Gluconeogenesis

Question 1

Which THREE enzymes (and steps) of glycolysis are bypassed in gluconeogenesis?

And why?

Answer 1

• Step 1: hexokinase
• Step 3:
  phosphofructokinase
• Step 10: pyruvate kinase

These three steps are the key driving force reactions of glycolysis, and cannot be easily reversed like some of the others.

Question 2

Define:

Gluconeogenesis

Answer 2

The anabolic pathway that synthesises glucose from certain non-carbohydrate but carbon-based metabolites.

Question 3

Where does gluconeogenesis mostly occur?

Answer 3

• Liver (~90%)
• Kidneys (~10%)

Question 4

List:

FOUR key substrate categories of gluconeogenesis in animals.

Answer 4

• Lactate
• Amino acids (barring Lys & Leu)
• Glycerol
• TCA cycle intermediates

Fatty acids do NOT directly contribute to gluconeogenesis, but even-numbered FAs yield acetyl-CoA which can feed into fatty acid oxidation, and the glycerol from a triacylglyceride may be converted to DHAP.

Question 5

What does FODMAP stand for?

Answer 5

• Fermentable
• Oligosaccharides
• Disaccharides
• Monosaccharides
• And
• Polyols

Question 6

How do plants and fungi differ from animals in regards to fatty acids and gluconeogenesis?

Answer 6

They can utilise FAs in the glyoxylate cycle (shunt), which bypasses the CO₂ generating steps of the TCA cycle.

In plants, glyoxysomes are the sites for this.

This essentially ‘short-circuits’ the catabolic network and shunts it back to an anabolic one.

Question 7

Why DON’T you have to bypass some steps of glycolysis (i.e. NOT 1, 3, & 10) during gluconeogenesis?

Answer 7

These steps are close to equillibrium, and so do NOT have a highly negative ΔG / are not highly exergonic like steps 1, 3, & 10.

In other words they are readily reversible.

Question 8

How many total reactions does it take to bypass steps 1, 3, & 10 of glycolysis during the gluconeogenesis pathway?

Answer 8

4 reactions

(These are bypasses IA, IB, II, & III)

Question 9

In order to make gluconeogenesis exergonic overall, what is the overall ‘cost’?

Answer 9

• 4ATP
• 2GTP
• 2NADH

Question 10

Identify:

The enzyme, substrate, end product, and any other important byproducts/reactants of Bypass IA during gluconeogenesis.

Answer 10

• Pyruvate decarboxylase
• ATP and bicarbonate are utilised to convert pyruvate into oxaloacetate.

This gives a 4C non-phosphorylated product.

Biotin (vitamin B₇) is a cofactor.

Question 11

Identify:

The enzyme, substrate, end product, and any other important byproducts/reactants of Bypass IB during gluconeogenesis.

Answer 11

• Phosphoenolpyruvate carboxykinase (PEP carboxykinase)
• Oxaloacetate is converted to phosphoenolpyruvate (PEP).
• This is coupled with GTP hydrolysis.

This generates CO₂ as a byproduct.

Question 12

Identify:

The enzyme, substrate, end product, and any other important byproducts/reactants of Bypass II during gluconeogenesis.

Answer 12

• Fructose-1,6-bisphosphatase
• Hydrolysis of phosphate ester link in fructose-1,6-bisphosphate to produce fructose-6-phosphate.

Note: NO ATP is produced though.

Reciprocal regulation compared to phosphofructokinase.

Question 13

Identify:

The enzyme, substrate, end product, and any other important byproducts/reactants of Bypass III during gluconeogenesis.

Answer 13

• Glucose-6-phosphatase
• Dephosphorylation of glucose-6-phosphate to form glucose.

This occurs in the ER lumen and then allows it to be transported out of the cell.

G-6-phosphatase is found only in the liver and kidneys.

Question 14

What is an allosteric activator of pyruvate carboxylase?

Answer 14

Acetyl-CoA

This is the oppposite of pyruvate kinase, which is in turn allosterically inhibited by acteyl-CoA.

Question 15

What is a general rule for allosteric regulation of glycolysis and gluconeogenesis?

Answer 15

Molecules that activate one, tend to inhibit the other.

As they are essentially opposite processes of each other.

Question 16

Where is pyruvate carboxylase located and why?

Answer 16

In the mitochondria, as this is where pyruvate is transported at the end of glycolysis.

Question 17

What are the THREE main potential fates of pyruvate once it has entered the mitochondria after glycolysis?

Answer 17

• Converted to acetyl-CoA for the TCA cycle OR fatty acid synthesis.
• Converted to oxaloacetate for gluconeogenesis.

Question 18

What must happen to the oxaloacetate produced in the mitochondria in order for gluconeogenesis to occur?

Answer 18

It must be reduced to malate, transported to the cytoplasm, and then converted back to oxaloacetate.

Question 19

Why is oxaloacetate transported to the cytoplasm during gluconeogenesis?

Answer 19

Many of the enzymes used in this pathway are the same as glycolysis, and so are located in the cytoplasm.

Question 20

Which step of gluconeogenesis burns ATP?

Answer 20

Step 4 (or 7 of glycolysis)

This is the opposite reaction of the one during glycolysis, which generates and ATP via phosphoglycerate kinase.

Question 21

Which step of gluconeogenesis consumes/oxidises NADH?

Answer 21

Step 5 (or 6 of glycolysis).

This is done to reduce the substrate.

Question 22

What are TWO allosteric inhibitors of fructose-1,6-bisphosphatase?

(Hint: from bypass II of gluconeogenesis).

Answer 22

• AMP
• Fructose-2,6-bisphosphate (strong inhibitor)

Both of these indicate low energy and build up of glycolytic substrates.

Question 23

Where is glucose-6-phosphatase located?

Answer 23

In the ER lumen of liver and kidney cells.

It is the enzyme that catalyses the final important bypass step of gluconeogenesis