Gluconeogenesis Flashcards
Gluconeogenesis
- needed to be able to synthesize glucose when glycogen stores are depleted
- mainly occurs in liver
- bypasses the 3 irreversible reactions of glycolysis
3 Irreversible Reactions of Glycolysis
- glucose to glc-6-P conversion
- hexokinase - fructose 1,6 P to fructose 1,6 bisphosphate
- phosphofructokinase-1 - phosphoenolpyruvate to pyruvate
- pyruvate kinase
Gluconeogeneis Replacement Reactions
- pyruvate converted to oxaloacetate
- pyruvate carboxylase - oxaloacetate to phosphoenolpyruvate
- PEP carboxykinase - fructose 1,6 bisphosphate to fructose 1,6 phosphate
- fructose 1,6 bisphosphatase - glucose 6 - P to free glucose
- glucose 6 phosphatase
Oxaloacetate Synthesis
- exergonic reaction
- bicarbonate + pyruvate using ATP
- pyruvate carboxylase
- synthesises the 4C compound oxaloacetate
Phosphoenolpyruvate Synthesis
- exergonic reaction
- oxaloacetate reacts with GTP to form phosphoenolpyruvate
- gives GDP and carbon dioxide
Pyruvate Carboxylase
- carboxylates pyruvate using biotin cofactor
- biotin attached via amide linkage to lysine
- biotin carboxylated using ATP
- carbon dioxide then attached to pyruvate
Net Gluconeogenesis Reaction
2 pyruvate + 4ATP + 2GTP + 2NADH + 2H+ + 4H20 == glucose + 4ADP + 2GDP + 2Pi + 2NAD+
- energetically costly but essential in the absence of carbs
Cori Cycle
- metabolic pathway in which lactate produced by anaerobic glycolysis in muscles is transported to the liver and converted to glucose, which then returns to the muscles and is cyclically metabolized back to lactate
- muscular activity uses ATP provided by glycogen breakdown
- releases G1P which is converted to G6P by phosphoglucomutase
- G6P feeds into glycolysis or pentose phosphate pathway to form ATP
- If oxygen is not enough to undergo oxidative phosphorylation, lactate is formed from pyruvate using lactate dehydrogenase
- NAD+ is regnerated to keep glycolysis going
- lactate produced is taken up by the liver, initiating the other half of the Cori cycle
- converts lactate into pyruvate and then glucose
- glucose supplied to muscle
Overall, the glycolysis steps of the cycle produce 2 ATP molecules at a cost of 6 ATP molecules consumed in the gluconeogenesis steps. Each iteration of the cycle must be maintained by a net consumption of 4 ATP molecules. As a result, the cycle cannot be sustained indefinitely. The intensive consumption of ATP molecules in the Cori cycle shifts the metabolic burden from the muscles to the liver.
Futile Cycle
- substrate cycling
- glycolysis goes from fructose 6P into fructose 1,6 BPG
- gluconeogenesis goes from fructose 1,6 BPG to fructose 6P
- if both run at the same time, you just have a net reaction of ATP and water reacting to give off heat and ADP
- this is stupid
- therefore, the cycles are regulated reciprocally to avoid waste
Glycolysis Regulation
Phosphofructokinase - activated by AMP/F 2,6 BP - inactivated by ATP/citrate/protons Pyruvate kinase - activated by F 1,6 BP - inactivated by ATP/alanine
Gluconeogenesis Regulation
Fructose 1,6 BP - inactivated by F 2,6 BP/AMP - activated by citrate PEP Carboxykinase - inactivated by ADP Pyruvate carboxylase - inactivated by ADP - activated by acetyl CoA
Fructose 2,6 BP
- regulator of PFK1/FBPase1
- synthesized from fructose 6P using ATP by PFK2 or hydrolyzed back to fructose 6P using water by FBPase 2
Reciprocal Regulation of PFK/FBPase
- activities combined into one bifunctional enzyme
- cAMP dependent kinase (activated by PKA/glucagon) phosphorylates enzyme and activates FBPase/inactivates PFK2
- phosphatase (insulin activated) dephosphorylates enzyme and activates PFK2/inactivates FBPase
concentration of F 2,6 BP:
- high concentration: inhibits gluconeogenesis, stimulates glycolysis
- low concentration: inhibits glycolysis, stimulates gluconeogenesis
Reducing Equivalents
- NAD+ and NADP+
- similar structures but NADP+ has an extra phosphate on the 2’OH of the ribose ring
- both carry 2 electrons but only retain one proton
NADH
Activated electron carrier for fuel oxidation
Reduced in oxidation of fuel molecules
Electrons passed to oxygen to generate ATP
- concentration high for catabolism (oxidation)
NAPH
Acts as an electron donor in reductive biosynthesis
HIgh potential electrons needed because precursors are more oxidised than the products like lipid synthesis
- concentration high for biosynthesis (reduction)
Pentose Phosphate
- produces NADPH
- 2 phases: oxidative and non-oxidative
- produces 2 NADPH molecules
Oxidative Phase of Pentose Phosphate Pathway
- produces NADPH
- ribulose 5 phosphate end product: isomerized to ribose
1. G6P oxidised (inihibited by high NADPH)
2. hydrolysis
3. oxidative decarboxylation - rate controlled by NADP+ concentrations: low levels limit regulatory first step because NADP+ is needed as an electron acceptor
Net Reaction of oxidative phase
G6P + 2NADP + 2H20 = 2NADPH + ribose-5-P + CO2 + 2H2
Glutathione
- prevents oxidative damage
- antioxidant in cytoplasm and reduces ROS like peroxides
- keeps SH groups in reduced form in the cytoplasm
- NADPH used to regenerate reduced form so that it can continue to react with ROS
Non-oxidative phase
- if nucleotides not needed, ribulose-5-P converted back into intermediates of glycolysis/gluconeogenesis
- 2 x fructose 6P
- glyceraldehyde 3P
- rate controlled by substrate availability
Fates of Glucose 6 Phosphate
- pentose phosphate pathway: ribose + NADPH
- glycolysis: pyruvate
- conversion to G1P: glycogen metabolism
- conversion to glucose: use by other tissues
