Chapter 2 - Glycolysis Flashcards
What is the process of Glycolysis? (What goes in, what comes out?)
One molecule of glucose (6 carbons) is degraded into two molecules of pyruvate (3 carbons each)
How is free energy stored in Glycolysis?
Free energy is stored as 2 molecules of ATP and 2 molecules of NADH
What kind of reaction (exergonic/endergonic) is glycolysis in standard conditions? Is it reversible?
In standard conditions, glycolysis is an exergonic reaction and is irreversible because of negative free energy (-G)
What is Pasteurs effect?
Aerobic growth requires less glucose than anaerobic conditions.
What is the difference between aerobic and anaerobic conditions?
In aerobic conditions, there is an abundance of Oxygen.
In anaerobic conditions, there is much less Oxygen than normal
How many reactions are in the entire process of glycolysis?
There are 10 enzyme-catalyzed reactions split into two stages.
What occurs in the first stage (Reactions 1-5) of Glycolysis?
Stage 1 is a preparatory stage where glucose is phosphorylated, converted into fructose, phosphorylated again, and cleaved into two molecules of glyceraldehyde-3-phosphate.
In this phase, there is an investment of 2 molecules of ATP
What occurs in the second stage (reactions 6-10) of Glycolysis
The two molecules of glyceraldehyde-3-phosphate produced in the first stage are converted to pyruvate with concomitant generation of 4 ATP molecules and 2 molecules of NADH.
There is a net gain of 2 ATP molecules per one molecule of glucose in glycolysis (start with 2, end with 4)
What are the three main importances of the phosphorylated intermediates in Glycolysis?
- Possession of a negative charge which inhibit their diffusion through membrane
- Conservation of free energy in high energy phosphate bond.
- Facilitation of catalysis.
What is the 1st reaction of Glycolysis?
Hexokinase Reaction: Phosphorylation of Hexoses (many glucose).
Hexokinase (facilitated by Mg-ATP) reacts with glucose and replaces the OH group on Carbon 6 with a phosphate group, resulting in Glucose-6-Phosphate or G6P)
What is the 2nd reaction of Glycolysis?
Phosphoglucose isomerase OR phosphohexose Isomerase reaction: The Isomerization of G6P (Glucose-6-Phosphate) into Fructose-6-Phosphate (F6P)
The enzyme requires Mg2+ to activate (which is made by reaction 1)
What is the 3rd reaction of Glycolysis?
Phosphofructokinase-1 Reaction: The transfer of a phosphoryl group from ATP to C-1 of F6P to produce Fructose 1,6, diphosphate.
This step is an important, irreversible, regulatory step
What is the 4th reaction of Glycolysis?
Aldolase Reaction: Cleavage of Fructose-1,6-biphosphate into glyceraldehyde-3-phosphate (an aldose) and dihydroxy acetone phosphate (a ketose)
What is the 5th reaction of Glycolysis?
Triose Phosphate Mutase Reaction: Conversion of dihydroxy acetone phosphate into a second glyceraldehyde-3-phosphate.
What is the 6th reaction of Glycolysis?
Glyceraldehyde-3-Phosphate dehydrogenase reaction (GAPDH): Conversion of GAP to 1,3-Biphosphoglyceraldehyde.
What evidence (3) arises to show that the 6th reaction of Glycolysis has occurred?
-Iodoacetate inhibits this reaction, indicating the involvement of Cystine residue of enzyme.
- Tritium from GAP is transferred to NAD, indicating the transfer of hydride ion in oxidation form.
-P exchanges with PO4, indicating acyl enzyme intermediate
What are the steps in the mechanism for reaction 6 of Glycolysis?
- Glyceraldehyde-3-Phosphate (GAP) binds to the enzyme
- Nucleophilic attack by Sulfhydryl group on CHO group, forming a thiohemiacytal
- Direct transfer of Hydride to NAD+, leading to the formation of a thioester. The energy of this oxidation is conserved in synthesis of Thioester and NADH
- Another molecule of NAD replaces NADH from the enzyme site.
- Nucleophilic attack on thioester by PO4 to form 1,3-biphosphoglycerate
What is the 7th Reaction of Glycolysis?
Phosphoglycerate Kinase Reaction: Transfer of Phosphoryl group from 1,3 biphosphoglycerate to ADP, generating ATP
This reaction combined with the 6th reaction generates ATP from the energy released by oxidation of 3-phosphoglyceraldehyde.
What is the 8th Reaction of Glycolysis?
Phosphoglycerate Mutase Reaction: Conversion of 3-phosphoglycerate to 2-phosphoglycerate (2-PG)
What is the 9th Reaction of Glycolysis?
Enolase Reaction: Dehydration of 2-phosphoglycerate (2-PG) to phosphoenolpyruvate (PEP)
What is the 10th Reaction of Glycolysis?
Pyruvate Kinase Reaction: Transfer of Phosphoryl Group from PEP to ADP generating ATP and Pyruvate.
What is produced in Glycolysis from one molecule of Glucose?
2 molecules of Pyruvate
2 molecules of ATP
2 molecules of NADH
What happens to pyruvate under anaerobic conditions?
Under anaerobic conditions, pyruvate is converted to Lactate in homolactic fermentation or in ethanol in alcoholic fermentation
What is Alcoholic Fermentation?
A two-step process in which microorganisms and yeast convert pyruvate to alcohol and carbon dioxide to regenerate NAD+ for glycolysis.
What are the two steps of Alcoholic Fermentation
- Pyruvate Decarboxylase (PDC) Reaction: This enzyme is Mg2+ dependant and requires an enzyme-bound cofactor, thiamine pyrophosphate (TPP). In this reaction, a molecule of CO2 is released, producing acetaldehyde.
- Alcohol Dehydrogenase reaction: Acetaldehyde is reduced to ethanol using NADH+ as a reducing power, thus regenerating NAD+
What is the reaction mechanism of PDC (Pyruvate Decarboxylase)?
- Nucleophilic attack on carbonyl group carbon of Pyruvate by TPP (thiamine pyrophosphate) anion
- Departure of CO2 leaving the carbanion TPP adduct
- Protonation of Carboanion
- Release of acetaldehyde following regeneration of TPP.
What are the two types of controls for metabolic reactions?
Substrate limited and Enzyme limited
What is a substrate-limited control?
When concentrations of reactant and products in the cell are near equilibrium, then it is the availability of substrate which decides the rate of reaction
What is an enzyme-limited control?
When the concentration of substrate and products are far away from equilibrium, then it is the activity of the enzyme that decides the rate of reaction. These reactions are the one which control the flux of the overall pathway.
What three steps in glycolysis have enzymes that regulate the flux of glycolysis?
The hexokinase step (reaction 1)
The phosphofructokinase (PFK) step (reaction 3)
The pyruvate kinase step (reaction 10)
What is the phosphofructokinase-1 (PFK-1) step (reaction 3) controlled by
complex allosteric regulation
How is PFK inhibited by ATP?
In the presence of high concentration of ATP, ATP binds to the inhibition site of PFK, and decreases the activity of the enzyme.
What acts as inhibitors of PFK-1 (phosphofructokinase-1)
Hint: Theres three
AMP
ADP
Fructose-2,6,-biphosphate
all act as allosteric inhibitors of PFK-1
What is the substrate cycle? (aka futile cycle)
A cycle in which FPBase (fructose-1,6-biphosphatase) degrades the product of the PFK reaction (FBP) (reaction 3) back to its substrate (F6P) (reaction 2 product)
This cycle slows the glycolysis reaction down so not too much is made at once.
Why is the substrate cycle a futile exercise?
The cell invests ATP to make FBP, which is then hydrolysed back into F6P by FBPase. (reaction 3 product -> reaction 2 product -> reaction 3 product -> continue forever)
Its a waste of ATP as the reaction just happens again, but its the price the cell needs to pay to keep glycolysis in check.
