Gluconeogenesis and Glycogen synthesis Flashcards
When does gluconeogenesis occur?
Occurs when blood glucose levels drop. The brain is dependent on glucose, so blood concentrations cannot change very much.
Humans use about 160 grams of glucose/day, 75% in the brain. Body fluids have about 20 grams of glucose and glycogen has about 180 grams of glucose.
Gluconeogenesis is responsible for 64% of glucose production in first 22 h of fasting and all thereafter.
Where does gluconeogenesis occur?
Primarily in the liver, some in the kidney, in cytosol
What amino acids can glucose be made from?
All except Leu (L) and Lys (K)
Can glycerol be used as a carbon source in gluconeogenesis?
Yes
Can fatty acids be used as a carbon source in gluconeogenesis?
No
How many of the enzymes from glycolysis are used in gluconeogenesis? Why are different enzymes needed?
Uses 7 of the 10 total enzymes from glycolysis.
Requires 4 new enzymes to bypass the 3 rate determining steps in glycolysis. Remember these rate-limiting steps have highly -∆G’s in the glycolytic direction
Which reverse enzyme replaces hexokinase in gluconeogenesis?
glucose-6-phosphatase
Which reverse enzyme replaces PFK-1 in gluconeogenesis? How is it regulated?
fructose 1,6-bisphosphatase-1 (FBPase-1)
allosterically inhibited by Fructose 2,6 bisphosphate, AMP. Activated by citrate.
Which reverse enzyme replaces pyruvate kinase in gluconeogenesis? How is it regulated?
two new enzymes: pyruvate carboxylase converts pyruvate to oxaloacetate, PEP carboxykinase converts oxaloacetate to phosphoenolpyruvate
Pyruvate carboxylase is upregulated by Acetyl-CoA, downregulated by ADP.
Phosphoenolpyruvate carboxykinase is downregulated by ADP.
How are glycolysis reactions reversable?
∆G of each reaction are very different between standard conditions and true cellular conditions.
∆G is related to the equilibrium constant, which in turn reflects the available concentrations of substrates and products.
In the cell, most reactions of glycolysis have very small ∆G values, and are usually at equilibrium. These reactions are readily reversible depending on concentrations.
Reactions that are not reversible: hexokinase, phosphofructokinase, pyruvate kinase. These need a different enzyme for gluconeogenesis.
What coenzymes are required for converting pyruvate to phosphoenolpyruvate?
ATP, GTP, biotin
What is the difference between PFK-1 and FBPase-1 aside from direction?
Not phosphoryl transfer, just simple phosphorylysis.
Hydrolysis of high energy phosphate ester, therefore favorable ∆G = -8.6 kJ/mol (exergonic)
Where does the glucose-6-phosphatase reaction occur?
Occurs at the ER so glucose can be secreted from cell, unique to liver and kidney. Exergonic (-5.1 kJ/mol) because its a simple phosphorylsis
FBPase-1 and PFK-1 are both regulated by what?
fructose 2,6-bisphosphate
Summarise gluconeogenesis
- Multistep process where glucose is produced from lactate, pyruvate or oxaloacetate or any other TCA cycle metabolite.
- Three irreversible steps are bypassed with four new enzymes.
- Formation of one molecule of glucose from pyruvate requires 4 ATP, 2 GTP and 2 NADH. Energetically expensive!
- Glycolysis and gluconeogenesis are reciprocally regulated. Have glucose and need energy -> glycolysis, need glucose - > gluconeogenesis.
Cori cycle
At what three points is glycolysis regulated?
Glycolysis is regulated at three major steps:
Hexokinase (allosterically by changing
Km and feed back inhibition by G-6-P).
Phosphofructokinase (allosterically by
ATP and Fructose 2,6 phosphate).
Pyruvate kinase (by covalent modification – phosphorylation by PKA).
How does fructose 2,6-bisphosphate regulate glycolysis/gluconeogenesis?
Allosterically actives phosphofructokinase 1
Allosterically inhibits fructose bisphosphate phosphatase
The level of Fructose-2,6-bisphosphate is controlled by an enzyme (not in either pathway) PFK-2/FBPase-2
The enzyme, PFK-2/FPBase-2 is regulated by
a. Allosteric inhibition and activation
b. Phosphorylation, when it is phosphorylated the kinase activity is inhibited and phosphatase activity activated, therefore [F-2,6-P] decreases
- The phosphorylation state of PFK-2/FPBase-2 is hormonally controlled by Glucagon, which is released when blood sugar is low.
- Therefore, the [F-2,6-P] and hence the relative “rates” of glycolysis and gluconeogenesis are controlled by blood sugar levels.
Cell energy charge
ENERGY CHARGE, which measures how much of the adenylate pool is in the ATP form.
Cells like their energy charge to be about 0.95. If is is lower, they will do glycolysis. If it is higher, they will put the glucose away for later use.
In energy charge, ATP counts as 2 because it has two phosphoanhydride bonds. ADP has 1 and counts as 1
What is the non-oxidative phase of the pentose phosphate pathway?
Nonoxidative phase recycles six molecules of the pentose into five molecules of the hexose glucose 6-phosphate, allowing continued production of NADPH and converting glucose 6-phosphate to CO2.
Glycogen
- Higher organisms protect themselves from potential fuel shortages by polymerizing glucose into glycogen.
- Glycogen is a glucose polymer with α1-4 glycosidic linkages and α1-6 branches every 8-14 residues.
- Muscle can be 1-2% and liver up to 10% by weight of glycogen.
- As a polymer, the concentration is 0.01 mM, but if the individual glucose units were free then 0.4 M.
- Glycogen granules also contain the enzymes that catalyze glycogen synthesis and degradation.
- Liver glycogen can be depleted in 12 – 24 hours.
What size and shape are glycogen particles?
Glycogen polymers exist as spherical particles that contain 10,000 – 60,000 glucose residues
glycogen phosphorylase a
- Catalyzes phosphorolysis (bond cleavage by the substitution of a phosphate) to yield glucose 1-phosphate. Only releases units that are at least 5 residues away from a branch point.
- Glycogen-n + Pi <-> glycogen-(n-1) + G-1-P
Although the reaction is reversible in solution, within the cell the enzyme only works in the forward direction as shown below because the concentration of inorganic phosphate is much higher than that of glucose-1-phosphate.
Glycogen phosphorylase can act only on linear chains of glycogen (α1-4 glycosidic linkage). Its work will immediately come to a halt four residues away from α1-6 branch (which are exceedingly common in glycogen). In these situations, a debranching enzyme is necessary, which will straighten out the chain in that area. In addition, the enzyme transferase shifts a block of 3 glucosyl residues from the outer branch to the other end, and then a α1-6 glucosidase enzyme is required to break the remaining (single glucose) α1-6 residue that remains in the new linear chain. After all this is done, glycogen phosphorylase can continue. The enzyme is specific to α1-4 chains, as the molecule contains a 30-angstrom-long crevice with the same radius as the helix formed by the glycogen chain; this accommodates 4-5 glucosyl residues, but is too narrow for branches. This crevice connects the glycogen storage site to the active, catalytic site.
Describe the mechanism of phosphoglucomutase?
Reaction catalyzed by phosphoglucomutase. The reaction begins with the enzyme phosphorylated on a Ser residue.
In step 1, the enzyme donates its phosphoryl group to C6 of glucose 1-phosphate, producing glucose 1,6-bisphosphate.
In step 2, the phosphoryl group at C-1 of glucose 1,6-bisphosphate (red) is transferred back to the enzyme, re-forming the phosphoenzyme and producing G-6-P.
glycogen synthase a
Catalyses glycogen synthesis (glycogenesis) using UDP-glucose (activated form of glucose conjugated to uridine diphosphate that contains two high energy bonds) and a glycogen chain of n residues.
How is glucose prepared to be added to a glycogen chain?
glucose-1-phosphate is added to UTP by UDP-glucose pyrophosphorylase, resulting in pyrophosphate and UDP-glucose.
How is pyrophosphate useful to the cell?
It can be broken down to individual Pi by inorganic pyrophosphatase, and the reaction has a negative ∆G°’ (=-33.6 kJ/mol, greater than ATP) so can be used to drive other reactions to completion
What happens to protein kinase A when adrenaline is supplied?
Protein Kinase A is activated by adrenaline and phosphorylates enzymes that promote glycogenolysis
How is muscle glycogen phosphorylase regulated?
By covalent modification. phosphorylase a phosphatase (PP1) dephosphorylates phosphorylase a (the active form), changing it to phosphorylase b (the less active form). Phosphorylase b kinase transfers 2 phosphate groups from 2 ATP to phosphorylase b (the less active form), changing it to phosphorylase a (the active form).
- Phosphorylase a phosphatase (PP1)* is activated by insulin (the fed state), which preserves glycogen.
- Phosphorylase b* kinase is activated by glucagon, Ca2+ and AMP.
- Phosphorylase a* undergoes a conformational change through allosteric binding by glucose, making it more easily dephosphorylated by PP1, reducing activity. Similar to product inhibition.
When phosphorylase a is active, what is happening in the cell?
Glycogen is being broken down to provide energy to myocytes for muscle contraction (via glycolysis) or to be converted to blood glucose by hepatocytes
What is the cascade mechanism of epinephrine and glycogen?
cAMP activates PKA, which phosphorylates phosphorylase b kinase, which phosphorylates phosphorylase b converting it to phosphorylase a.
Because of this domino effect, 1 molecule of hormone yields 10,000 glucose molecules
UDP-glucose pyrophosphorylase
Converts UTP and glucose-1-phosphate into UDP-glucose for use by glycogen synthase a
