Gluconeogenesis

Question 1

What are the roles of gluconeogenesis?

Answer 1

To produce glucose when blood glucose levels are low (i.e., fasting, glycogen store depletion)

Question 2

Is gluconeogenesis an anabolic or catabolic pathway?

Answer 2

Anabolic, meaning that it requires energy, is reductive, and is divergent

Question 3

What are the carbon sources for gluconeogenesis?

Answer 3

Pyruvate, lactate, glycerol, and some amino acids (glucogenic)

Question 4

Why is gluconeogensis simply NOT the reverse reactions of glycolysis?

Answer 4

1. Glycolysis itself is irreversible because some steps are irreversible
2. Need for method of regulation

Question 5

Which reactions are different in gluconeogenesis (as compared to glycolysis)?

Answer 5

Steps 1, 3, and 10 of glycolysis

Question 6

Where does gluconeogenesis primarily occur in mammals?

Answer 6

In the cytosol of hepatocytes in the liver

Question 7

What is the first bypass reaction in gluconeogenesis?

Answer 7

Pyruvate is transported into the mitochondrial matrix via pyruvate transporter and acted upon by pyruvate carboxylase with the help of biotin, its covalently attached coenzyme

Question 8

Pyruvate is produced in the cytosol, but the first bypass reaction of gluconeogensis occurs in the mitochondrial matrix. How does this occur?

Answer 8

Pyruvate moves through porins on the outer mitochondrial membrane into the mitochondrial intermembrane space and is transported across the inner mitochondrial memberane into the mitochondrial matrix via the pyruvate transporter

Question 9

Once in the mitochondrial matrix, what happens to pyruvate?

Answer 9

It is acted upon by pyruvate carboxylase and biotin, where it is converted into oxaloacetate

Question 10

The product of catalysis by pyruvate carboxylase is oxaloactetate, which must be moved back into the cytosol to continue gluconeogensis. How does the cell accomplish this?

Answer 10

Through the malate-oxaloacetate shuttle system

Question 11

How does the malate-oxaloacetate shuttle system function?

Answer 11

1. Oxaloacetate is converted to malate via malate dehydrogenase
2. Malate is transported out of the mitochondria via the malate transporter
3. Once in the cytosol, malate is converted back into oxaloacetate by malate dehydrogenase

Question 12

Malata dehydrogenase is part of what class of enzymes?

Answer 12

Oxidoreductase

Question 13

To convert oxaloacetate into malate, what coenzyme is required?

Answer 13

NADH

Question 14

To convert malate into oxaloacetate, what coenzyme is required?

Answer 14

NAD⁺

Question 15

How many active sites does biotin have?

Answer 15

Two - in one active site, biotin picks up carbon dioxide; in the other active site, biotin transfers carbon dioxide

Question 16

How is biotin covalently linked to pyruvate carboxylase?

Answer 16

Via an amide linkage at a lysine residue

Question 17

What reactant is required for pyruvate to be converted to oxaloacetate via pyruvate carboxylase?

Answer 17

Carbonate

Question 18

Is the conversion of pyruvate to oxaloacetate via pyruvate carboxylase energy-requiring or energy-producing?

Answer 18

Energy-requiring; it requires ATP

Question 19

What is the mechanism for pyruvate carboxylase?

Answer 19

1. The oxygen atom on bicarbonate acts a nucleophile, attacking the electrophilic phosphorus of the gamma phosphate on ATP, releasing ADP and a proton (from bicarbonate)
2. The resulting molecule is an “activated molecule of carbon dioxide”
3. The nitrogen atom in biotin now functions as a nucleophile and attacks the carbon atom of the carbonyl group, ultimately “kicking out” phosphate, which is a good leaving group
4. A hydrogen atom from pyruvate’s methyl group deprotonates, forming a carbanion that attacks the carbonyl carbon of carboxy-biotin, ultimately kicking out biotin and reforming the enzyme by protonating nitrogen on its way out

Question 20

Once oxaloacetate moves back into the cytosol it is acted up by which enzyme?

Answer 20

PEP carboxykinase

Question 21

Oxaloacetate is converted to _________________________ via PEP carboxykinase.

Answer 21

Phosphoenolpyruvate

Question 22

What is the mechanism of PEP carboxykinase?

Answer 22

It is a decarboxylation followe by a phosphoryl transfer (from GTP)

Question 23

What cofactor is required for PEP carboxykinase to function?

Answer 23

GTP

Question 24

What does GTP stand for?

Answer 24

Guanosine triphosphate

Question 25

What is the structure of GTP?

Answer 25

Question 26

What type of enzyme catalyzes the conversion of oxaloacetate to malate?

Answer 26

Dehydrogenase

Question 27

What cofactor is required for the dehydrogenation of oxaloacetate to malate?

Answer 27

NADH

Question 28

The conversion of oxaloacetate to malate operates under what type of mechanism?

Answer 28

Redox

Question 29

What is the mechanism for the interconversion of oxaloacetate and malate?

Answer 29

Add picture

Question 30

Identify this compound.

Answer 30

Oxaloacetate

Question 31

Identify this compound.

Answer 31

"Malate"

Question 32

What is the mechanism for the conversion of oxaloacetate to phosphoenolpyruvate?

Answer 32

Question 33

The first bypass reaction of gluconeogenesis is the conversion of pyruvate to oxaloacetate to phosphoenolpyruvate via the oxaloacetate-malate shuttle system. What is the second bypass reaction of gluconeogenesis?

Answer 33

The hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate and inorganic phosphate

Question 34

What is the mechanism for the conversion of fructose-1,6-bisphosphate to fructose-6-phosphate in gluconeogenesis?

Answer 34

Phosphoester hydrolysis

Question 35

What is the name of the enzyme that catalyzes the hydrolysis of fructose-1,6-bisphosphate to fructose-6-phosphate in gluconeogenesis?

Answer 35

Fructose-1,6-bisphosphatase

Question 36

What is the third bypass reaction of gluconeogenesis?

Answer 36

The conversion of glucose-6-phosphate to glucose via glucose-6-phosphatase

Question 37

What is the mechanism of action for glucose-6-phosphatase?

Answer 37

Phosphoester hydrolysis

Question 38

What are the last two reactions of gluconeogenesis (catalyzed by fructose-1,6-bisphosphatase and glucose-6-phosphatase) not ATP-producing reactions?

Answer 38

Because the reactants are not high-energy compounds

Question 39

What is the energetic cost of gluconeogenesis?

Answer 39

Total: 6 ATP All result from the conversion of pyruvate to oxaloacetate

Question 40

What is the first mechanisnistic step of pyruvate carboxylase?

Answer 40

The activation of carbon dioxide from bicarbonate by enzymatic addition of a phosphate group

Question 41

What does the "business end" of biotin facilitate in the pyruvate carboxylase mechanism?

Answer 41

The transfer of carbon dioxide from one active site to another via covalent bonding

Question 42

Why is a phosphate group attached to carbon dioxide in the pyruvate carboxylase mechanism?

Answer 42

Because it is functions as an appropiate leaving group, enabling biotin to "capture" the carbon dioxide

Question 43

Why is ATP needed in the pyruvate carboxylase mechanism?

Answer 43

As a source of phosphate

Question 44

What is the carrier of carbon dioxide in the pyruvate carboxylase mechanism?

Answer 44

Biotin

Question 45

Which molecule ultimately abstracts carbon dioxide from the carboxybiotin intermediate?

Answer 45

Pyruvate (C3 carbanion)

Question 46

What is the intermediate named in pyruvate carboxylase called?

Answer 46

Carboxybiotin

Question 47

Does pyruvate carboxylase facilitate the addition or removal of a carbon dioxide molecule?

Answer 47

The addition (to pyruvate)

Question 48

What chemical changes does pyruvate carboxylase catalyze? In other words, what happens to pyruvate?

Answer 48

A carbon dioxide molecule (COO-) is added to pyruvate, resulting in a four carbon molecule with an additional terminal COO- group

Question 49

What does malate dehydrogenase do to oxaloacetate?

Answer 49

Reduces the carbonyl to a hydroxyl group

Question 50

Does PEP carboxykinase facilitate a carboxylation or decarboxylation?

Answer 50

Decarboxylation

Question 51

What is the first step in the PEP carboxykinase mechanism?

Answer 51

Removal of CO2

Question 52

What is the second step in the PEP carboxykinase mechanism?

Answer 52

Phosphoryl transfer from GTP

Question 53

PEP carboxykinase catalyzes the conversion of oxaloacetate to what intermediate of glycolyis?

Answer 53

Phosphoenolpyruvate

Question 54

Describe the mechanism of PEP carboxykinase?

Answer 54

The oxygen anion on C4 kicks back and expels CO2 A carbanion forms on C3, which then kicks back, forming an alkene with C2, and the carbonyl oxygen becomes phosphorylated

Question 55

What is the second step in the pyruvate carboxylase mechanism?

Answer 55

A nitrogen atom in biotin is deprotonated and becomes a nucleophile, attacking the carbonyl in the activated CO2 molecule and expelling Pi

Question 56

What is the third step in the pyruvate carboxylase mechanism?

Answer 56

After bicarbonate has been phosphorylated and dephosphorylated, forming the carboxybiotin intermediate, C3 of pyruvate is deprotonated; the resulting carbanion attacks the carbonyl of the carboxybiotin, ultimately transfering the carboxyl group to pyruvate and restoring biotin's original configuration