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Biochemistry II > Gluconeogenesis > Flashcards

Gluconeogenesis Flashcards

1
Q

What are the roles of gluconeogenesis?

A

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

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2
Q

Is gluconeogenesis an anabolic or catabolic pathway?

A

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

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3
Q

What are the carbon sources for gluconeogenesis?

A

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

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4
Q

Why is gluconeogensis simply NOT the reverse reactions of glycolysis?

A
  1. Glycolysis itself is irreversible because some steps are irreversible
  2. Need for method of regulation
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5
Q

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

A

Steps 1, 3, and 10 of glycolysis

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6
Q

Where does gluconeogenesis primarily occur in mammals?

A

In the cytosol of hepatocytes in the liver

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7
Q

What is the first bypass reaction in gluconeogenesis?

A

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

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8
Q

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

A

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

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9
Q

Once in the mitochondrial matrix, what happens to pyruvate?

A

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

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10
Q

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?

A

Through the malate-oxaloacetate shuttle system

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11
Q

How does the malate-oxaloacetate shuttle system function?

A
  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
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12
Q

Malata dehydrogenase is part of what class of enzymes?

A

Oxidoreductase

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13
Q

To convert oxaloacetate into malate, what coenzyme is required?

A

NADH

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14
Q

To convert malate into oxaloacetate, what coenzyme is required?

A

NAD+

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15
Q

How many active sites does biotin have?

A

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

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16
Q

How is biotin covalently linked to pyruvate carboxylase?

A

Via an amide linkage at a lysine residue

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17
Q

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

A

Carbonate

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18
Q

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

A

Energy-requiring; it requires ATP

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19
Q

What is the mechanism for pyruvate carboxylase?

A
  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
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20
Q

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

A

PEP carboxykinase

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21
Q

Oxaloacetate is converted to _________________________ via PEP carboxykinase.

A

Phosphoenolpyruvate

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22
Q

What is the mechanism of PEP carboxykinase?

A

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

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23
Q

What cofactor is required for PEP carboxykinase to function?

A

GTP

24
Q

What does GTP stand for?

A

Guanosine triphosphate

25
Q

What is the structure of GTP?

A
26
Q

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

A

Dehydrogenase

27
Q

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

A

NADH

28
Q

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

A

Redox

29
Q

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

A

Add picture

30
Q

Identify this compound.

A

Oxaloacetate

31
Q

Identify this compound.

A

“Malate”

32
Q

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

A
33
Q

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?

A

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

34
Q

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

A

Phosphoester hydrolysis

35
Q

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

A

Fructose-1,6-bisphosphatase

36
Q

What is the third bypass reaction of gluconeogenesis?

A

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

37
Q

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

A

Phosphoester hydrolysis

38
Q

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

A

Because the reactants are not high-energy compounds

39
Q

What is the energetic cost of gluconeogenesis?

A

Total: 6 ATP

All result from the conversion of pyruvate to oxaloacetate

40
Q

What is the first mechanisnistic step of pyruvate carboxylase?

A

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

41
Q

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

A

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

42
Q

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

A

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

43
Q

Why is ATP needed in the pyruvate carboxylase mechanism?

A

As a source of phosphate

44
Q

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

A

Biotin

45
Q

Which molecule ultimately abstracts carbon dioxide from the carboxybiotin intermediate?

A

Pyruvate (C3 carbanion)

46
Q

What is the intermediate named in pyruvate carboxylase called?

A

Carboxybiotin

47
Q

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

A

The addition (to pyruvate)

48
Q

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

A

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

49
Q

What does malate dehydrogenase do to oxaloacetate?

A

Reduces the carbonyl to a hydroxyl group

50
Q

Does PEP carboxykinase facilitate a carboxylation or decarboxylation?

A

Decarboxylation

51
Q

What is the first step in the PEP carboxykinase mechanism?

A

Removal of CO2

52
Q

What is the second step in the PEP carboxykinase mechanism?

A

Phosphoryl transfer from GTP

53
Q

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

A

Phosphoenolpyruvate

54
Q

Describe the mechanism of PEP carboxykinase?

A

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

55
Q

What is the second step in the pyruvate carboxylase mechanism?

A

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

56
Q

What is the third step in the pyruvate carboxylase mechanism?

A

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

57
Q
A

Biochemistry II (19 decks)

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