Chapter 18 - GLYCOLYSIS

Question 1

What does Glycolysis mean?

Answer 1

Glycolysis

Breakdown of sugar

Glyco - sugar

Lysis - breaking down

Question 2

Explain what enzymes are

Answer 2

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

Question 3

Show how the graph looks like with an enzyme and without an enzyme

Answer 3

Question 4

Explain how enzymes function

Answer 4

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

Question 5

Explain the induced fit model of enzymes

Answer 5

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

Question 6

Explain enzyme activation

Answer 6

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

Question 7

Explain enzyme inhibition

Answer 7

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

Question 8

What are the essential features of Glycolysis?

Answer 8

• 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
• Products of glycolysis are pyruvate, ATP and NADH.
• There are three major fates for pyruvate.

Question 9

How many reactions does glycolysis consist of?

Answer 9

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

Question 10

How many phases is glycolysis divided into and explain each phase.

Answer 10

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.

Question 11

What are the products of glycolysis?

Answer 11

Products of glycolysis are pyruvate, ATP and NADH.

Question 12

How many major fates does pyruvate have?

Answer 12

There are three major fates for pyruvate.

Question 13

Overview of the all the steps in glycolysis and the phases of glycolysis

Answer 13

Question 14

What are the different fates of pyruvate after glycolysis?

Answer 14

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)

Question 15

What can the pyruvate from glycolysis be concerved into under aerobic and anerobic conditions?

Answer 15

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.

Question 16

Why are coupled reactions important in glycolysis?

Answer 16

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.

Question 17

Explain reaction 1 of glycolysis

Answer 17

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

Question 18

What are the advantages of phosphorylation of Glucose

Answer 18

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

Question 19

Explain the enzyme Hexokinase

Answer 19

• Requires Mg 2+ for the reaction
  
  • Substrate is MgATP2-
• Four Isozymes of hexokinase
  
  • Hexokinase I (Brain)
  • Hexokinase II (Skeletal)
  • Hexokinase III
  • Glucokinase (Pancreas liver)

Question 20

Explain the regulation of Hexokinase

Answer 20

• Allosterically inhibited by glucose-6-phosphate
  
  • At high levels of G-6-P, hexokinase activity is downregulated
    
    • Inhibition is reversible
• As G-6-P is consumed through subsequent glycolysis reactions, hexokinase becomes activated again

Question 21

In liver, how is glucose converted when glucose levels are high and when glucose levels are low

Answer 21

• 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

Question 22

Explain Reaction 2 of glycolosis

Answer 22

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

Question 23

Explain reaction 3 of glycolysis

Answer 23

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

Question 24

Which reaction step is known as the most regulated reaction in glycolysis pathway

Answer 24

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.

Question 25

Why is PFK reaction the most important step in glycolysis

Answer 25

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.

Question 26

Explain PFK

Answer 26

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!

Question 27

What is the first priming reaction of glycolysis?

Answer 27

Reaction 1 - Hexokinase It is known as the first priming reaction of glycolysis Phosphorylation of glucose to Glucose-6-phosphate coupled with ATP.

Question 28

What is known as the second priming reaction of glycolysis?

Answer 28

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

Question 29

List the many ways Phosphofructokinase be regulated

Answer 29

Regulation of PFK-1 ATP AMP Citrate PFK-2

Question 30

Explain in details all the different ways PFK-1 be regulated

Answer 30

1. Phosphofuctokinase-1 is regulated by the cells ATP supply 1. ATP is an allosteric inhibitor of PFK. 2. PFK activity goes up when ATP supply is depleted - ATP broken down into ADP or AMP 3. PFK activity is inhibited whenever ATP is abundant in the cell 4. High levels of ATP turn glycolysis off! 5. ATP is both a substrate for PFK-1 and an endpoint for glycolysis ATP binds both an active site and an allosteric site 6. ATP binds to an allosteric site on the enzyme Induces a conformational change that lowers the affinity of PFK-1 for Fructose-6-phosphate 2. 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 3. Citrate is an allosteric inhibitor of PFK. Citrate is produced by the citric acid cycle 1. High citrate concentration also binds allosterically and increases inhibition of PFK-1 activity 2. If citrate is high, then glycolysis is functioning and sending pyruvate into the TCA cycle 4. PFK-2 catalyzes the formation of fructose 2,6-biphosphate from fructose-6-phosphate 1. Increasing the concentration of 2,6 biphosphate strongly activates PFK-1

Question 31

How is PFK-1 regulated by ATP?

Answer 31

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

Question 32

How is PFK-1 regulated by both ATP and AMP?

Answer 32

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

Question 33

How is PFK-1 regulated by citrate?

Answer 33

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

Question 34

How is PFK-1 regulated by PFK-2

Answer 34

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.

Question 35

Explain Reaction 4 of glycolysis

Answer 35

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

Question 36

Explain the reaction 5 of glycolysis

Answer 36

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

Question 37

What reactions does the phase one of glycolysis consist of?

Answer 37

Phase one of glycolysis consists of Reaction 1 Reaction 2 Reaction 3 Reaction 4 Reaction 5

Question 38

Explain the overall energetics of phase 1 of glycolysis

Answer 38

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

Question 39

What are the main features of the second phase of glycolysis?

Answer 39

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

Question 40

Explain reaction 6 of glycolysis

Answer 40

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

Question 41

Explain reaction 7 of glycolysis

Answer 41

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.

Question 42

Explain reaction 8 of glycolysis

Answer 42

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

Question 43

What are mutases

Answer 43

Mutases Enzymes that catalyze migration of functional groups

Question 44

Explain reaction 9 in glycolysis

Answer 44

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

Question 45

Explain reaction 10 in glycolysis

Answer 45

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

Question 46

Explain pyruvate kinase

Answer 46

**Pyruvate kinase** ## Footnote 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

Question 47

What other metabolites can enter the glycolytic pathways?

Answer 47

Mannose, galactose, fructose, and other simple metabolites can enter the glycolytic pathway.

Question 48

Are the following pathways aerobic or anaerobic? Glycolysis TCA cycle

Answer 48

Glycolysis - Anaerobic does not require oxygen TCA cycle depends on oxygen Aerobic

Question 49

Does presence of oxygen give us more energy per glucose or less energy per glucose?

Answer 49

Presence of oxygen gives us more energy per glucose

Question 50

Explain Hypoxia

Answer 50

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

Question 51

How do cells respond to hypoxic stress?

Answer 51

When oxygen is limiting (Hypoxia), cells adapt to carry out more glycolysis. Hypoxia causes changes in gene expression that increases levels of glycolytic enzymes.

Question 52

Explain hypoxia inducible factor

Answer 52

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!