Glycolysis Flashcards
Glycolysis
Anaerobic pathway for ATP generation -ancient -conserved -can operate aerobically 10 enzyme-catalyzed reactions occur in cytosol One glucose is broken down to 2 pyruvate
Stage 1 of Glycolysis
Energy Investment
- glucose needs to be activated
- ATP is consumed
- involves hexose sugars
Stage 2 of Glycolysis
Energy Payout
- energy is harvested in the form of ATP
- NADH is also generated
- involves triose sugars
Glucose -> Glucose 6-Phosphate
ATP investment Irreversible Coupled reaction Phosphate transfer Catalyzed by hexokinase Regulated but not rate limiting
Glucose 6-Phosphate -> Fructose 6-Phosphate
Isomerization
Reversible
Fructose 6-Phosphate -> Fructose 1,6-bisphosphate
ATP investment Coupled reaction Phosphate transfer Catalyzed by phosphofructokinase-1 Regulated and rate limiting
Fructose 1,6-bisphosphate -> GAP
Lysis
Reversible
Fructose 1.6-bisphosphate -> DHAP -> GAP
Isomerization
Reversible
Second molecule of GAP produced via a different pathway
One molecule of Glucose Produces Two Molecules of GAP
Every reaction described from GAP to pyruvate happens twice per glucose
GAP -> 1,3 bisphosphoglycerate
Oxidation
Reversible
Energy Capture (NADH)
Catalyzed by GAPDH
1,3-BPG
High energy intermediate
-acyl phosphate
-stabilized by resonance
large phosphate-transfer potential
1,3-Bisphosphoglycerate -> 3-Phosphoglycerate
Substrate-level phosphorylation Reversible Coupled (ATP synthesis) Energy capture step (ATP) Phosphate transfer
3-Phosphoglycerate -> 2-Phosphoglycerate
Isomerization
Reversible
2-Phosphoglycerate -> Phosphoenolpyruvate (PEP)
Dehydration
Reversible
PEP
High energy intermediate
PEP -> Pyruvate
Substrate-level phosphorylation Irreversible Coupled (ATP synthesis) Catalyzed by pyruvate kinase (regulated) Energy capture step (ATP)
Overall Glycolysis Reaction
Glucose + 2 ADP + 2 NAD+ + 2 Pi = 2 Pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
Net ATP production per glucose
2 ATP
Why is Glycolysis Regulated
Ensures that energy needs are met
Glucose is not wasted when ATP is abundant
Means of Regulation in Glycolysis
Substrate availability -glucose import Enzyme Regulation -hexokinase -phosphofructokinase-1 (rate-limiting) -pyruvate kinase
Hexokinase Inhibition
G6P acts as a negative allosteric effector for hexokinase
Product Inhibition
PFK-1 Regulation
Allosterically regulated by ADP/AMP and PEP
If there is more ADP/AMP than ATP, then the cell needs more ATP
Elevated PEP indicates that the products of glycolysis are not being consumed
Feedback Inhibition
PFK-1 Regulation Graph
Check cheat sheet
PFK-1 Regulation and FGP
FGP is a homoallosteric activator
PFK-1 Regulation and AMP
AMP is a heteroallosteric activator
PFK-1 Regulation and PEP
PEP is a heteroallosteric inhibitor
Pyruvate Kinase Regulation
Allosteric enzyme
inhibited by ATP (product inhibition)
Activated by fructose-1,6-bisphosphate (feed-forward activation)
Effect of F-1,6-BP on Pyruvate Kinase
Heteroallosteric activator
Feed forward activation
Effect of F-1,6-BP on Pyruvate Kinase Graph
Check cheat sheet
ATP as an Inhibitor
PFK-1 and PK are both inhibited by ATP
- most enzymes catalyze reversible reactions
- synchronous regulation of reversible reactions
- maintain steady-state for intermediates
Glycogen Metabolism
Glycogen is synthesized from glucose-6-phosphate (anabolic)
Breakdown of glycogen uses inorganic phosphate to break glycosidic bonds
When we start from glycogen, we get more ATP/glucose (3) because no ATP is used to generate G-6-P from glycogen
Why is an anaerobic fate for pyruvate required?
To regenerate NAD+ for the oxidation reaction in glycolysis under anaerobic conditions
Glycolysis Produces:
2 pyruvate
2 NADH
Net of 2 ATP
Why does NADH need to be reoxidized to NAD+?
For glycolysis to continue Aerobic -oxidative phosphorylation Anaerobic -pyruvate reduction (ethanol, lactate)
Production of Lactate
NOT an acid
A dead-end product in skeletal muscle (during anaerobic activity)
Pyruvate converted to lactate via lactate dehydrogenase
Lactate Production Equation
Pyruvate + NADH + H+ = L-Lactate
Lactate Exportation from the Muscle
Lactate and protons are exported from the muscle to the blood via a specific membrane transporter protein
- lowers pH
- encourages O2 release from hemoglobin = the Bohr effect
Production of Ethanol
Does not occur in vertebrates
Occurs in yeast
-decarboxylation and reduction
-final products = CO2, ethanol and NAD+
Pyruvate Dehydrogenase Reaction
Catalyzed by pyruvate dehydrogenase complex
-PDH, PDC
Links glycolysis to the citric acid cycle
-connects the two processes, part of neither
Occurs inside the mitochondria, in the matrix
What occurs in the matrix?
Pyruvate dehydrogenase
Citric acid cycle
Oxidative phosphorylation
beta-oxidation (fatty acids)
Transport of Pyruvate
Glycolysis generates pyruvate in the cytosol
Pyruvate is converted to acetyl-CoA in the mitochondrial matrix
Transport across the inner mitochondrial membrane requires the transporter protein pyruvate translocase
A proton is transported with the pyruvate
How is pyruvate converted to acetyl-CoA
Via the pyruvate dehydrogenase complex (PDC)
Pyruvate Dehydrogenase Reaction
Pyruvate + HS-CoA + NAD+ = Acetyl-CoA + CoA + CO2 + NADH
What is Acetyl-CoA?
Acetyl group attached via a thioester bond
-high energy molecule
Is the formation of Acetyl-CoA reversible?
No, the formation of acetyl-CoA is a key irreversible step in carbohydrate metabolism
More on the Pyruvate Dehydrogenase Reaction
Oxidative decarboxylation Transacetylation Irreversible Catalyzed by PDH -requires 5 cofactors including NAD+, FAD, CoA
Pyruvate Dehydrogenase Complex
Multienzyme complex that contains
- multiple copies of three catalytic enzymes (decarboylate, transfer to CoA and oxidation)
- 5 cofactors
- regulated by kinases and phosphatases
Advantages of Multienzyme Complexes
Speeds up reaction times
Limits number of side reactions
Enzymes controlled as a single unit
Regulation of PDH
Highly regulated Irreversible Reaction -Acetyl-CoA cannot be used to make glucose Sensitive to ATP requirements Regulated by: NAD+/NADH ratio Ca++ concentration Acetyl-CoA
How NAD+/NADH Ratio Regulates PDH
Substrate/product effect
NADH inhibits PDH
-allostery (inhibitor)
-protein kinase activation (phosphorylation of PDH)
Acetyl-CoA and PDH Regulation
Inhibitor
Protein kinase activation (phosphorylation of PDH)
Ca++ and PDH Regulation
Protein phosphatase activation
-dephosphorylation of PDH
How is PDH Regulated by Reversible Phosphorylation
Switched off when energy levels are high
Phosphorylation (via a kinase) switches off the activity of the complex (inactive)
-protein kinase is activated by NADH + Acetyl-CoA
Dephosphorylation (via phosphatase) activates the complex (active)
-protein phosphate activated by Ca++
Inhibition of PDH
NADH and acetyl-CoA
Activation of PDH
NAD+ and HS-CoA
