Gluconeogenesis vs. Glycolysis
- Created by: Tooba
- Created on: 19-04-13 21:48
Introduction
Gluconeogenesis = process which synthesises glycose from pyruvate (comes from noncarbs like lactate, amino acids and glycerol)
Glycolysis = breakdown of glucose to pyruvate to generate ATP anaerobically
Both pathways have some common reactions, but aren't exact opposites (due to irreversible steps of glycolysis-bypassed in gluconeogenesis)
WHY DO WE USE THEM???
2 x molecule
Irreverible reaction
Step joining/cleaving 2 molecules
Overview of gluconeogenesis
Pyruvate --> Oxaloacetate (converted via ATP in mitochondria)
Oxaloacetate --> Phosphoenolpyruvate (decarboxylated/phosphorylated-GTP in cytosol)
PEP --> 2-phosphoglycerate (hydrated)
2-PG --> 3-phosphoglycerate (isomerised)
3-PG --> 1, 3-bisphosphoglycerate (phosphorylated)
1, 3-BPG --> Glyceraldehyde-3-phosphate + orthophosphate (cleaved)
GA-3-P --> Dihydroxyacetone phosphate (readily interconverts)
GA-3-P + DHAP --> Fructose-1, 6-bisphosphate (joined)
F-1, 6-BP --> Fructose-6-phosphate (dephosphorylated)
F-6-P --> Glucose-6-phosphate (easily converted) * Free glucose easily diffuses in/out
G-6-P --> Glucose (dephosphorylated in liver and kidney ER)
Overview of glycolysis
Glucose --> Glucose-6-phosphate (phosphorylated)
G-6-P --> Fructose-6-phosphate (isomerised)
F-6-P --> Fructose 1, 6-bisphosphate (phosphorylated)
F-1, 6-BP --> Glyceraldehyde-3-phosphate + Dihydroxyacetone phosphate (cleaved)
DHAP --> GA-3-P (isomerises)
GA-3-P + Orthophosphate --> 1, 3-bisphosphoglycerate (joined)
1, 3-BPG --> 3-phosphoglycerate (dephosphorylated)
3-PG --> 2-phosphoglycerate (isomerised)
2-PG --> Phosphoenolpyruvate (dehydrated)
PEP --> Pyruvate (converted)
Enzymes involved
Pyruvate decarboxylase ||| Pyruvate kinase
PEP carboxykinase ||| ---
Enolase ||| Enolase
PG mutase ||| PG mutase
PG kinase ||| PG kinase
GA-3-P dehydrogenase ||| GA-3-P dehydrogenase
Triose phosphate isomerase ||| Triose phosphate isomerase
Aldose ||| Aldose
F-1, 6-BPase ||| Phosphofructokinase
Phosphoglucose isomerase ||| Phosphoglucose isomerase
Glucose-6-phosphatase ||| Hexokinase
Energy considerations
Glucose --> G-6-P; Hexokinase
F-6-P --> F-1, 6-BP; Phosphofructokinase
PEP --> Pyruvate; Pyruvate kinase
Have large, -ve delta G, so eqm lies to pyruvate formation side, so enzymes differ:
G-6-P --> Glucose; Glucose-6-phosphatase
F-1, 6-BP --> F-6-P; Fructose-1, 6-bisphosphatase
Pyruvate --> Oxaloacetate; Pyurvate carboxylase
Oxaloacetate --> Malate --> Oxaloacetate (moves from mitochondria to cytoplasm)
Oxaloacetate --> PEP; PEP carboxykinase
*Reversal of glycolysis = +84kJ/mol, but gluconeogenesis = -48kJ/mol:
*Hydrolysis of 4NTPs (2ATP + 2GTP) coupled to make unfavourable process, favourable.
Reciprocal regulation
Glycolysis dependent on [glucose], gluconeogenesis depends on [glucose precursors]
Cell requires energy --> glycolysis predominates, surplus of energy --> gluconeogenesis
Reciprocal regulation = pathways controlled so one inactive while other is highly active
Rate and direction of both processes controlled at the points where they differ, because backwards and forwards reactions can be independently regulated
Biosynthetic and degradative pathways differ in 1 reaction min. so both thermodynamically favourable under similar physiological conditions (same steps in both processes near eqm under cellular conditions, so when 1 process is favoured, reactions take place in its direction)
Avoids futile cycle where 2ATP and 2GTP wastefully hydrolysed
Mechanisms of regulation
1. F-6-P <--> F-1, 6-BP
High AMP (energy needed): PFK stimulated and F-1, 6-BPase inhibited
High ATP + citrate (high energy charge): inhibit PFK and citrate activates F-1, 6-BPase
2. PEP <--> Pyruvate
High ADP (low energy status): Pyruvate carboxylase + PEP carboxykinase inhibited
High acetyl CoA (biosynthetic precursors from citric cycle): activates pyruvate carboxylase
ATP + alanine (allosteric effectors-high energy charge): inhibit pyruvate kinase
3. LIVER: both respond to level of blood glucose via F-2, 6-BP
F-2, 6-BP --> F-6-P --> F-2, 6-BP; FBPase2/PFK2
Low glucose = glucagon --> cAMP cascade --> ProteinKinaseA phosphorylates enzyme's Ser --> PFK2 activity inhibited/FBPase2 activity promoted --> less F-2, 6-BP = gluconeogenesis
Cori cycle
In limited Oxygen, more NADH formed than can be oxidised to NAD+ by respiratory chain
NAD+ formed from pyruvate --> lactate by lactate dehydrogenase
Lactate: muscle --> blood --> liver --> Pyruvate (by LDH) --> Glucose (by gluconeogenesis)
Glucose: liver --> blood --> muscle --> Broken down (by glycolysis)
Opposite pathways occur in different locations, where conditions favour each = cooperative
Relationship between pathways
Glycolysis = direct energy, substrate for citric acid cycle + oxidative phosphorylation and intermediates for biosynthetic pathways
Gluconeogenesis = maintain blood glucose levels during starvation + vigorous exercise and energy for brain + erythrocytes
Very similar = early organisms produce and break down glucose w/o lots of extra energy
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