Lecture 2 (W1) - Glycolysis Flashcards
What are the functions of glycolysis? [2]
- Generate energy (ATP) from rearrangement of atoms in glucose
- Partially breakdown glucose into pyruvate, providing a starting point for the complete oxidation of glucose in the TCA cycle.
Which organs does glucolysis occur? [3]
- Liver
- Brain
- Muscle
Which organelle in a cell does glycolysis usually occur?
Cytosol
(NOT cytoplasm! Cytoplasm includes organelles ;; while cytosol is the liquid portion of the cell)
Cytoplasm = cytsol + other organelles (excluding nucleus)
State the overall equation for glycolysis.
C6H12O6 + ADP + Pi + NAD -> 2CH3COCOO- + 2 NADH + 2 ATP + 2H+
What 2 main stages are glycolysis split into?
- Energy consumption / investment stage
- Energy recovery phase
Draw a flow chart of the energy investment stage, labelling parts where ATP is consumed and enzymes involved.
refer to notes
Draw a flowchart of the energy recovery phase, indicating where ATP is generated.
- intermediates (GAP, 1,3-BPG, 3PG, 2PG, PEP, pyruvate)
Refer to notes
What is the net energy / ATP produced from the 2 stages of glycolysis?
- Stage 1 : consume 2 ATP
- Stage 2 : produce 2 ATP (but repeated twice ;; 2x 3C intermediate) = 4 ATP
Thus, net 2 ATP is produced from glycolysis
What is meant by priming?
Which enzymes catalyse priming in glycolysis?
Priming is the initial phase where energy in the form of ATP is used to add phosphate groups to glucose, essentially “activating” the molecule to proceed through the subsequent steps of the glycolytic pathway
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- Hexokinase (glucose –> G6P)
- Phosphofructokinase, PFK (F6P –> F-1,6BP)
What class of enzyme in the glycolytic pathway is responsible for cleavage?
Aldolase
- Cleaves Fructose-1,6 biphosphate (F-1,6 BP) into Glyceraldehyde-6-phosphate (GAP) and dihydroxyacetone phosphate (DHAP)
Both hexokinase and glucokinase converts glucose into G6P. What is the difference in which :
- the tissues they are found in?
- Affinity for glucose (substrate)?
- how they are regulated?
- Hexokinase is found in most tissues except the liver ; while glucokinase is exclusive to liver
<br></br> - Hexokinase has a high affinity for glucose (esp for organs like the brain which needs glucose for energy);; while glucokinase has a significantly lower affinity for glucose.
<br></br> - Hexokinase is regulated by product inhibition, where high concentrations of product will bind to active site and prevent further substrate conversion ;; while glucokinase is allosterically regulated with inhibitors and activators binding to allosteric sites
What is meant by substrate level phosphorylation?
The formation of ATP (or GTP) by transferring a phosphate group from a high energy compound to ADP / GDP molecule.
Why is ATP formation (ADP + Pi → ATP) considered endergonic?
ATP formation has a positive ΔG (+30.5 kJ/mol), meaning it consumes energy and is not spontaneous.
Bond forming -> consumes energy -> endogonic
How does substrate-level phosphorylation overcome the positive ΔG of ATP synthesis from ADP?
By coupling ATP formation with an exergonic reaction, such as the cleavage of a high-energy phosphate group from another substrate molecule, which has a negative ΔG. This ensures the overall reaction still has an overall negative ΔG
What are the 3 irreversible steps in glycolysis, and the enzymes that catalyse each reaction?
Step 1 : glucose → G6P (hexokinase)
Step 3 : F6P → F-1,6 BP (phosphofructokinase)
Step 10 : phosphoenolpyruvate (PEP) → pyruvate (pyruvate kinase)
Why are irreversible reactions, rather than reversible reactions, targets for regulation in metabolism?
- Reversible reactions exist almost in equilibrium
- Irreversible reactions serve as commitment points to drive metabolic pathways forward and determine the rate of metabolic pathways.
Consider the 3 irreversible reactions for glycolysis :
<br></br>
Step 1 : glucose → G6P (hexokinase)
Step 3 : F6P → F-1,6 BP (phosphofructokinase)
Step 10 : phosphoenolpyruvate (PEP) → pyruvate (pyruvate kinase)
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Which step is the first committed step in glycolysis and why?
Step 3. Step 1 is not the first committed step as G6P can serve as a substrate for other metabolic pathways as well. Step 10 is too late into the glycolytic pathway for it to be a first committed step.
What is meant by a first committed step in a metabolic pathway?
The first committed step is the first irreversible reaction in a metabolic pathway where a substrate is converted into a metabolite that is dedicated exclusively to that pathway. Once this step occurs, the metabolite cannot enter any other pathway and must proceed through the rest of the pathway.
The first committed step in a metabolic pathway is always the first irreversible reaction. True or False?
False.
The second irreversible reaction is the first committed step for glycolysis;; but in general, it is true that the first irreversible reaction is the first committed step for other metabolic pathways.
The enzymes which catalyse the irreversible steps in glycolysis are regulated allosterically.
List out some activators and inhibitors of:
1. hexokinase
2. Phosphofructokinase-1 (PFK-1)
3. Pyruvate kinase
Refer to notes
Why is F-2,6 BP an activator for phosphofructokinase 1 (PFK-1), which converts F6P into F-1,6BP?
Usually, F-2,6 BP is not synthesized. The fact that it is synthesized suggests that there is an excess of F6P (substrate), thus activating PFK-
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- F-2,6BP binds to PFK-1, increasing its affinity for F6P and reducing the inhibitory effect of ATP, thereby accelerating the conversion of F6P to F-1,6BP and promoting glycolysis.
Phosphofructokinase, PFK
- How many subunits does it have?
- What are its conformations?
- PFK is a tetramer , indicating that it has 4 protein subunits
- Conformations : relaxed state (R) and tense state (T)
What are the 2 roles that ATP plays in glycolysis?
- It serves as a source of energy (by breaking the phosphoanhydride bond and hydrolysising ATP into ADP + Pi, it releases energy)
<br></br> - It acts as an allosteric inhibitor for enzymes such as PFK. (if too much ATP, means body is not utilising energy that fast, slow down glycolysis to conserve resources)
What are the 3 fates of pyruvate after glycolysis?
1. TCA cycle (precursor to oxidative phosphorylation)
2. Homolactic fermentation to produce lactate
3. Alcoholic fermentation to produce ethanol + CO2 → in microbial cells, not as impt
