glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
- More NADH in the mitochondria than in the cytosol
- Need NADH for further reactions, so you take electrons by storing it on malate in mitochondria and then using malate to form oxaloacetate and NADH to produce more NADH in the cytosol (since there is so little)
4 Characteristics of gluconeogenesis reaction 1
- reducing equivalents (stored on NAD+) are transported from the mitochondria to the cytosol
- These reducing equivalents (NADH) will be used later by the glyceraldehyde-3-phosphate dehydrogenase (GAPDH).
- More NADH in the mitochondria than in the cytosol
- Need NADH for further reactions, so you take electrons by storing it on malate in mitochondria and then using malate to form oxaloacetate and NADH to produce more NADH in the cytosol (since there is so little)
Reaction 2 gluconeogenesis
Phosphorylation and decarboxylation of oxaloacetate in phosphoenolpyruvate catalyzed by phosphoenolpyruvate carboxykinase (PEPCK).
Gluconeogensis reaction 2 characteristics
- The CO2 that was attached by the pyruvate carboxylase in reaction 1 will be released during this reaction which will provide the electrons necessary for the P-O bond
- PEP becomes a better substrate for future/following reactions (formed by removing the CO2 from a previous molecules (in this case oxaloacetate) to provide necessary electrons to form phosphate-oxygen bonds
-GTP = high energy (treat it like an ATP) = needs investment of 1 GTP
Energy profile of the first bypass 4 characteristics
- Under intracellular conditions: ΔG = - 25kJ/mol
- Two high energy phosphates must be invested for the synthesis of one phosphoenolpyruvate.
- Remember that we need 2 molecules of phospho-enolpyruvate to produce one molecule of glucose.
- 2 ATP and 2 GTP needed/molecule of glucose.
Name and describe the alternative path for the first bypass
This alternative path is mainly used when lactate is the substrate. The main reason being that cytosolic NADH is generated by the lactate dehydrogenase.
**No need to carry reducing equivalents (NADH) outside of the mitochondria.
In image: Left side is what we just covered, so right image is the new alternative path = uses lactate to convert to pyruvate and produces PEP in the mitochondria that is transported out through a PEP transporter into the cytosol to continue the pathway
-Malate transporter is regulated which determines which of the 2 pathways is used (if malate transporter is turned off then we will use lactate)
Reactions 3-8 gluconeogenesis
Same as glycolysis just backwards
Gluconeogensis reaction 9
Hydrolysis (use of water) of fructose-1,6-bisphosphate into fructose-6-phosphate catalyzed by fructose-1,6-bisphosphatase (FBPase).
**Bypass glycolysis reaction 3 (Phosphofructokinase)
fructose-1,6-bisphosphatase (FBPase) converting fructose-1,6-bisphosphate into fructose-6-phosphate
Removes 1 phosphate on carbon number 1
-Error in structure (image) should have another oxygen (OH) on C2 of both structures (LOOK UP STRUCUTRE TO CONFIRM)
**Fun fact: The fructose 1,6 bisphosphatase was shown to be a key
enzyme in regulating the metabolic rate of hibernating animals,
the enzyme activity is temperature sensitive (less active when it
is cold and therefore, inhibiting gluconeogenesis).
Gluconeogensis Reaction 10
Gluconeogenesi reaction 11
Bypass glycolysis reaction 1 (Hexokinase)
Reaction 11: Hydrolysis of glucose-6-phosphate into glucose catalyzed by the glucose-6-phosphatase.
Energy balance sheet for gluconeogenesis
Table only shows standard delta G = doesn’t actually represent at cellular level, but standard delta G = favourable to move in forward direction for gluconeogenesis
-The actual delta G is are -25 kJ/mol
Why cant glycolysis operate in reverse
Costs 4 ATP, 2 GTP, and 2 NADH because without it its +79.9kJ/mol and the reaction will not proceed in the forward direction for gluconeogenesis
Explain when gluconeogensis is needed (graph-wise)
1) After a few hours after breakfast, glycogenolysis will kick in which is our storage of glucose to release more glucose because all our energy from breakfast has been used
2) Then after we used our glucose stores, gluconeogenesis will kick in to start breaking down non-carbohydrates to produce glucose for activities until we eat again
-Eg. If we only have 2 meals a day (eg. Ramadan) then you will have a greater peak of gluconeogenesis during the day because less glucose intake from meals
Gluconeogeneis from lactate
1) muscle and erythrocytes produce lactate.
2) This lactate from the muscles cells and erythrocytes travel to the liver.
3) In the liver lactate is converted back into glucose (Lactate is re-oxidized to pyruvate by the liver lactate dehydrogenase (LDH)) and return to the blood where it is available for the muscle cells/erythrocytes (Cori Cycle)
**Note there is a PEP alternative pathway
Gluconeogensis from glycerol
1) When our stock of carbohydrates is depleted (glucose and glycogen), fatty acids will become the principal metabolic fuel to fulfill the energy demand.
2) Catabolism (through hydrolysis)of triacylglycerides leads to the production of glycerol and free fatty acids:
->These fatty acids will be used as fuel for the TCA cycle.
->Glycerol will be converted to dihydroxyacetone phosphate. This molecule will further be converted to glyceraldehyde-3-phosphate (Reaction 7 of gluconeogenesis).
