Gluconeogenesis

Question 1

What purpose does gluconeogenesis serve?

Answer 1

Converts pyruvate and related 3 & 4 carbon compounds to glucose when supplies are low.

Question 2

T or F: Fatty acids can be used to synthesize glucose

Answer 2

F

Question 3

What makes a compound glucogenic?

Answer 3

Net synthesis. Conservation of all carbons in a metabolic pathway.

Question 4

What cellular compartments are involved in gluconeogenesis?

Answer 4

Mitochondrial
Cytosolic
ER (lumenal side)

Question 5

What is the overall reaction of gluconeogenesis?

Answer 5

2 pyruvate + 4ATP + 2GTP + NADH + 6water —-> Glucose + 4ADP + 2GDP + 6Pi + 2(NAD+) + 2(H+)

Question 6

Gluconeogenesis is basically the reversal of glycolysis with three major exceptions. What are those?

Answer 6

The three control steps in glycolysis are bypassed:

1. Pyruvate kinase bypassed by Pyruvate carboxylase & PEPCK
2. PFK-1 bypassed by Fructose-1,6-bisphosphatase
3. Hexokinase bypassed by Glucose-6-phosphatase

Question 7

What are the 4 unique reactions of gluconeogenesis?

Answer 7

Pyruvate carboxylase
PEPCK (phosphoenolpyruvate carboxykinase)
Fructose-1,6-bisphosphatase
Glucose-6-phosphatase

Question 8

What role does biotin play in pyruvate carboxylase?

Answer 8

Pyruvate carboxylase converts pyruvate to oxaloacetate during gluconeogenesis where biotin, a common CO2 carrier, acts as a prosthetic group.

Question 9

Pyruvate carboxylase activity requires what?

Answer 9

AcCoA which comes from pyruvate

Question 10

How is OAA (oxaloacetate) moved into the cytosol during gluconeogenesis?

Answer 10

Malate dehydrogenase

Question 11

Describe the rolls of the two malate dehydrogenase isozymes.

Answer 11

Mitochondrial malate dehydrogenase uses NADH to reduce OAA to malate so it can be transported into the cytosol, then cytosolic malate dehydrogenase reoxidizes malate to get OAA and NADH.

Question 12

What is another pathway to move OAA into the cytosol besides malate dehydrogenase?

Answer 12

Aspartate aminotransferase which is utilized in the urea cycle.
Note: It does not transfer NADH into cytosol.

Question 13

Describe the rolls of the two PEPCK isozymes?

Answer 13

The main differences are what has to be transported out.

Mitochondrial PEPCK decarboxylates oxaloacetate to PEP directly in mitochondria, then PEP is transported out.

Cytosolic PEPCK decarboxylates OAA to facilitate PEP formation. Transport of NADH to cytosol important for other reactions.

Question 14

What is the phosphoryl requirement for PEPCK?

Answer 14

GTP

Question 15

Which enzyme involved in gluconeogenesis reverses the hexokinase reaction from glycolysis?

Answer 15

Glucose-6-phosphatase

Question 16

PEPCK, Fructose-1,6-bisphosphatase and glucose-6-phosphatase all require what cofactor for activation?

Answer 16

Mg2+

Question 17

What makes a compound glucogenic?

Answer 17

Conservation of all carbons through the TCA cycle.

Anything AFTER the a-ketogluterate dehydrogenase reaction is glucogenic.

Question 18

Why is regulating glycolysis and gluconeogenesis so important?

Answer 18

To avoid futile cycles

Question 19

How does substrate availability affect regulation of gluconeogenesis and glycolysis?

Answer 19

AcCoA and pyruvate are needed to initiate/inactivate these metabolic processes.

Question 20

How does glucagon affect enzymatic activity?

Answer 20

Glucagon has a positive effect on gluconeogenic enzymes because it is a starved state hormone. This means it would have a negative effect on PFK-1 & pyruvate kinase.

Question 21

How does insulin affect enzymatic activity?

Answer 21

Insulin has a positive effect on PFK-1 & pyruvate kinase while having a negative effect on PEPCK because insulin is a “fed state” hormone.

Question 22

How is fructose-1,6-bisphosphatase allosterically regulated?

Answer 22

Positive effectors: Citrate

Negative effectors: AMP, F2,6-bisphosphate (stimulates glycolysis, but inhibits gluconeogenesis)

Question 23

Phosphorylation of fructose-6-phosphate is catalyzed by

Answer 23

PFK-2 and Fructose-2,6-bisphosphatase.

The balance of PFK-2 and F26-bisphosphatase activities determine concentration of F26-bisphosphate, all regulated by glucagon & insulin.

Question 24

What are the positive and negative effectors of pyruvate carboxylase?

Answer 24

Positive: AcCoA (apart of pyruvate)
Negative: ADP

Question 25

Provide three sources of the carbons needed to make glucose.

Answer 25

3 carbon compounds: Lactate, pyruvate, glycerol

Question 26

What is the primary difference between cytosolic and mitochondrial forms of PEPCK?

Answer 26

Mitochondrial PEPCK is utilized when lactate is the precursor for glucose synthesis & cytosolic PEPCK is utilized when pyruvate is the precursor for glucose synthesis.