Module 2 - Glycolysis

Question 1

Glycolysis serves two major functions

Answer 1

First, it generates ATP.

Second, a number of the intermediates of this pathway serve as building blocks for the biosynthesis of other biomolecules such as amino acids and fatty acids.

Question 2

What is the main reason for glycolysis?

Answer 2

it is involved in metabolizing glucose, which is a fuel used by almost all organisms.

Question 3

What is the net gain of glycolysis

Answer 3

generates a net of 2 ATP per molecule of glucose.

Question 4

Where does most of the glucose comes from that enters our cells to start glycolysis?

Answer 4

Lactose - Milk

Sucrose - Table Sugar

Maltose - a product of starch breakdown and rich in foods where fermentation by yeast occurs such as in breads and brewed beverages

Question 5

glucose doesn’t just diffuse across the cell membrane to get into the cell; rather, there are specific transporters that assist in this process. - They are…

Answer 5

Question 6

What are kinases?

Answer 6

enzymes that phosphorylate biomolecules using ATP as the phosphate donor.

Question 7

What is the first step of glycolysis?

Answer 7

phosphorylation of glucose

glucose is rapidly phosphorylated by hexokinase to form glucose-6-phosphate

This is a very important step since phosphorylation of glucose traps it in the cell and prevents it from being transported out

Question 8

Second step of glycolysis

Answer 8

Glucose-6-P is converted to fructose-6-P:

this is an isomerization reaction, meaning that there are no atoms lost, but only a rearrangement of the atoms occurs.

this is a reversible reaction

Question 9

Third Step of glycolysis

Answer 9

Fructose-6-P is phosphorylated at a second carbon to form fructose-1,6-bisphosphate

irreversible reaction

Regulatory Step

Question 10

Fourth Step of glycolysis

Answer 10

Cleavage of fructose 1,6-bisphosphate to two different 3-carbon molecules

it is important to note that glyceraldehyde-3-P is the molecule that proceeds directly onward in the glycolytic pathway.

Question 11

Sixth step of glycolysis

Answer 11

Oxidation of glyceraldehyde-3-P powers the formation of 1,3-bisphosphogycerate which has high phosphoryl-transfer activity

Question 12

dehydrogenases

Answer 12

enzymes that catalyze redox reactions.

Question 13

The fifth step of glycolysis

Answer 13

The dihydroxyacetone phosphate is not wasted or lost from the pathway, since it is readily converted to glyceraldehyde-3-P by an isomerase in a reversible reaction.

Question 14

The seventh step of hydrolysis

Answer 14

Phosphoryl transfer from 1,3-bisphosphoglycerate to ADP to form ATP

the first energy-producing step in glycolysis

Remember that there are actually two ATPs formed at this step for every molecule of glucose started with, since one glucose generates two 3-carbon molecules that flow this pathway.

ATP formed in this manner is termed substrate-level phosphorylation

Question 15

Steps 8-10 of glycolysis

Answer 15

3-phosphoglycerate is converted to 2-phosphoglycerate by phosphoglycerate mutase

2-phosphoglycerate is then converted to phosphoenolpyruvate (or PEP)

PEP is a high phosphoryl-transfer compound, and is able to donate its phosphate group to ADP in the last step of glycolysis to produce pyruvate

This is another example of substrate-level phosphorylation

Question 16

What are mutases?

Answer 16

isomerases that reposition phosphate groups within a molecule

Question 17

What is the final product of glycolysis?

Answer 17

2 pyruvate molecules

Question 18

What is the net equation of glycolysis?

Answer 18

Glucose + 2 Pi +2 ADP + 2 NAD+

Yields

2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O

Question 19

What happens to the pyruvate that is produced by glycolysis?

Answer 19

Pyruvate can be converted to ethanol

Pyruvate can be directly converted to lactate

Pyruvate can be converted to acetyl CoA

Question 20

How is pyruvate converted to ethanol?

Answer 20

This pathway occurs in some microorganisms and in yeast

important in the production of beer and wine

NAD+ is regenerated in the second step of this conversion

a process called fermentation

Question 21

How is pyruvate converted to lactate?

Answer 21

NADH is used to reduce pyruvate to lactate

occurs in muscle when oxygen supply becomes limiting

As lactate levels build up in the muscle, muscle fatigue sets in

the lactate lowers the pH of the muscle to as low as 6.3 in extreme exercise, which inhibits phosphofructokinase, the main regulatory point of glycolysis

Question 22

How is pyruvate converted to acetyl CoA

Answer 22

this is the pathway whereby the most energy is obtained from pyruvate, since acetyl CoA is the entry point into the citric acid cycle and the electron transport chain, which oxidizes glucose all the way to CO2 and H2O in an aerobic manner

Question 23

Fructose and Galactose

Answer 23

Fructose is a component of sucrose, or table sugar, as well as in corn syrup which is used as a sweetener in many foods and drinks

Galactose is a component of lactose, known as milk sugar

Both of these sugars are metabolized by glycolysis, although enter the pathway at different points

Question 24

Fructose metabolism in the liver

Answer 24

the vast majority of fructose is metabolized by the liver

In liver, fructose is first phosphorylated to fructose -1-P by fructokinase. This is then split into two 3-carbon sugars, dihydroxyacetone phosphate and glyceraldehyde by a special aldolase.

While dihydroxyacetone phosphate is an intermediate of glycolysis and thus can directly enter the pathway, glyceraldehyde has to be phosphorylated to glyceraldehyde-3-P in order for it to enter glycolysis

Question 25

Enzyme regulation in glycolysis

Answer 25

enzymes that catalyze irreversible reactions are potential sites of control

In glycolysis, there are three such enzymes: hexokinase,
phosphofructokinase (Most important)
pyruvate kinase

Question 26

Glycolysis in the muscle

Answer 26

glycolysis is primarily controlled by the energy state of the cell, which is represented by the ATP:AMP ratio. The higher the ratio, the greater the energy state and less need for glycolysis; the lower the ratio, the lower the energy state and thus more need for glycolysis.

Question 27

glycolysis has two roles

Answer 27

to degrade glucose to generate ATP.

to provide building blocks for biosynthetic processes, such as the formation of glycogen, fatty acids, and amino acids

Question 28

How does the enzyme phosphofructokinase (PFK) control glycolysis in muscle?

Answer 28

ATP binds to a site on PFK, and inhibits its catalytic activity by decreasing the enzyme’s affinity for one of its substrates, fructose-6-P

AMP competes with ATP for the same site, but when it itself is bound, does not inhibit the enzyme

a drop in pH in the muscle also inhibits PFK by enhancing the effect of ATP

Question 29

How does the enzyme hexokinase control glycolysis in muscle?

Answer 29

Hexokinase plays a lesser role than PFK

If glucose-6-P levels get too high, it means that there is less flux through glycolysis occurring and thus a signal that there is sufficient ATP available. It makes sense, therefore to slow the first step of glycolysis, which will lead to glucose buildup in the muscle, which will slow the uptake of glucose from the blood into the muscle.

Question 30

Why isn’t hexokinase the main point of control for glycolysis in the muscle?

Answer 30

You might be wondering why hexokinase isn’t the major point of control since it is the first reaction of glycolysis, and often the first step in a pathway is the one controlled. There is a very good reason for this. It turns out that the phosphorylation of glucose to glucose-6-phosphate, while being the first step in glycolysis, is not the first committed step for this pathway. That’s because glucose-6-P can enter other pathways, including the pentose phosphate pathway and glycogen synthesis.

Question 31

How does the enzyme pyruvate kinase control glycolysis in muscle?

Answer 31

It is allosterically inhibited by ATP and activated by fructose-1,6-bisP.

The inhibition by ATP makes sense from an energy state perspective, but why activation by fructose-1,6-bisP?

This metabolite is an intermediate in glycolysis, and the product of phosphofructokinase, the major rate-limiting enzyme.

A rise in fructose-1,6-bisP is a clear indication of an increased flux through the pathway, and thus it makes sense that pyruvate kinase activity would be increased to handle the increased flux that is headed in its direction.

Question 32

What is one of the major functions of the liver?

Answer 32

maintain glucose levels in the blood.

The glucose that it takes up is either stored as glycogen (a polymer of glucose);

used to generate reducing power in the form of NADPH which is used for biosynthesis (this is done by the pentose phosphate pathway);

or converted via glycolysis to molecules that serve as building blocks for the synthesis of other biomolecules.

Question 33

How does the enzyme phosphofructokinase (PFK) control glycolysis in liver?

Answer 33

Because glycolysis in liver is primarily used to provide building block molecules, signals indicating whether these building blocks are abundant or scarce are the primary signals.

Citrate is a major inhibitor of PFK in liver, since citrate is formed from acetyl CoA which in turn is a product of pyruvate metabolism.

Thus, a high level of citrate is an indicator that biosynthetic precursors are at sufficient levels, and metabolizing more glucose through glycolysis can be slowed.

Question 34

how does the liver respond to high blood sugar levels?

Answer 34

When glucose rises in the blood after a meal, the liver takes up much of this;

as a result, there is more flux through glycolysis which leads to build up of fructose-6-P levels

some of the fructose-6-P is converted to fructose-2,6-P

Fructose-2,6-bisP is an allosteric activator of PFK, which it does by increasing its affinity for one of its substrates and by blunting the inhibitory effect of ATP

Question 35

How does the enzyme hexokinase control glycolysis in liver?

Answer 35

liver has a unique isoform of hexokinase, called glucokinase

Glucokinase is unique from muscle hexokinase in two key ways

glucokinase has a 50-fold higher Km for glucose than hexokinase, which means that glucose-6-P is formed only when glucose is abundant, such as after a meal

The second unique feature is that glucokinase is not inhibited by its product, glucose-6-P. This feature also facilitates the liver to be able to use the excess glucose that it takes up from the blood so it is not wasted