Chapter 18 - GLYCOLYSIS Flashcards
What does Glycolysis mean?
Glycolysis
Breakdown of sugar
Glyco - sugar
Lysis - breaking down
Explain what enzymes are
Enzymes
- Proteins that function to speed a chemical reaction
- Enzymes serve as catalysts
- Participates in chemical reaction, but is not used up by the reaction
- Lower the Energy of Activation (Ea)
- Energy required to start a chemical reaction
- Energy must be added to get one reactant to react with another
Show how the graph looks like with an enzyme and without an enzyme
Explain how enzymes function
How Enzymes Function
Enzyme binds substrate to form a complex
Substrate binds to active site on enzyme
E + S —–> ES ——-> E + P
Enzyme + Substrate —–> enzyme-substrate comple
—–> enzyme + product
Explain the induced fit model of enzymes
Induced fit model of enzymes
- Substrate and active site shapes don’t match exactly
- Active site is induced to undergo a slight change in shape to accommodate substrate binding
- Change in shape facilitates reaction
Explain enzyme activation
Enzyme Activation
- Cell regulates metabolism by regulating which enzymes are active
- Genes producing enzymes can be turned on or off to regulate enzyme concentration
- Enzyme can be modified by adding or removing phosphates – changes shape
Explain enzyme inhibition
Enzyme Inhibition
- Occurs when enzyme cannot bind its substrate
- Many enzymes are regulated by feedback inhibition
- When product is abundant it binds to the enzyme’s active site and blocks further production
- When product is used up, it is removed from the active site
- Enzyme begins to function again
What are the essential features of Glycolysis?
- Essentially all cells carry out glycolysis.
- Glycolysis consists of ten reactions - essentially the same in all cells but with different reates
- Two phases:
- First phase converts glucose to two G-3-P
- Investm nt Stage
- Primin Reactions
- Second phase produces two pyruvates
- Payoff Stage
- First phase converts glucose to two G-3-P
- Products of glycolysis are pyruvate, ATP and NADH.
- There are three major fates for pyruvate.
How many reactions does glycolysis consist of?
Glycolysis consists of ten reactions - essentially the same in all cells but with different reates
Two phases:
First phase converts glucose to two G-3-P
Investment Stage
Primin Reactions
Second phase produces two pyruvates
Payoff Stage
How many phases is glycolysis divided into and explain each phase.
Glycolysis consists of ten reactions. These are divided into two phases.
The first phase:
Energy requiring phase
converts glucose to two molecules of G-3-P
2 ATPs are used to prime these reactions
First phase is divided into 2 stages: investment stage and priming reactions
The second phase
Energy producing phase
generates two pyruvates
Conversion of glyceraldehyde-3-phosphate to pyruvate and coupled formation of 4 ATP and 2 NADH
Second phase has one stage and it is known as the payoff stage.
What are the products of glycolysis?
Products of glycolysis are pyruvate, ATP and NADH.
How many major fates does pyruvate have?
There are three major fates for pyruvate.
Overview of the all the steps in glycolysis and the phases of glycolysis
What are the different fates of pyruvate after glycolysis?
2 pyruvate can go into or can be used for 3 different processes:
Under aerobic conditions:
2 pyruvate can go into the TCA cycle
Under anaerobic conditions:
2 pyruvate can used in lactic acid fermentation (in contracting muscle)
or
2 pyruvate can be used in alcoholic fermentation (in yeast)
What can the pyruvate from glycolysis be concerved into under aerobic and anerobic conditions?
Pyruvate produced in glycolysis can be utilized by cells in several ways.
In animals, pyruvate is normally converted to acetyl-coenzyme A, which is then oxidized in the TCA cycle to produce CO2.
When oxygen is limited, pyruvate can be converted to lactate.
Alcoholic fermentation in yeast converts pyruvate to ethanol and CO2.
Why are coupled reactions important in glycolysis?
Coupled reactions involving ATP hydrolysis are used to drive the glycolytic pathway.
Coupled reactions convert some, but not all of the metabolic energy of glucose into ATP.
Under cellular conditions, approximately 5% of the energy of glucose is released in glycolysis.
Explain reaction 1 of glycolysis
Reaction 1 of glycolysis:
Enzyme: hexokinase or glucokinase
Phosphorylation of glucose
Glucose to Glucose - 6 - phosphate (G-6-P)
Glucose + ATP —–> G-6-P + ADP
Known as the first priming reaction of glycolysis
ΔGo‘ for the 1st step is large and negative and the reaction is extremely favorable
Energy required to make G-6-P comes from the hydrolysis of ATP
What are the advantages of phosphorylation of Glucose
Advantages of phosphorylation of Glucose:
It keeps intracellular concentration of glucose low so reaction proceeds forward
Glucose is kept in the cell by phosphorylation to G-6-P, which cannot easily cross the plasma membrane
Explain the enzyme Hexokinase
- Requires Mg 2+ for the reaction
- Substrate is MgATP2-
- Four Isozymes of hexokinase
- Hexokinase I (Brain)
- Hexokinase II (Skeletal)
- Hexokinase III
- Glucokinase (Pancreas liver)
Explain the regulation of Hexokinase
- Allosterically inhibited by glucose-6-phosphate
- At high levels of G-6-P, hexokinase activity is downregulated
- Inhibition is reversible
- At high levels of G-6-P, hexokinase activity is downregulated
- As G-6-P is consumed through subsequent glycolysis reactions, hexokinase becomes activated again
In liver, how is glucose converted when glucose levels are high and when glucose levels are low
- In liver: Glucokinase converts glucose to glucose-6-phosphate and directs it to a pathway to be converted to glycogen
- Only active when glucose levels are high (Km is high)
- Induced by insulin
- Under normal conditions (lower levels of glucose) hexokinase I is active in liver and directs glucose to glycolysis
Explain Reaction 2 of glycolosis
Reaction 2 of Glycolysis
Enzyme: Phosphoglucoisomerase
Phosphoglucoisomerase catalyzes the isomerization of G6P to fructose-6-phosphate
Reaction occurs at a ΔG that is near 0 so it’s near equilibrium and it is freely reversible
Explain reaction 3 of glycolysis
Reaction 3 of Glycolysis
Enzyme: Phosphofructokinase
Phosphorylation of Fructose-6-phosphate to Fructose-1,6-biphosphate
This reaction is coupled to the hydrolysis of ATP.
Known as the second priming reaction of glycolysis
Reaction 3 is the major point of regulation during glycolysis
The phosphofructokinase catalyzed reaction commits the glucose to the glycolysis pathway
Which reaction step is known as the most regulated reaction in glycolysis pathway
Reaction 3 - phosphofructokinase (PFK)
The phosphofructokinase (PFK) catalyzed reaction commits the glucose to the glycolysis pathway (i.e. it will not be converted to another sugar, or stored as glycogen).
The PFK reaction, therefore, is the most important step to regulate in the glycolysis pathway.
Why is PFK reaction the most important step in glycolysis
Reaction 3 of glycolysis - PFK - Phosphofructokinase
The phosphofructokinase (PFK) catalyzed reaction commits the glucose to the glycolysis pathway (i.e. it will not be converted to another sugar, or stored as glycogen).
The PFK reaction, therefore, is the most important step to regulate in the glycolysis pathway.
Explain PFK
Phosphofructokinase
The enzyme that catalyzes reaction 3 in glycolysis
It phosphoralates F6P to Fructose-1,6-bisphosphate
Phosphofructokinase reaction commits the cells to metabolizing glucose
- You’ve put too much energy in to back track
- Reaction is irreversible under cellular conditions
Branch point
Major point of regulation during glycolysis!
What is the first priming reaction of glycolysis?
Reaction 1 - Hexokinase
It is known as the first priming reaction of glycolysis
Phosphorylation of glucose to Glucose-6-phosphate coupled with ATP.
What is known as the second priming reaction of glycolysis?
Reaction 3 - Phosphofructokinase
Known as the second priming reaction of glycolysis
Phosphorylation of F6P to Fructose-1,6-bisphosphate
Coupled to the hydrolysis of ATP
List the many ways Phosphofructokinase be regulated
Regulation of PFK-1
ATP
AMP
Citrate
PFK-2
Explain in details all the different ways PFK-1 be regulated
- Phosphofuctokinase-1 is regulated by the cells ATP supply
- ATP is an allosteric inhibitor of PFK.
- PFK activity goes up when ATP supply is depleted - ATP broken down into ADP or AMP
- PFK activity is inhibited whenever ATP is abundant in the cell
- High levels of ATP turn glycolysis off!
- ATP is both a substrate for PFK-1 and an endpoint for glycolysis
ATP binds both an active site and an allosteric site - ATP binds to an allosteric site on the enzyme
Induces a conformational change that lowers the affinity of PFK-1 for Fructose-6-phosphate
- AMP bind allosterically to PFK-1 and relieve inhibition by ATP
- Thus, the energy status of the cell, as reflected in the relative concentrations of ATP and AMP, serves to regulate glycolysis by regulating the action of PFK
- Citrate is an allosteric inhibitor of PFK. Citrate is produced by the citric acid cycle
- High citrate concentration also binds allosterically and increases inhibition of PFK-1 activity
- If citrate is high, then glycolysis is functioning and sending pyruvate into the TCA cycle
- PFK-2 catalyzes the formation of fructose 2,6-biphosphate from fructose-6-phosphate
- Increasing the concentration of 2,6 biphosphate strongly activates PFK-1
How is PFK-1 regulated by ATP?
Phosphofuctokinase-1 is regulated by the cells ATP supply
ATP is an allosteric inhibitor of PFK.
PFK activity goes up when ATP supply is depleted - ATP broken down into ADP or AMP
PFK activity is inhibited whenever ATP is abundant in the cell
High levels of ATP turn glycolysis off!
ATP is both a substrate for PFK-1 and an endpoint for glycolysis
ATP binds both an active site and an allosteric site
ATP binds to an allosteric site on the enzyme
Induces a conformational change that lowers the affinity of PFK-1 for Fructose-6-phosphate
AMP bind allosterically to PFK-1 and relieve inhibition by ATP
Thus, the energy status of the cell, as reflected in the relative concentrations of ATP and AMP, serves to regulate glycolysis by regulating the action of PFK
How is PFK-1 regulated by both ATP and AMP?
Phosphofuctokinase-1 is regulated by the cells ATP supply
ATP is an allosteric inhibitor of PFK.
PFK activity goes up when ATP supply is depleted - ATP broken down into ADP or AMP
PFK activity is inhibited whenever ATP is abundant in the cell
High levels of ATP turn glycolysis off!
ATP is both a substrate for PFK-1 and an endpoint for glycolysis
ATP binds both an active site and an allosteric site
ATP binds to an allosteric site on the enzyme
Induces a conformational change that lowers the affinity of PFK-1 for Fructose-6-phosphate
AMP bind allosterically to PFK-1 and relieve inhibition by ATP
Thus, the energy status of the cell, as reflected in the relative concentrations of ATP and AMP, serves to regulate glycolysis by regulating the action of PFK
How is PFK-1 regulated by citrate?
Citrate is an allosteric inhibitor of PFK. Citrate is produced by the citric acid cycle
High citrate concentration also binds allosterically and increases inhibition of PFK-1 activity
If citrate is high, then glycolysis is functioning and sending pyruvate into the TCA cycle
How is PFK-1 regulated by PFK-2
PFK-2 catalyzes the formation of fructose 2,6-biphosphate from fructose-6-phosphate
Increasing the concentration of 2,6 biphosphate strongly activates PFK-1
Phosphofructokinase is regulated by fructose-2,6-bisphosphate, a potent allosteric activator that increases the affinity of phosphofructokinase for the substrate fructose-6-P.
Explain Reaction 4 of glycolysis
Reaction 4 of glycolysis
Enzyme: Fructose biphosphate aldolase
Cleavage of fructose-1,6-bisphosphate by fructose bisphosphate aldolase
Fructose bisphosphate aldolase cleaves fructose-1,6-bisphosphate between the C3 and C4 carbons to produce: dihydroxy acetone phosphate (DHAP) and glyceraldehyde-3-phosphate.
ΔG is very positive! and not energetically favorable
Explain the reaction 5 of glycolysis
Reaction 5 of glycolysis
Enzyme - Triose phosphate isomerase
Converts DHAP to G-3-P
This enzyme allows both the products from reaction 4 to continue through the glycolytic pahtway
First phase of glycolysis is over!
Overall reaction sequence has ΔGo‘ = 2.2 kJ/mol
Hydrolysis of ATP makes overall reaction exothermic
What reactions does the phase one of glycolysis consist of?
Phase one of glycolysis consists of
Reaction 1
Reaction 2
Reaction 3
Reaction 4
Reaction 5
Explain the overall energetics of phase 1 of glycolysis
The net DG0’ for the five steps is +2.2 kJ/mol, or slightly unfavorable.
Starting from glucose, we now have two molecules of glyceraldehyde 3-phosphate
It took two ATP molecules to get this far - 2 ATPs are consumed in phase I of glycolysis
What are the main features of the second phase of glycolysis?
4 ATPs are produced in phase II of glycolysis
This phase invloves in going from 2 glyceraldehyde 3-phosphate to 2 pyruvate.
Net ATP yield for glycolysis is two ATP (4 ATP from phase II - 2 ATP from phase I)
The second phase of glycolysis involves two very high energy phosphate intermediates
1,3-bisphosphoglycerate
phosphoenolpyruvate
Explain reaction 6 of glycolysis
Reaction 6 of glycolysis
Enzyme: Glyceraldehyde-3 Phosphate dehydrogenase
Glyceraldehyde-3-phosphate (G-3-P) is oxidized to 1,3-bisphosphoglycerate (1,3-BPG)
This is the first redox reaction in the glycolytic pathway: the oxidation of G3P with associated reduction of NAD+
Energy yield from converting an aldehyde to a carboxylic acid is used to make 1,3-BPG and NADH
Oxidation is highly exergonic
Explain reaction 7 of glycolysis
Reaction 7 of glycolysis
Enzyme: phosphoglycerate kinase
Substrate-lvel phosphorylation of ADP by 1,3-bisphosphoglycerate
1,3-bisphosphoglycerate + ADP —–> ATP + 3-phosphoglycerate
Phosphoryl group is transferred to make ATP
Substrate for the reaction MgADP-
Glycolysis has broken even at this point with ATP.
Explain reaction 8 of glycolysis
Reaction 8 of glycolysis
Enzyme: Phosphoglycerate mutase
shuffling of phosphate group
3-phosphoglycerate (3-PG) > 2-phosphoglycerate (2-PG)
Can get 2 more ATPs
Phosphoglycerate mutase:
3-phospho is moved from C-3 to C-2
Mutases
Enzymes that catalyze migration of functional groups
What are mutases
Mutases
Enzymes that catalyze migration of functional groups
Explain reaction 9 in glycolysis
Reaction 9 of glycolysis
Enzyme: Enolase
Generating phosphoenolpyruvate
2-phophoglycerate (2-PG) —> Phosphoenolpyruvate
Reaction forms substrate into a form where more potential energy can be released
Explain reaction 10 in glycolysis
Reaction 10 of glycolysis
Enzyme: Pyruvate Kinase
Generates final 2 ATPs
Goes from phosphoenolpyruvate to pyruvate
One ATP is produced for each substrate level phosphorylation of ADP by PEP. Since 2 PEP molecules are ultimately produced by each glucose molecule, the net production at this step is 2 molecules of ATP for each glucose.
These two ATP (from one glucose) can be viewed as the “payoff” of glycolysis
Large, negative G – indicating that this reaction is subject to regulation
Explain pyruvate kinase
Pyruvate kinase
Final regulatory step in glycolysis
Activating factors
AMP
Fructose-1,6-phosphate
Inhibiting Factors
ATP
High concentrations allosterically inhibit
Decreases its affinity for PEP
Acetyl-CoA
What other metabolites can enter the glycolytic pathways?
Mannose, galactose, fructose, and other simple metabolites can enter the glycolytic pathway.
Are the following pathways aerobic or anaerobic?
Glycolysis
TCA cycle
Glycolysis - Anaerobic does not require oxygen
TCA cycle depends on oxygen
Aerobic
Does presence of oxygen give us more energy per glucose or less energy per glucose?
Presence of oxygen gives us more energy per glucose
Explain Hypoxia
Hypoxia
Oxygen Limitation
Leads to a variety of changes in the gene expression
- Increased angiogenesis
- Increased synthesis of red blood cells
- Increased levels of glycolytic enzymes
How do cells respond to hypoxic stress?
When oxygen is limiting (Hypoxia), cells adapt to carry out more glycolysis.
Hypoxia causes changes in gene expression that increases levels of glycolytic enzymes.
Explain hypoxia inducible factor
Hypoxia inducible factor
DNA binding protein
Two subunits
HIF-1β-Constitutively expressed
HIF-1α- Inducibly expressed
HIF-1α
O2 – Prolyl hydroxylases (PHDs) hydroxylate subunit
Ubiquitin E3 ligase leading to ubiquitylation and degradation by the host 26S proteosome
PHD is oxygen dependent!
