Biochemistry - Glyoxalate Cycle

Front 1

1. What are the unique enzymatic reactions in the glyoxylate cycle? What are the names of the enzymes and describe the reactions they catalyze?

Back 1

1. Malate Synthase: Acetyl CoA + glycoaldehyde æÆ malate + CoA
Isocitrate Lyase: Isocitrate æÆ succinate + glyoxylate

The structural formulas and reactions are seen in Figure 16-20, p. 624 in Lehninger.

Front 2

2. What does the glyoxylate cycle allow the organism to do with lipids and acetyl CoA? Explain why metabolism of Acetyl CoA via the TCA cycle cannot give a net synthesis of glucose?

Back 2

The glyoxylate cycle allows an organism to do net gluconeogenesis with fatty acids and acetate. The fatty acids are converted to acetyl CoA which in the above reactions in answer1 can give net synthesis of a TCA cycle intermediate, succinate . Succinate is converted to oxaloacetate which in turn is converted to PEP via the PEP carboxykinase reaction. PEP via gluconeogenic reactions is then converted to glucose. In the TCA cycle condensation of acetyl CoA with oxaloacetate via citrate synthase yields citrate. However two CO2 molecules are lost in the eventual conversion of citrate to oxaloacetate via the TCA cycle reactions. Thus there is no net addition of carbons in the TCA cycle starting with acetyl CoA.

Front 3

3. What is the name of the intracellular compartment or body where the glyoxylate cycle is compartmented in plants?

Back 3

In germinating seeds there is an intracellular compartment called the glyoxysome. The glyoxysome contains the unique glyoxylate cycle enzymes, isocitrate lyase and malate synthase. The source of fatty acids which are converted to acetyl CoA in the glyoxysomes are from lipid bodies.

Front 4

4. How is the regulation of citric acid and glyoxylate cycles coordinately regulated? In your answer explain at what step in the glyoxylate cycle where inhibition by TCA cycle and glycolytic intermediates or by AMP and ADP occurs and at what step in the TCA cycle does covalent modification regulate TCA cycle activity. Also explain how TCA cycle and glycolytic intermediates or ADP and AMP regulate the covalent modification.

Back 4

4. In the glyoxylate cycle isocitrate lyase is inhibited by excess TCA cycle and glycolytic intermediates or by excess AMP and ADP.

These are signals that formation of energy (ATP) is required and the glycolytic and TCA metabolites are at high concentrations due to active glycolysis and TCA cycle activity.

These high concentrations of metabolites, particularly AMP and ADP will activate the TCA cycle by activating a protein phosphatase which would dephosphorylate isocitrate dehydrogenase (ICDH) and make it active.

In turn activation of the protein phosphatase activates TCA cycle activity.

At the same time the high concentration of metabolites, particularly AMP and ADP would inhibit a protein kinase that phosphorylates ICDH to make it inactive.

If ATP concentrations are high (meaning AMP and ADP concentrations are low) TCA cycle activity would be at a minimum and thus the TCA cycle and glycolytic intermediates are low.

Under these conditions the protein phosphatase would be low in activity and the protein kinase high in activity.

Thus, the protein kinase would phosphorylate the ICDH and make it inactive. Thus TCA cycle activity would be low and glyoxylate cycle activity high.

Front 5

The glyoxalate cycle produces what from acetate.

Back 5

4-carbon compounds

Front 6

Vertebrates cannot convert fatty acids or acetate to
carbohydrates. Thus acetate cannot serve as?

Back 6

a precursor for glucose.

Front 7

Net Reaction of the Glyoxalate Cycle is?

Back 7

2 Acetyl CoA + NAD+ ===> succinate + 2 CoA + NADH + H+

Front 8

In plants and some bacteria, acetate can provide
net carbons for gluconeogenesis.

This is done by?

Back 8

the glyoxalate cycle

Front 9

In plants and some bacteria, acetate can provide net carbons for?

Back 9

gluconeogenesis.

Front 10

The 2 unique reactions of the Glyoxalate Cycle are?

Back 10

isocitrate lyase and malate synthase.

Front 11

In coordinated regulation of glyoxylate and citric acid cycles, when isocitrate dehydrogenase (ICDH) is inactivated by phosphorylation by a protein kinase, What happens to isocitrate?

Back 11

isocitrate is shunted to the glyoxylate cycle

Front 12

In coordinated regulation of glyoxylate and citric acid cycles, regulation of isocitrate dehydrogenase (ICDH) activity determines?

Back 12

partitioning of isocitrate between the 2 cycles.

Front 13

In coordinated regulation of glyoxylate and citric acid cycles, when the inactive ICDH is dephosphorylated and activated by the protein phosphatase, What happens to isocitrate?

Back 13

isocitrate then is metabolized by the TCA cycle.

Front 14

What does the glyoxylate cycle allow the organism to do with lipids and acetyl CoA? (short answer)

Back 14

The glyoxylate cycle allows an organism to do net gluconeogenesis with fatty acids and acetate.

Front 15

How does the glyoxylate cycle allows an organism to do net gluconeogenesis with fatty acids and acetate?

Back 15

The fatty acids are converted to acetyl CoA which in the above reactions in answer1 can give net synthesis of a TCA cycle intermediate, succinate . Succinate is converted to oxaloacetate which in turn is converted to PEP via the PEP carboxykinase reaction. PEP via gluconeogenic reactions is then converted to glucose.

Front 16

How does the glyoxylate cycle allows an organism to do net gluconeogenesis with fatty acids and acetate? (short answer)

Back 16

fatty acids ~~> acetyl CoA ~~> Succinate ~~> Oxaloacetate ~~> PEP ~~> glucose

Front 17

What are the steps in the glyoxylate cycle that allows an organism to do net gluconeogenesis with fatty acids and acetate?

Back 17

fatty acids are converted to acetyl CoA

acetyl CoA can give net synthesis of a TCA cycle intermediate, succinate .

Succinate is converted to oxaloacetate

oxaloacetate is converted to PEP via the PEP carboxykinase reaction.

PEP is then converted to glucose via gluconeogenic reactions

Front 18

In the glyoxylate cycle, oxaloacetate is converted to PEP via?

Back 18

the PEP carboxykinase reaction.

Front 19

In the glyoxylate cycle, PEP is then converted to glucose via?

Back 19

gluconeogenic reactions

Front 20

In the TCA cycle condensation of acetyl CoA with oxaloacetate via citrate synthase yields?

Back 20

citrate.

Front 21

In the TCA cycle condensation of acetyl CoA with oxaloacetate from the glyoxykinase happens via what pathway to yields citrate.

Back 21

citrate synthase

Front 22

What type of molecules are lost in the eventual conversion of citrate to oxaloacetate via the TCA cycle reactions.

Back 22

two CO2

Front 23

two CO2 molecules are lost in the eventual conversion of citrate to oxaloacetate via?

Back 23

the TCA cycle reactions.

Front 24

How many addition net of carbons in the TCA cycle starting with acetyl CoA.

Back 24

no net addition of carbons

Front 25

What is the name of where the glyoxylate cycle is compartmented in plants?

Back 25

In germinating seeds there is an intracellular compartment called the glyoxysome.

Front 26

The glyoxysome contains the unique glyoxylate cycle enzymes, _____ & ______

Back 26

isocitrate lyase and malate synthase.

Front 27

The source of fatty acids which are converted to acetyl CoA in the glyoxysomes are from?

Back 27

lipid bodies.

Front 28

How is the regulation of citric acid and glyoxylate cycles coordinately regulated?

Back 28

Regulation of isocitrate dehydrogenase (ICDH) activity determines partitioning of isocitrate between the 2 cycles.

Front 29

When the inactive ICDH is dephosphorylated and activate by the protein phosphatase, isocitrate then is metabolized by the TCA cycle. When ICDH is inactivated by phosphorylation by a protein kinase, isocitrate is?

Back 29

shunted to the glyoxylate cycle.

Front 30

Regulation of isocitrate dehydrogenase (ICDH) activity determines partitioning of isocitrate between the 2 cycles. When the inactive ICDH is dephosphorylated and activate by the protein phosphatase, isocitrate is?

Back 30

metabolized by the TCA cycle.

Front 31

Introducing extra carbons into the citric acid cycle as Acetyl-CoA is futile since the two carbons are eventually lost as CO2. How would we add extra carbons into Citric acid?

Back 31

Find a way to introduce extra carbons to the citric acid cycle AFTER CO2 release

Front 32

Extra Acetyl CoA turns into _______ and is added to TCA

Back 32

succunate

Front 33

Differences btwn Glyoxylate and TCA?

Back 33

2 Acetyl-CoA loss to TCA as CO2

No net loss of Acetyl-CoA in glyoxylate

Front 34

The real function for glyoxylate cycle?

Back 34

To produce extra succinate

Front 35

What is added to the glyoxylate cycle?

Back 35

2 Acetyl-CoA

Front 36

2 carbons are added to TCA in the form of ?

Back 36

Acetyl-CoA

Front 37

What are the 5 steps in the Glyoxylate cycle?

Back 37

Acetyl-CoA ~~> citrate by Citrate synthase

Citrate ~~> Isocitrate by Aconitase

Isocitrate ~~> Glyoxylate by isocitrate lyase

- Succinate is formed and enters TCA

Glyoxylate ~~> Malate by Malate Sythase

Malate ~~> Oxaloacetate by Malate dehydrogenese

Front 38

In the glyoxylate cycle, Acetyl-CoA ~~> citrate by?

Back 38

Citrate synthase

Front 39

In the glyoxylate cycle, Citrate ~~> Isocitrate by?

Back 39

Aconitase

Front 40

In the glyoxylate cycle, Isocitrate ~> Glyoxylate by?

Back 40

isocitrate lyase

Front 41

In the glyoxylate cycle, Glyoxylate ~~> Malate by?

Back 41

Malate Sythase

Front 42

In the glyoxylate cycle, Malate ~~> Oxaloacetate by?

Back 42

Malate dehydrogenese

Front 43

What step of the Glyoxylate Cycle is the Citrate synthase catalyst involved with?

Back 43

Acetyl-CoA ~~> citrate

Front 44

What step of the Glyoxylate Cycle is the Aconitase catalyst involved with?

Back 44

Citrate ~~> Isocitrate

Front 45

What step of the Glyoxylate Cycle is the isocitrate lyase catalyst involved with?

Back 45

Isocitrate ~~> Glyoxylate

Front 46

What step of the Glyoxylate Cycle is the Malate Sythase catalyst involved with?

Back 46

Glyoxylate ~~> Malate

Front 47

What step of the Glyoxylate Cycle is the Malate dehydrogenese catalyst involved with?

Back 47

Malate ~~> Oxaloacetate