Module 4 Flashcards
Aerobicity: Glycolysis
Anaerobic
How does Glycolysis generate energy?
Catabolism of Glucose
Location: Glycolysis
Cytoplasm
How does Glycolysis obtain free energy from Glucose without Oxygen being present?
Glycolysis involves two sequential stages:
- Activation of Glucose via Phosphorylation (“ATP Investment”)
- Collection of Energy from High-Energy Intermediates (“ATP Earnings”)
Glycolysis: Stage 1
“ATP Investment”
ATP is used to produce activated 3-Carbon sugar compounds.
Glycolysis: Stage 2
“ATP Earnings”
ATP is derived from the oxidation of 3-Carbon sugar compounds.
Glycolysis Stage 1: Net Macromolecule Reaction
1 Glucose → 2 Glyceraldehyde-3-P
Overall Reaction: 1 Glucose + 2 ATP → 2 Glyceraldehyde-3-P + 2 ADP + 2 Pi
Glycolysis Stage 2: Net Macromolecule Reaction
2 Glyceraldehyde-3-P → 2 Pyruvate
Overall Reaction: 2 G3P + 4 ADP + 4 Pi + 2 NAD+ + 2 H+ → 2 Pyruvate + 4 ATP + 2 NADH + 2 H2O
How does Hexokinase-facilitated Glucose phosphorylation trap the former Glucose molecule within the cell?
The GLUT4 membrane transport protein cannot bind/recognize Glucose-6-P, so it only transports Glucose across the cell membrane.
How does the phosphorylation of Glucose alter the compound’s free energy?
Phosphorylation increases the free energy of Glucose.
Glucose phorphorylation is highly thermodynamically favorable (i.e. irreversible).
Glycolysis 1: Hexokinase Phosphorylation
Hexokinase/Glucokinase catalyzes the phosphorylation of Glucose to generate Glucose-6-P (via coupling to an ATP hydrolysis reaction).
Glycolysis 2: Phosphoglucoisomerase Conversion
Phosphoglucoisomerase catalyzes the isomerization of Glucose-6-P (6-Carbon ring) to generate Fructose-6-P (5-Carbon ring).
Thermodynamics: Glycolysis 1
Hexokinase Phosphorylation
Highly Thermodynamically Favorable
Irreversible
Thermodynamics: Glycolysis 2
Phosphoglucoisomerase Conversion
Slightly Thermodynamically Favorable
Reversible
Thermodynamics: Glycolysis 3
Phosphofructokinase-1 Phosphorylation
Highly Thermodynamically Favorable
Irreversible
Glycolysis 3: Phosphofructokinase-1 Phosphorylation
Phosphofructokinase-1 catalyzes the phosphorylation of Fructose-6-P to generate Fructose-1,6-BP.
Which step of Glycolysis is the major regulatory/commitment step?
Glycolysis 3: Phosphofructokinase-1 Phosphorylation
Thermodynamics: Glycolysis 4
Aldolase Cleavage
Slightly Thermodynamically Favorable
Reversible
Why does the Aldose Cleavage reaction readily occur in the cell despite being highly thermodynamically unfavorable under standard conditions?
The products of the Aldose Cleavage reaction are continuously being used/consumed in other processes, so this reaction is constantly shifted toward the products (per Le Chatelier’s Principle).
The concentrations of the Aldose Cleavage reaction metabolites in the cell results in a mass action ratio that favors the cleavage reaction.
Glycolysis 4: Aldolase Cleavage
Aldolase cleaves Fructose-1,6-BP (between C-3 and C-4) to form Glyceraldehyde-3-P (3-Carbon) and Dihydroxyacetone-P (3-Carbon).
Glycolysis 5: Triose Phosphate Isomerase Isomerization
Triose Phosphate Isomerase catalyzes the reversible isomerization of Dihydroxyacetone-P to Glyceraldehyde-3-P (via the Enediol intermediate).
Thermodynamics: Glycolysis 6
Glyceraldehyde-3-P Dehydrogenase Oxidation-Phosphorylation
Slightly Thermodynamically Favorable
Reversible
Thermodynamics: Glycolysis 5
Triose Phosphate Isomerase Isomerization
Slightly Thermodynamically Unfavorable
Reversible
Thermodynamics: Glycolysis 7
Phosphoglyercerate Kinase Phosphorylation
Slightly Thermodynamically Favorable
Reversible
Thermodynamics: Glycolysis 8
Phosphoglycerate Mutase Isomerization
Slightly Thermodynamically Unfavorable
Reversible
Thermodynamics: Glycolysis 9
Enolase Dehydration
Slightly Thermodynamically Favorable
Reversible
Thermodynamics: Glycolysis 10
Pyruvate Kinase Phosphorylation
Highly Thermodynamically Favorable
Irreversible
At which steps of Glycolysis does substrate-level phosphorylation occur?
- Step 7: Phosphoglycerate Kinase Phosphorylation
- Step 10: Pyruvate Kinase Phosphorylation
Glycolysis 6: Glyceraldehyde-3-P Dehydrogenase Oxidation-Phosphorylation
Glyceraldehyde-3-P Dehydrogenase catalyzes the coupled oxidation-phorylation reaction that converts Glyceraldehyde-3-P to 1,3-Biphosphoglycerate.
Glycolysis 7: Phosphoglycerate Kinase Phosphorylation
Phosphoglycerate Kinase catalyzes the dephosphorylation of 1,3-Biphosphoglycerate to generate ATP (via substrate-level phosphorylation) and 3-Phosphoglycerate.
Glycolysis 8: Phosphoglycerate Mutase Isomerization
Phosphoglycerate Mutase catalyzes the isomerization of 3-Phosphoglycerate to generate 2-Phosphoglycerate (via phosphoryl transfer).
Glycolysis 9: Enolase Dehydration
Enolase catalyzes the dehydration/condensation of 2-Phosphoglycerate to generate the higher-energy Phosphoenolpyruvate.
Glycolysis 10: Pyruvate Kinase Phosphorylation
Pyruvate Kinase catalyzes the dephosophorylation of Phosphenolpyruvate to generate ATP (via substrate-level phosphorylation) and Pyruvate.
Which steps of Glycolysis are thermodynamically unfavorable?
- Step 5: Triose Phosphate Isomerase Isomerization
- Step 8: Phosphoglycerate Mutase Isomerization
Glycolysis: Net Reaction
1 Glucose + 2 NAD+ + 2 ADP + 2 Pi → 2 Pyruvate + 2 NADH + 2 ATP + 2 H+ + 2 H2O
Glycolysis: 10 Steps
- Hexokinase Phosphorylation
- Phosphoglucoisomerase Isomerization
- Phosphofructokinase-1 Phosphorylation
- Aldolase Cleavage
- Triose Phosphate Isomerase Isomerization
- Glyceraldehye-3-P Dehydrogenase Oxidation-Phosphorylation
- Phosphoglycerate Kinase Phosphorylation
- Phosphoglycerate Mutase Isomerization
- Enolase Dehydration
- Pyruvate Kinase Phosphorylation
Glycolysis: Sugar Compounds in Sequence
- Glucose
- Glucose-6-P
- Fructose-6-P
- Fructose-1,6-BP
- Dihydroxyacetone-P
- Glyceraldehyde-3-P
- 1,3-Biphosphoglycerate
- 3-Phosphoglycerate
- 2-Phosphoglycerate
- Phosphoenolpyruvate
- Pyruvate
What process regenerates NAD+ under anaerobic conditions?
Fermentation
Fermentation: Animals vs. Microorganisms
- Animals: Fermentation converts Pyruvate to Lactate
- Microorganisms: Fermentation converts Pyruvate to Ethanol.
Mechanism: Glyceraldehyde-3-P Dehydrogenase Oxidation-Phosphorylation
What is the regulation of Glycolysis dependent on?
Energy Charge Within the Cell
During which reactions does the regulation of Glycolysis within muscle tissue occur?
- Hexokinase Phosphorylation
- Phosphofructokinase-1 Phosphorylation
- Pyruvate Kinase Phosphorylation
These three reactions are highly exergonic (i.e. irreversible).
How does Hexokinase regulation in muscle occur?
Allosteric Inhibition via Glucose-6-P
The binding of Glucose-6-P to Hexokinase’s regulatory binding site causes an enzymatic conformational change that inhibits ATP-Hexokinase binding.
Detailed Mechanism: Glyceraldehyde-3-P Dehydrogenase Oxidation-Phosphorylation
- The Sulfhydryl group of G3PD’s active site undergoes nucleophilic attack on G3P’s carbonyl Carbon to generate a thiohemiacetal intermediate.
- π-electron rearrangement at the thiohemiacetal Oxygen creates a Hydride ion that undergoes nucleophilic attack on NAD+’s benzene ring to generate NADH. (The thiohemiacetal intermediate is converted to a acyl thioester intermedicate.)
- NADH and H+ leave the G3PD active site.
- NAD+ and Pi enter the G3PD active site.
- The Pi group’s anionic Oxygen undergoes nucleophilic attack on the acyl thioester Carbon to cleave the S—C bond and generate 1,3-Bisphosphoglycerate.
4-Step Mechanism: Glyceraldehyde-3-P Dehydrogenase Oxidation-Phosphorylation
- Nucleophilic attack by G3PD’s Sulfhydril group on G3P’s carbonyl Carbon generates a thiohemiacetal intermediate.
- π-electron rearrangement creates a Hydride ion that nucleophilically attacks NAD+’s to generate NADH and an acyl thioester intermedicate.
- NADH and H+ leave the G3PD active site; NAD+ and Pi enter the G3PD active site.
- The Pi nucleophilically attacks the acyl thioester Carbon to cleave the S—C bond and generate 1,3-Bisphophoglycerate.
How does Phosphofructokinase-1 regulation in muscle occur?
- Allosteric Inhibition via ATP
- Allosteric Stimulation via ADP/AMP
- When there is high energy charge in the cell, ATP binds to Phosphofructokinase-1’s allosteric site to inhibit (reduce F6P binding) the enzyme’s activity.
- When there is low energy charge in the cell, AMP/ADP binds to Phosphofructokinase-1’s allosteric site to stimulate (increase F6P binding) the enzyme’s activity.
Which reactions of Glycolysis generate ATP?
- Glycolysis 7: Phosphoglycerate Kinase Phosphorylation
- Glycolysis 10: Pyruvate Kinase Phosphorylation
Which reaction of Glycolysis generates NADH?
Glycolysis 6: Glyceraldehyde-3-P Dehydrogenase Oxidation-Reduction
Which reaction of Glycolysis requires/consumes ATP?
Glycolysis 3: Phosphofructokinase-1 Phosphorylation
Conformations: Phosphofructokinase-1
- T-State: Inactive (ATP Bound)
- R-State: Active (AMP/ADP Bound)
How does regulation of Pyruvate Kinase in muscle occur?
- Allosteric Inhibition via ATP/Alanine/Acetyl-CoA/LCFA
- Allosteric Stimulation via F16BP
What process generates Alanine from Pyruvate?
Transamination
How does the Liver function to maintain blood Glucose levels?
- When blood Glucose levels are high, Glucose is stored as Glycogen or converted into fatty acids (for eventual delivery into adipose tissue).
- When blood Glucose levels are low, Glucose is produced de novo (via Gluconeogenesis) or mobilized from Glycogen stores.
Which type of Hexokinase functions in the Liver?
Glucokinase
Hexokinase IV
How does Glucokinase differ from Hexokinase?
- Glucokinase has lower affinity for Glucose than Hexokinase.
- Glucokinase is not inhibited by G6P binding (unlike Hexokinase).
How does Phosphofructokinase-1 regulation in the Liver occur?
- Allosteric Inhibition via ATP/Citrate
- Allosteric Stimulation via ADP/AMP/F26BP
Which compound is the most important activator of Phosphofructokinase-1 activity in the Liver?
Fructose-2,6-Biphosphate
F26BP binding to Phosphofructokinase-1 increases PFK affinity for F6P and decreases ATP inhibition of PFK (to increase rates of Glycolysis when Glucose is abundant).
How does regulation of Pyruvate Kinase in the Liver occur?
- Allosteric Inhibition via ATP/Alanine/Acetyl-CoA/LCFA
- Allosteric Stimulation via F16BP
- Covalent Inhibition via Phosphorylation
- Covalent Stimulation via Dephosphorylation
Which enzyme catalyzes the phosphorylation of Pyruvate Kinase in the Liver?
Protein Kinase A
PKA
Fate of Pyruvate: Anaerobic vs. Aerobic
- Anaerobic: Converted to Lactate/Ethanol (in Cytoplasm)
- Aerobic: Used to Generate Acetyl-CoA (in Mitochondria)
What is the function of the Pyruvate Dehydrogenase Complex?
Formation of Acetyl-CoA from Pyruvate
The PDH Complex catalyzes the irreversible conversion of Pyruvate to Acetyl-CoA (and the formation of NADH).
Subunits: Pyruvate Dehydrogenase Complex
- E1: Pyruvate Dehydrogenase
- E2: Dihydrolipoyl Transacetylase
- E3: Dihydrolopoyl Dehydrogenase
E1: Pyruvate Dehydrogenase
A tetrameric protein that decarboxylates Pyruvate (to generate Hydroxyethyl-TPP and CO2) and transfers a Hydroxyethly group to the E2 Lipoamide (to generate TPP).
E2: Dihydrolipoyl Transacetylase
A trimeric protein that oxidizes a Hydroxyethyl group (to generate Acetate) and transfers Acetate to Coenzyme A (to generate Acetyl-CoA).
E3: Dihydrolipoyl Dehydrogenase
A dimeric protein that oxidizes the E2 Dihydrolipoamide (to generate Lipoamide), catalyzes the reduction of FAD (to generate FADH2), and catalyzes the reduction of NAD+ (to generate NADH).
Drugs for Increasing Insulin Sensitivity
Treatments for Diabetes
- α-Glucosidase Inhibitors (Miglitol)
- Sulfonylurea Drugs (Glipizide)
- AMPK-Activating Drugs (MetFormin)
- PPaRγ Agonists (Thiazolidinedione)
MetFormin
An AMPK-activating drug that stimulates increased Glucose uptake and utilization.
Glipizide
A Sulfonylurea drug that stimulates increased Insulin secretions by inhibiting K+ leak channels (in the plasma membranes).
Miglitol
An α-Glucosidase inhibitor that blocks carbohydrate degredation in the small intestine (to lower blood Glucose levels).
Thiazolidinedione
A PPaRγ agonist that improves Insulin sensitivity (in liver/muscle cells)
Steady State
A condition of metabolic stability in which the rate of catabolism, the rate of anabolism, and the concentration of substrates are maintained at constant levels.
Not Equilibrium: The rate of catabolism and the rate of anabolism are NOT equal.
Which reactions of a metabolic pathway give the pathway its directionality?
Exergonic Reactions
Equilibrium: Committed Step
Far from Equilibrium
Exergonic
How can a reaction have a positive/unfavorable ∆G, but a negative/favorable ∆G°’?
- ∆G takes into consideration the concentration of reactant and products, which shift the reaction away from standard conditions.
- ∆G°’ is a measure of free energy at standard conditions, which does not consider differing concentrations of reactants and products.
Why is ADP + Pi more stable than ATP?
- Greater Charge Separation (Less Charge Repulsion)
- More Stable Resonance Structures
- Greater Solvation of Compounds
What molecules is the Inner Mitochondrial Membrane permeable to?
What molecules can pass through the Outer Mitochondrial Membrane?
Equation: Standard Reduction Potential
Equation: ∆G (Concentrations)