Module 2 - Glycolysis Flashcards
Glycolysis serves two major functions
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.
What is the main reason for glycolysis?
it is involved in metabolizing glucose, which is a fuel used by almost all organisms.
What is the net gain of glycolysis
generates a net of 2 ATP per molecule of glucose.
Where does most of the glucose comes from that enters our cells to start glycolysis?
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
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…
What are kinases?
enzymes that phosphorylate biomolecules using ATP as the phosphate donor.
What is the first step of glycolysis?
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
Second step of glycolysis
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
Third Step of glycolysis
Fructose-6-P is phosphorylated at a second carbon to form fructose-1,6-bisphosphate
irreversible reaction
Regulatory Step
Fourth Step of glycolysis
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.
Sixth step of glycolysis
Oxidation of glyceraldehyde-3-P powers the formation of 1,3-bisphosphogycerate which has high phosphoryl-transfer activity
dehydrogenases
enzymes that catalyze redox reactions.
The fifth step of glycolysis
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.
The seventh step of hydrolysis
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
Steps 8-10 of glycolysis
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
What are mutases?
isomerases that reposition phosphate groups within a molecule
What is the final product of glycolysis?
2 pyruvate molecules
What is the net equation of glycolysis?
Glucose + 2 Pi +2 ADP + 2 NAD+
Yields
2 pyruvate + 2 ATP + 2 NADH + 2 H+ + 2 H2O
What happens to the pyruvate that is produced by glycolysis?
Pyruvate can be converted to ethanol
Pyruvate can be directly converted to lactate
Pyruvate can be converted to acetyl CoA
How is pyruvate converted to ethanol?
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
How is pyruvate converted to lactate?
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
How is pyruvate converted to acetyl CoA
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
Fructose and Galactose
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
Fructose metabolism in the liver
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
Enzyme regulation in glycolysis
enzymes that catalyze irreversible reactions are potential sites of control
In glycolysis, there are three such enzymes: hexokinase,
phosphofructokinase (Most important)
pyruvate kinase
Glycolysis in the muscle
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.
glycolysis has two roles
to degrade glucose to generate ATP.
to provide building blocks for biosynthetic processes, such as the formation of glycogen, fatty acids, and amino acids
How does the enzyme phosphofructokinase (PFK) control glycolysis in muscle?
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
How does the enzyme hexokinase control glycolysis in muscle?
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.
Why isn’t hexokinase the main point of control for glycolysis in the muscle?
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.
How does the enzyme pyruvate kinase control glycolysis in muscle?
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.
What is one of the major functions of the liver?
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.
How does the enzyme phosphofructokinase (PFK) control glycolysis in liver?
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.
how does the liver respond to high blood sugar levels?
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
How does the enzyme hexokinase control glycolysis in liver?
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
