19 - Regulation of Glycolysis

Question 1

What are the three potential regulatory steps during glycolysis?

Answer 1

1. hexokinase conversion of glucose to glucose-6-phosphate
2. phosphofructokinase (pfk-1) conversion of fructose-6-phosphate to fructose-1,6-bisphosphate
3. Pyruvate kinase conversion of phosphoenol pyruvate (PEP) to pyruvate

Question 2

What is free energy like during glycolysis?

Answer 2

Change in free energy is always negative or zero in glycolysis under physiological conditions. Under standard conditions, free-energy changes are up and down (shows how physiological conditions make glycolysis a spontaneous reaction by providing a certain type of environment)

Question 3

Why is hexokinase not the committed step in glycolysis?

Answer 3

Because the resulting glucose-6-phosphate can be used in other pathways, such as the pentose phosphate pathway and gluconeogenesis.

Question 4

What is phosphorylation of glucose for?

Answer 4

Phosphorylation to glucose-6-phosphate is to provide a prerequisite for glycolysis and retention in the cell

Question 5

Hexokinase is inhibited by what?

Answer 5

glucose-6-phosphate

Question 6

Where is hexokinase found?

Answer 6

In most tissue

Question 7

Where is glucokinase found?

Answer 7

liver and beta cells of liver, gut and brain

Question 8

What does glucokinase do?

Answer 8

It phosphorylates glucose to G6P, like hexokinases, but it has a lower binding affinity for glucose. It serves to detect glucose after a meal or any other cause for a raise in blood sugar levels. It makes up for its low binding affinity by not being inhibited by G6P. It uses only glucose, where hexokinase can use different hexoses beside glucose.

Question 9

What inhibits phosphofructokinase-1? What activates it? (hint, Pfk-1 catalyzes conversion of fructose-6-phosphate to fructose-1,6-bisphosphate in glycolysis)

Answer 9

Inhibits

• ATP (because there is sufficient energy there is no need for glycolysis)
• Citrate (allosteric) indicates that there is already a lot of acetyl-CoA entering TCA cycle

Activates

• AMP (indicates low energy, even a little AMP can overcome the inhibition of ATP
• Fructose-6-phosphate (substrate activation)
• Fructose-2,6-bisphosphate activates phosphofructokinase-1 allosterically, it is made by Pfk-2, which is also regualted by phosphorylation (also important in regulation of gluconeogenesis by inhibiting FBPase-1)

Question 10

What two molecules regulate glycolytic rate by existing in a ratio to each other which inhibits or activates phosphofructokinase-1?

Answer 10

ATP (inhibits) and AMP (activates)

Question 11

What does Pfk-1 do?

Answer 11

Phosphorylates at carbon 1 of fructose-6-phosphate to produce the glycolytic intermediate fructose-1,6-phosphate

Question 12

What does Pfk-2 do?

Answer 12

Phosphorylates at carbon 2 of fructose-6-phosphate to produce the allosteric activator of Pfk-1 and inhibitor of FBPase-1, fructose-2,6-phosphate.

Question 13

How is Pfk-2 regulated?

Answer 13

Through enzyme modification under hormonal control.

• Glucagon stimulates phosphorylation of Pfk-2 (active) to Pfk2-phospho (inactive) with protein kinase A
• Insulin stimulates the dephosphorylation of Pkf-2-phospho (inactive) to Pfk-2 (active) with phosphoprotein phosphatase

Question 14

What allows the coordination of glycolysis with whole body glucose homeostasis?

Answer 14

Hormonal control with glucagon and insulin

Question 15

How is pyruvate kinase regulated? (hint, drives the conversion of phosphoenolpyruvate (PEP) to pyruvate)

Answer 15

• Allosteric product inhibition by ATP and acetyl-CoA
• Feedforward activation by fructose-1,6-bisphosphate ensures that glycolysis runs through to pyruvate
• Regulation through phosphorylation

Question 16

Is the last regulatory step of glycolysis (pyruvate kinase making pyruvate) reversible?

Answer 16

No, it’s an irreversible reaction. The only way to convert pyruvate back to phosphoenolpyruvate is by phosphoenolpyruvate carboxykinase (PEPCK)

Question 17

Which regulatory point of glycolysis is regulated by product inhibition?

Answer 17

Hexokinase is inhibited by glucose-6-phosphate (glucokinase is not however).

Question 18

What regulation point of glycolysis is activated by feedforward activation?

Answer 18

pyruvate kinase is feedforward activated by fructose-1,6-bisphosphate

Question 19

What two regulatory steps are regulated by feedback inhibition?

Answer 19

phosphofructokinase-1- and pyruvate kinase are inhibited by ATP

Question 20

In most tissue (excluding the liver) how is fructose metabolized?

Answer 20

Hexokinase phosphorylates fructose to fructose-6-phosphate (a glycolytic intermediate).

Question 21

How is fructose metabolized in the liver?

Answer 21

Glucokinase cannot phosphorylate fructose. Fructose is phosphorylated by 1-fructokinase to fructose-1-phosphate (not a glycolytic intermediate).

Fructose-1-phosphate can be converted to glyceraldehyde then glyceraldehyde-3-phosphate. Glyceraldehyde-3-phosphate is a glycolytic intermediate.

Glyceraldehyde-3-phosphate enters glycolysis after main regulatory step

There is no feedback regulation of fructose metabolism

Question 22

How is galactose metabolized?

Answer 22

Galactose is converted in several steps into glucose-1-phosphate and to then glucose-6-phosphate, which enters glycolysis.

Question 23

Why is too much fructose not good?

Answer 23

The liver glucose transporters also transport fructose. Fructokinase phosphorylates fructose very rapidly. Most fructose is metabolized in the liver and the liver can get overwhelmed with fructose breakdown products, which can enter fat synthesis pathways.

Question 24

What is the main difference in enzymatic kinetics of hexokinase and glucokinase?

Answer 24

Glucokinase is slow to start, but not easily overwhelmed. Hexokinase works even at low concentrations of glucose, but is easily maxed out (Vmax).

Question 25

Because of the high capacity of glucokinase, the liver can use all excess glucose and do what with it?

Answer 25

Store it as glycogen or turn it into fat.

Question 26

How can glucokinase be used as a glucose sensor in pancreatic beta cells, which secrete insulin?

Answer 26

In the range of normal blood glucose, glucokinase activity increases nearly linearly with glucose levels.

Question 27

What is the main regulatory step (committed) step of glycolysis?

Answer 27

Through phosphofructokinase (Pfk-1)’s conversion of fructose-6-phosphate to fructose-1,6-phosphate