Glycolysis

Question 1

Let’s remind ourselves, what is the point of glycolysis?

Answer 1

Glycolysis is the pathway we use to convert glucose to pyruvate, and it occurs in the cytosol of every cell in the body

Question 2

First of all, we need to get that glucose into the cell. How do we do it?

Answer 2

With transport proteins.

Insulin stimulates glucose transport into muscle and adipose cells by causing glucose transport proteins (GLUT4) within cells to move to the cell membrane. Doesn’t really affect other organs like the brain, RBCs, or liver.

Question 3

What is the first thing to happen to glucose in glycolysis and what enzymes are used?

Answer 3

The first thing is to turn glucose to glucose-6-phosphate.

We use the enzymes hexokinase (found in all tissues) and glucokinase (in the liver and pancreas) and we use up our first ATP.

Question 4

So we’ve made our Glucose 6 phosphate from glucose using hexokinase. Now what is the next step?

Answer 4

The next step is to turn this glucose-6-phosphate to fructose, because glucose has gotten too much attention already. Fructose isn’t that much different from glucose so turning it into fructose-6-phosphate only requires phosphoglucose isomerase.

Question 5

Alright so now Fructose-6-phosphate has a chance to shine. What happens to this guy?

Answer 5

How come nothing is happening to the second half of this molecule? Let Fructose rein for another around since glucose had two shots.

Using phosphofructokinase 1, we phosphorylate Fructose-6-Phosphate to Fructose 1-6-bisphosphonate.

This step is the second to use ATP, but is the first committed step of glycolysis.

Question 6

What do you mean we committed?! What happens next to our newly formed fructose-1-6-bisphosphonate?

Answer 6

Fructose-1-6-bisphosphonate is angry that it committed against its will, and just wants to explode. It breaks down via aldolase to make triose phosphates, glyceraldehyde-3-phosphate, and DHAP.

But in reality, we actually just get two glyceraldehyde-3-phosphates cause we use triose phosphate isomerase to turn DHAP into a second glyceraldehyde-3-phosphate

Question 7

Great! We have twins! More mouths to feed! What do we do to glyceraldehyde-3-phosphate?

Answer 7

We need to start doing electron stuff. We oxidize glyceraldehyde 3 phosphate with NAD+ and react it with some floating inorganic phosphate to make 1,3-Bisphosphoglycerate and NADH using glyceraldehyde-3-phosphate dehydrogenase, which turns glyceraldehyde 3 phosphate into a carboxyllic acid which forms a high-energy anhydride with inorganic phosphate

Special electron stuff means we need a something fancy
- cysteine residue at the active site is essential

Question 8

So we did the fancy electron stuff to make 1,3-Bisphosphoglycerate. Now what?

Answer 8

After the electron makeover, 1,3-Bisphosphoglycerate feels fat and wants to slim down, so with the help of phosphoglycerate kinase, it reacts with some leftover ADP to make 3-phosphoglycerate and ATP

Question 9

What happens with our new slimmed down 3-phosphoglycerate?

Answer 9

Time to take off some phosphates:

1. Phosphoglyceromutase to turn 3 to 2
2. Enolase to dehydrate the 2-Phosphoglycerate to Phosphoenolpyruvate (PEP) which has a lot of phosphate energy

Question 10

So now we have this super reactive PEP, what do we do with it?

Answer 10

It reacts with ADP to make pyruvate and ATP as the final reaction using pyruvate kinase

Question 11

What does a deficiency in pyruvate kinase lead to?

Answer 11

This means we get fewer ATP from glycolysis, so RBCs have less ATP for their pumps, causing a hemolytic anemia. Glycolysis goes backwards a little since it can’t finish and we get a buildup of 2,3-bisphosphoglycerate in the red blood cells, which promotes oxygen release from the cells in a greater extent than usual (decreases affinity of hemoglobin for oxygen)

Question 12

When is hexokinase most active?

Answer 12

It gets inhibited by its product, glucose-6-phosphate, so it works best when glucose-6-phosphate is being readily utilized.

Question 13

Glucokinase is also important for our first step in glycolysis and gets used only in the liver and pancreas. When is it most active?

Answer 13

Only after a meal, when insulin levels are high.

It is NOT inhibited by its product, but it is regulated by a glucokinase sequester protein that holds it in the nucleus until glucose levels go up.

Question 14

Discuss PFK-1, the enzyme that makes our Fructose 6 phosphate fatter

Answer 14

It makes it fatter because it is actually reinforced by its own product. It too is also stimulated when glucose is high and when muscles need ATP.

Question 15

Discuss the fed vs fasted state in regards to the PFK-1 step that makes our fructose fatter

Answer 15

So after a meal, technically Fructose-6-phosphate is made fatter by PFK-2, not 1. The fatter fructose stimulates the PFK-1 to wake up and start up glycolysis.

When fasting, PFK2 is phosphorylated by protein kinase A (activated by cAMP). This new evil phosphorylated form turns our fat fructose back to fructose-6-phosphate, and since there is less fat fructose, PFK-1 levels drop back down.

In the fed state, insulin is floating around causing phosphatases to dephosphorylate our PFK-2 to cause it to be more ready to make fat fructose and thus stimulate PFK-1

Question 16

How does muscle regulate this insanely emotional PFK-1 who keeps getting yanked around?

Answer 16

Activated by AMP in muscle when it needs ATP, which is why during exercise, AMP levels go up (also, we are using the ATP and turning it to AMP).

But as we make more ATP (and more citrate), PFK-1 gets inhibited to keep things in check. (The reason we say citrate is because a high amount of citrate means we are sending more than enough product to the TCA cycle, so glycolysis can slow down)

Question 17

Pyruvate kinase, our lovely enzyme at the end of glycolysis that we need to make pyruvate and ATP, is activated and inhibited by what?

Answer 17

Activated by fructose1-6-bisphosphonate and inhibited by alanine and the liver during fasting via phosphorylation.

This guy is activated by the fed state

Question 18

What are the fates of pyruvate?

Answer 18

1. Convert it to lactate in order to turn NADH to NAD+ using lactate dehydrogenase. This is particularly important under anaerobic conditions
2. Convert to acetyl-CoA - Pyruvate can enter the mitochondria and be converted by pyruvate dehydrogenase to acetyl-CoA which can then enter the TCA cycle.
3. Convert to oxaloacetate - Via pyruvate carboxylase to replenish the TCA cycle
4. Convert it to Alanine

Question 19

Quick overview of what happens when one glucose molecule goes into glycolysis

Answer 19

One glucose gets you two moles of pyruvate and 4 ATPs although 2 get used so only a net gain of 2 ATP.

We also make 2 cytosolic NADH

Question 20

What does converting glucose to lactate do for us?

Answer 20

If NADH from glycolysis is used to reduce pyruvate to lactate the net gain is 2 moles of ATP per mole of glucose converted to lactate.

Question 21

If we go the route of turning Glucose to pyruvate and then the acetyl CoA route (recall we can turn pyruvate to alanine, oxaloacetate, lactate, or acetyl CoA), what ATP do we gain and what happens next?

Answer 21

The glucose we turned to pyruvate, we generate the 2 NADH which with shuttles gives us 3 - 5 ATP in addition to the 2 ATP we gain from glycolysis itself.

The 2 pyruvate we make via pyruvate dehydrogenase gets turned to 2 Acetyl CoA, in the process generating to more 2 NADH, which will turn to 5 ATP.

The 2 Acetyl CoA go to the TCA cycle to make 4 CO2, 6NADH, 2 FADH2 and 2GTP which will translate to another 20 ATP.

This gives us a total of 30 - 32 ATP

Question 22

Back when we turned glucose to pyruvate, we generated 2 NADH that then “got shuttled” to give us 3 - 5 ATP.

What two shuttles do we use and why?

Answer 22

NADH can’t cross the mitochondrial membrane to participate in the ETC. We use two shuttles to give us the ATP right at home.

Glycerol phosphate shuttle

• NADH turns DHAP to glycerol-3-phosphate
• Glycerol-3-phosphate reacts with FAD linked dehydrogenase to make FADH2 and regenerate DHAP
• Oxidative phosphorylation turns FADH2 to 1.5 moles of ATP

Malate aspartate shuttle

• NADH turns oxaloacetate to malate via malate dehydrogenase
• Malate enters the mitochondrion and is turned back to oxaloacetate my mitochondrial malate dehydrogenase, generating NADH in the matrix
• Each NADH makes about 2.5 moles of ATP, so 5 in total