Glycolysis Flashcards
Define metabolism
The sum total of all the reactions that take place in a living cell
What are the reactions in metabolism for?
- extract energy & materials from the environment (catabolism)
- energy & materials used to produce new molecules (anabolism)
- result - cell sustained & can propagate itself
What are reactions arranged into?
Pathways
- product for one reaction is the reactant (substrate) for the next
What is each step in a reaction pathway catalysed by?
catalysed by an enzyme
Non-spontaneous reactions are driven by …
coupling them to spontaneous reactions.
Energy is needed to …
drive metabolism
What are the 3 different arrangements of pathways?
- linear
- circular
- spiral
(slide 6)
What are intermediates in reaction pathways?
Temporary molecules consumed in further steps in the reaction pathway - neither reactants/products of the overall reaction.
What is ATP?
- a molecule within the cell
- that allows for quick & easy access to energy
- when needed by the cell’s organelles
Draw the structure of ATP
(slide 9)
- 3 phosphate groups (triphosphate chain)
- ribose sugar
- adenine (nitrogenous base)
Draw the structure of ADP
(slide 10)
- 2 phosphate groups (diphosphate chain)
- ribose sugar
- adenine (nitrogenous base)
Draw the structure of AMP
(slide 10)
- 1 phosphate group
- ribose sugar
- adenine (nitrogenous base)
What is one of the common links between catabolism and anabolism?
ATP - used to shuttle chemical energy from catabolism to anabolism
What are coenzymes in metabolism?
- biological oxidation/reduction agents NAD+ and FAD
- used to shuttle energy from the favourable oxidations that take place in catabolism
- to the unfavourable reductions that take place in anabolism (redox reactions)
What are the Stages of Cellular Respiration?
- Glycolysis
- The Krebs Cycle
- The Electron Transport Chain
(all examples of metabolic pathways)
Is glycolysis catabolic or anabolic?
Catabolic pathway
What are the characteristics of glycolysis?
- series of 10 coupled reactions
- anaerobic
What are the 2 roles of gylcolysis?
- supply energy
- produce intermediates for biosynthetic pathways
Where does glycolysis occur?
Cytoplasm
What are the 3 stages of glycolysis?
- investment stage
- cleavage phase
- harvesting stage
What is the 1st stage of glycolysis?
Investment stage:
- 2x ATP are consumed for each molecule of glucose
- glucose converted to fructose-1,6-bisphosphate
- glucose is trapped inside the cell and at the same time converted to an unstable form that can be readily cleaved into 3-C units
(slide 16)
What is the 2nd stage of glycolysis?
Cleavage stage:
- fructose-1,6-bisphosphate is cleaved into …
- 2x 3-C units of glyceraldehyde-3-phosphate
(slide 16)
What is the 3rd stage of glycolysis?
Harvesting stage:
- 4x ATP and 2x NADH are gained from each molecule of glucose
(- this ATP is a result of substrate-level phosphorylation)
- glyceraldehyde-3-phosphate is oxidized to pyruvate
(slide 16)
What is reaction 1 of glycolysis?
Phosphorylation of glucose to glucose 6-phosphate
glucose + ATP → G-6P + ADP + H+
- enzyme: hexokinase
What reaction type is reaction 1?
Phosphoryl transfer
What enzyme is used in reaction 1 of glycolysis?
Hexokinase
- has a low Km for glucose; thus, once glucose enters the cell, it gets phosphorylated
What happens in reaction 1 of glycolysis?
Reaction requires energy
- so coupled to the hydrolysis of ATP to ADP and Pi
Is reaction 1 of glycolysis reversible or irreversible?
Irreversible step
- ∴ glucose gets trapped inside cell
- (glucose transporters transport only free glucose, not phosphorylated glucose)
What is reaction 2 of glycolysis?
Isomerization of glucose 6-phosphate to fructose 6-phosphate
G-6P ⇌ F-6P
What reaction type is reaction 2?
Isomerization
What enzyme is used in reaction 2 of glycolysis?
phosphoglucose isomerase
What happens in reaction 2 of glycolysis?
Aldose sugar converted into the keto- isoform
Is reaction 2 of glycolysis reversible or irreversible?
Reversible reaction
- F-6P quickly consumed
- and the forward reaction is favoured
What is reaction 3 of glycolysis?
Phosphorylation of the hydroxyl group on C1 (of F-6P), forming fructose 1,6-bisphosphate
What reaction type is reaction 3?
Phosphoryl transfer
What enzyme is used in reaction 3 of glycolysis?
phosphofructokinase
- allosteric enzyme regulates pace of glycolysis
What happens in reaction 3 of glycolysis?
Reaction is coupled to the hydrolysis of an ATP to ADP and Pi
Is reaction 3 of glycolysis reversible or irreversible?
irreversible reaction of the glycolytic pathway
What is reaction 4 of glycolysis?
Begins Stage 2
fructose-1,6-bisphosphate ⇌ dihydroxyacetone phosphate + glyceraldehyde 3-phosphate
F-1,6-BP ⇌ DHAP + GAP
What reaction type is reaction 4?
Aldol cleavage
What enzyme is used in reaction 4 of glycolysis?
Aldolase
What happens in reaction 4 of glycolysis?
Fructose-1,6-bisphosphate splits into:
- 2x 3-carbon molecules
- one aldehyde
- one ketone:
dihyroxyacetonephosphate (DHAP) and glyceraldehyde 3-phosphate (GAP)
What is reaction 5 of glycolysis?
dihydroxyacetone phosphate ⇌ glyceraldehyde 3-phosphate
What reaction type is reaction 5?
Isomerization
What enzyme is used in reaction 5 of glycolysis?
triose phosphate isomerase
What happens in reaction 5 of glycolysis?
- DHAP and GAP are isomers of each other
- so readily inter-convert by the enzyme
- GAP is a substrate for the next step in glycolysis
- so all of the DHAP is eventually depleted
- ∴ 2 molecules of GAP are formed from each molecule of glucose
What is reaction 6 of glycolysis?
Begins Stage 3.
- GAP is dehydrogenated by enzyme (GAPDH)
- NAD+ is reduced
GAP + NAD+ + Pi ⇌ 1,3-BPG + NADH + H+
What reaction type is reaction 6?
Phosphorylation coupled to oxidation
What enzyme is used in reaction 6 of glycolysis?
glyceraldehyde 3-phosphate dehydrogenase (GAPDH)
What happens in reaction 6 of glycolysis?
- Up to this step, 2 molecules of ATP were required for each molecule of glucose being oxidized
- Since there are 2 GAP molecules generated from each glucose, each of the remaining reactions occur twice for each glucose molecule being oxidized
What is reaction 7 of glycolysis?
1,3-BPG + ADP + H+ ⇌ 3-Phosphoglycerate + ATP
What reaction type is reaction 7?
Phosphoryl transfer
What enzyme is used in reaction 7 of glycolysis?
phosphoglycerate kinase
What happens in reaction 7 of glycolysis?
1,3-BPG has:
- a mixed anhydride
- a high energy bond at C1
- this high energy bond is hydrolysed to a carboxylic acid
- and the energy released is used to generate ATP from ADP
What is reaction 8 of glycolysis?
3-Phosphoglycerate ⇌ 2-Phosphoglycerate
What reaction type is reaction 8?
Phosphoryl shift
What enzyme is used in reaction 8 of glycolysis?
phosphoglycerate mutase
What happens in reaction 8 of glycolysis?
The phosphate shifts from C3 to C2 to form 2-phosphoglycerate
What is reaction 9 of glycolysis?
2-Phosphoglycerate ⇌ Phosphenolpyruvate + H2O
What reaction type is reaction 9?
Dehydration
What enzyme is used in reaction 9 of glycolysis?
enolase (lyase)
What happens in reaction 9 of glycolysis?
- dehydration catalyzed by enolase (a lyase)
- H2O molecule removed to form phosphoenolpyruvate
- which has a double bond between C2 and C3
What is reaction 10 of glycolysis?
Phosphoenolpyruvate + ADP + H+ → Pyruvate + ATP
What reaction type is reaction 10?
Phosphoryl transfer
What enzyme is used in reaction 10 of glycolysis?
pyruvate kinase
What happens in reaction 10 of glycolysis?
- Enolphosphate is a high energy bond
- hydrolyzed to form the enolic form of pyruvate with the synthesis of ATP
- Enol pyruvate quickly changes to keto pyruvate which is far more stable.
State the reactants/products of each step of glycolysis
- glucose
- glucose 6-phosphate
- fructose 6-phosphate
- fructose 1,6-biphosphate
- dihydroxyacetone ⇌ glyceraldehyde 3-phosphate
- 1,3-Biphosphoglycerate
- 3-Phosphoglycerate
- 2-Phosphoglycerate
- Phosphoenolpyruvate
- Pyruvate
What is Gibbs free energy ΔG?
Thermodynamic property that measures the amount of energy available to do useful work in a system (at constant T & P)
- used to determine whether a reaction is spontaneous or non-spontaneous
What are the ΔG values of a spontaneous and non-spontaneous reaction?
spontaneous = ΔG<0
non-spontaneous = ΔG>0
What are the ΔG values of a spontaneous and non-spontaneous reaction?
spontaneous = ΔG<0
non-spontaneous = ΔG>0
When does a negative ΔG occur?
- ΔH is negative
and/or - ΔS is positive
What does ΔG tell us about the direction of a reaction?
Tells us which direction a reaction will favour
- does not mean it will go to completion, rather to equilibrium
What does a negative and positive ΔH mean?
-ΔH = exothermic
+ΔH = endothermic
What does a negative and positive ΔG mean?
-ΔG = reaction feasible
+ΔG = reaction not feasible
What does it mean when ΔG = 0?
reaction is in equilibrium
What happens as ΔG gets more negative in glycolysis?
- ΔG gets more negative
- moves further away from equilibrium
- harder to reverse
Why do we need to control pathways?
- so opposing pathways don’t run at same time
- desired product is not made in sufficient quantities
- waste ATP
- important to stop futile cycle
What is the enzyme hexokinase regulated by?
excess glucose-6-phosphate (G6P)
How is hexokinase regulated by G6P?
- G6P accumulates in cell (meaning there is enough glucose)
- feedback inhibition of hexokinase
- until G6P is consumed
What is the reverse reaction pathway of glycolysis called?
gluconeogenesis
What is the rate-limiting step of glycolysis?
phosphofructokinase step
(step 3)
What activates and inhibits the phosphofructokinase enzyme?
activates: high AMP/ADP levels
inhibits: high ATP levels
How is phosphofructokinase regulated by feed-back inhibition by citrate?
- citrate = allosteric regulator of PFK
- overproduction of citrate = more than enough ATP
- negative feedback mechanism of citrate inhibits PFK
- this prevents overproduction of ATP in 3rd reaction of glycolysis
Define allosteric
- inhibition or activation of an enzyme
- by a small regulatory molecule
- that interacts at a site (allosteric site) other than the active site (at which catalytic activity occurs)
Give an example of allosteric feed-forward activation involving phosphofructokinase
- positive effector of phosphofructokinase = fructose-2,6-bisphosphate (F-2,6-BP)
- formed by hormone-stimulated phosphorylation of F-6-P
When is pyruvate kinase regulated?
If glycolysis gets past phosphofructokinase step
When is pyruvate kinase activity inhibited?
Under low glucose conditions by covalent phosphorylation
What drives the pyruvate kinase reaction forwards?
If fructose-1,6-bisphosphate is formed
- acts as an allosteric feedforward activator
What are positive and negative effectors in the regulation of pyruvate kinase?
- positive = AMP and ADP
- negative = ATP (binds to allosteric sites)
What is a negative effector of catabolism?
alanine (amino acid derived from pyruvate)
What do alanine levels signal?
Alanine levels signal the anabolic state of a cell
What do high alanine levels indicate?
- the cell has enough starting material for anabolic reactions
- so catabolism can be paused
What are the 3 ‘fates’ of pyruvate produced by glycolysis?
- anaerobic (lactic acid fermentation)
- aerobic oxidation
- anaerobic (alcoholic fermentation)
