Bio 26 Glycolysis

Question 1

What are the three major priorities for glucose in the liver in the fed state?

Answer 1

1) Glycolysis, TCA, ETC (to meet the energy needs of the cell)
2) Glycogenesis (to store energy)
3) Lipogenesis (Fatty acid synthesis & TAG Synthesis)

Question 2

What is beta oxidation?

Answer 2

The breakdown of fatty acids into Acetyl-CoA and NADH and FADH2.

Question 3

What are the general pathways that are active during the short term fasting stage?

Answer 3

1) Glycogenolysis (makes glucose for export)
2) Gluconeogenesis (makes glucose for export)
3) Beta-Oxidation to make Acetyl-CoA which makes Ketones for export
4) TCA and ETC are always active

Question 4

In the fed state, which tissues use glycolysis?

Answer 4

When fed, all tissues use glycolysis. Glycolysis is used for energy production and to produce substrates for the TCA cycle for cells with mitochondria.

Question 5

In the fasting state, what is the energy source for liver, muscle, brain, and RBC cells?

Answer 5

Brain, neurons, RBC’s rely on glycolysis always, so they use glucose as fuel even in the fasting state. This glucose comes from liver glycogenolysis and gluconeogenesis.

The liver and muscles use fatty acid oxidation as their fuels.

Question 6

Describe GLUT-1&3

Answer 6

Both are found in the brain and in other cells, only GLUT-1 is in RBC’s.
Both have a Km=1.0mM (18mg/dL) so they have a high affinity for glucose and will uptake it at all times.

Question 7

What are the blood glucose ranges for fed, short and long term fasting?

Answer 7

Fed: 10-20 mM (180-360mg/dL)
Short Fast: 5-8 mM (90-140mg/dL)
Long Fast: 3-5 mM (70-100mg/dL)

Question 8

Describe GLUT-2

Answer 8

Found in liver and pancreatic beta cells
Km=15-20mM (highest Km of the glut transporters)
GLUT-2 as well as glucokinase in the pancreas act as glucose sensors for the release of insulin

Question 9

Describe GLUT-4

Answer 9

Found in muscle and adipose tissues
Insulin dependent
Km=2.5-5mM

Question 10

What are the overall products of aerobic glycolysis?

Answer 10

2 ATP
2 NADH
2 Pyruvate

Question 11

When is ATP consumed in glycolysis?

Answer 11

Step 1) glucose–>G6P (hexokinase or glucokinase)

Step 3) F6P–>F1,6bP (PFK-1)

Question 12

When is ATP produced during glycolysis?

Answer 12

Step 7) production of 3-Phosphoglycerate
Step 10) production of Pyruvate (pyruvate kinase)

These are called substrate level phosphorylation.

Question 13

What are the products of anaerobic glycolysis and when is this pathway used?

Answer 13

Used in cells without mitochondria or in periods when deprived of oxygen.
Produces 2 ATP and 2 Lactate, but consumes 2 NADH.

Question 14

Describe the reaction for the production of Lactate.

Answer 14

Pyruvate is converted to Lactate with lactate dehydrogenase and the use of one NADH. Produces one ATP.

Question 15

What are the three irreversible steps in glycolysis and therefore the rate limiting steps, and what enzymes are involved?

Answer 15

Steps 1, 3, 10
Production of G6P, F1,6bP, and Pyruvate
Enzymes are Glucokinase, PFK-1, Pyruvate Kinase

PFK-1 is the rate limiting enzyme for glycolysis as a whole. The other two act as points of regulation.

Question 16

Where are hexokinase and glucokinase active, and what is the difference between the two?

Answer 16

Hexokinase is in most tissues, glucokinase is in the liver and pancreatic beta cells.

Hexokinase has low Km and low Vmax (Vmax is reached even at fasting glucose levels), and is inhibited by its product G6P.
Glucokinase has high Km and a high Vmax, and is NOT inhibited by G6P.

Question 17

How would a glucokinase deficiency affect the insulin levels in the blood?

Answer 17

GLUT-2 and glucokinase in Beta-cells act as a sensor for blood glucose. The higher the blood glucose, the higher the cytosol glucose in Beta-cells will be. This in turn causes the Beta-cells to release insulin into the blood. A deficiency in glucokinase would result in hyperglycemia because insulin would never be released.

Question 18

How is PFK-1 regulated?

Answer 18

It is allosterically regulated.

Inhibition: High cytosolic ATP, High cytosolic citrate
Activation: High AMP, High F2,6bP

Question 19

How do insulin and glucagon signal the cell?

Answer 19

Insulin: RTK pathway that favors dephosphorylation
Glucagon: G-coupled receptor that increases cAMP and PKA activity and favors phosphorylation

Question 20

Describe the function of F2,6bP.

Answer 20

Strongest activator of PFK-1
Is active when dephosphorylated
Controlled by Insulin/Glucagon ratio
High I/G ratio dephosphorylates PFK-2 which makes F2,6bP from F6P. F2,6bP activates PFK-1 which activates glycolysis and inhibits FBPase-1 which is the rate-limiting enzyme for gluconeogenesis.

Question 21

What would be the result of an absence of F2,6-bisP?

Answer 21

PFK-1 ( the rate limiting step of glycolysis) would not be as active and FBPase-1 (rate limiting step for gluconeogenesis) would not be inhibited.
So, glycolysis would slow, gluconeogenesis would increase.

Question 22

Why are patients asked to be on a high carb diet several days before a glucose tolerance test?

Answer 22

High I/G ratio stimulates the production of glycolytic proteins such as those needed for glycolysis. Examples are hexokinase, glucokinase, PFK-1, and pyruvate kinase.
A low carb diet would result in a decrease of these proteins being produced and the body’s ability to process high blood glucose levels would decrease and make the patient look like a diabetic when in reality they just aren’t used to a high carb diet.

Question 23

How is pyruvate kinase regulated?

Answer 23

It is allosterically activated by F1,6-bisP through feed-forward activation.
It is inhibited by phosphorylation and activated by dephosphorylation. So, high I/G=active, low I/G=inactive.

Question 24

When oxygen in the cell is low, the ETC cannot run. This causes a build up of NADH which is a product of glycolysis and will slow down glycolysis. What is one way that NADH can be used and NAD+ produced so that glycolysis can continue?

Answer 24

Pyruvate is reduced by lactate dehydrogenase and NADH to produce Lactate, ATP, and NAD+.
The NAD+ is used to allow glycolysis to continue.

Question 25

Can NADH cross the mitochondrial membrane?

Answer 25

NO

Question 26

How does the energy from NADH get into the mitochondria?

Answer 26

Glycerol-3P Shuttle: transfers the energy to a FAD molecule in the mitochondria to form FADH2

Malate-Aspartate Shuttle: transfers the energy to another NADH molecule in the mitochondria