L17_1B BioChemistry 2014

8/9/2019 L17_1B BioChemistry 2014

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Monday, Nov. 4, 2013

Last time:Pyruvate Dehydrogenase

Today:Citric Acid Cycle

Wednesday:Electron Transport: brief reviewOxidative Phosphorylation

ATP synthaseReview (if time) of metabolicdiseases.


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Overview of the citric acid cycle

1. Hub of cellular metabolism.

2. The net reaction of the citric acid cycle is:

Acetyl CoA + 3NAD++ FAD + GDP + Pi+ 2H2O

2CO2+ 3NADH + FADH

2+ GTP + 2H++ CoA

3. Citric acid cycle occurs in mitochondria

4. Carbon atoms of 2CO2 generated in one round are not from the acetyl

group that entered in the same round.

1. TCA intermediates are precursors for other compounds.

2. 4 pairs of electrons are transferred to oxidize Acetyl CoA to CO2.


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Glucose

Pyruvate

Acetyl CoA

Citric Acid Cycle

3NADHFADH2

GTP

ATP

GlycolysisFatty acids

!-oxidation

Amino acids


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2CO2+ 3NADH + FADH2+ GTP + 2H++ CoA

3. Citric acid cycle occurs in mitochondria






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Citrate

"-ketoglutarate

Isocitrate

Succinyl-CoASuccinate

Fumarate

Malate

Oxaloacetate

NAD+

NADH+ H+

NAD+

NADH+ H+

CO2

CoASH

CoASH

GDP+ P

GTP

H2O

FAD

FADH2

CO2

NAD+

NADH+ H+ CoASH

Acetyl CoA


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Citrate

"-ketoglutarate

Isocitrate


Fumarate

Malate

Oxaloacetate

Acetyl CoA

Citratesynthase

aconitase

Isocitratedehydro-

genase

"-ketoglutarate

dehydrogenase

Succinyl CoAsynthetase

Succinatedeyhdrogenase

fumarase

Malatedeyhdrogenase


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Reaction mechanism of succinyl-CoAsynthetase another example ofsubstrate-level phosphorylation

.

What does this

tell about

His-PO3 energy?

Substrate-level phosphorylation is a type of chemical reaction in which ATP is formed bythe direct transfer of a phosphate group to ADP from a substrate or reactive intermediate.


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Citrate

"-ketoglutarate

Isocitrate


Fumarate

Malate

Oxaloacetate

Acetyl CoA

Citratesynthase

aconitase

Isocitratedehydro-

genase

"-ketoglutarate

dehydrogenase

Succinyl CoAsynthetase

Succinatedeyhdrogenase

fumarase

Malatedeyhdrogenase

The only enzyme in

this pathway that ismembraneassociated


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2CO2+ 3NADH + FADH

2+ GTP + 2H++ CoA

3. Regulation occurs at steps with largest free energy drop






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Table 17-2

Note how the citrate synthase free energy change compensates for the

malate DH energy change!


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Regulation of the Citric Acid Cycle Citrate synthase

Inhibited by citrate, NADH, succinyl CoA, ATP

Activated by ADP

Isocitrate dehydrogenase:

Inhibited by: ATP

Activated by: Ca2+ADP

"-ketoglutarate dehydrogenase

Inhibited by succinyl CoA, NADH

Activated by Ca2+

Substrate availability; product inhibition; competitive feedback inhibition;

Allosteric regulation


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Why is the citric acid cycle so effective at extracting energy from Acetyl CoA?

Because the two carbons from Acetyl CoA are fully oxidized to

CO2

Calculate the oxidation numbers of the two carbons of acetyl CoA(from the acetyl group) before and after they have gone through

the citric acid cycle.

Carbon atoms of 2CO2 generated in one round are not directly

from the acetyl group that entered in the same round.


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Citrate

"-ketoglutarate

Isocitrate

Oxaloacetate

Acetyl CoA

Citratesynthase

Isocitratedehydro-

genase

"-ketoglutarate

dehydrogenase

CO2

Blue =positionof labeled

carbonduringfirstround

Red =positionsof labeledcarbonsduring

secondround


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4. The two carbons from Acetyl CoA are fully oxidized to CO25. Citric Acid Cycle intermediates are precursors for other metabolites.



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Cataplerotic

Depletes intermediates

Anaplerotic

Builds up intermediates

Pyruvate Carboxylase


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4. The two carbons from Acetyl CoA are fully oxidized to CO25. Citric Acid Cycle intermediates are precursors for other metabolites.



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Isocitrate Dehydrgoenase

"-Ketoglutarate Dehydrogenase

Succinate Dehydrogenase

Malate Dehydrogenase

NADH

NADH

NADH

FADH2


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3


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4

Gycolysis:Glucose + 2Pi+ 2ADP + 2NAD

+

2 pyruvate + 2ATP + 2NADH + 2H++ 2H2O

Pyruvate Dehydrogenase Reaction:Pyruvate + NAD++ CoA

acetylCoA + CO2+ NADH + H+

Citric Acid Cycle:Acetyl CoA + 3NAD++ FAD + GDP + Pi+ 2H2O

2CO2

+ 3NADH + FADH2

+ GTP + 3H++ CoA


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Balance SheetATP NADH FADH2

Pyruvate to Acetyl CoA

(per mole pyruvate)

citric acid cycle

(per mole pyruvate)

Total/mole pyruvate

Total/mole glucose

Glycolysis

(per mole glucose)

Overall total/moleglucose

0 1 0

1

3

1

1 4 1

2

8

2

2 2 0

4 10 2

mitochondria

cytoplasm


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Eight of the NADHs were made in the mitochondria, but

what happens to the two NADH that were made in thecytoplasmby glycolysis?

1. The glycerophosphate shuttle2. The malate aspartate shuttle


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Glycerophosphate shuttle

(Restores NAD+for more

glycolysis)


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Malate-aspartate shuttle.This shuttle is particularly important in heart and liver. It isa shuttle for NADH that does NOT cost ATP. The malate dehydrogenase (MDH)occurs as separate enzymes in mitochondria and cytosol (cMDH & mMDH).Transamination reactions will be covered in more detail next quarter.

Transaminase

CMDH

Transaminase

MMDH

Mod. From Stryer 5/e


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Glutamic acid

"-ketoglutarate


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In reality, the Citric Acid Cycle is#

It is more like this:Not really like this:


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Key concepts of the citric acid cycle1. Hub of cellular metabolism.2. The net reaction of the citric acid cycle is:



3. Regulation occurs at steps with largest free energy drop.

4. The two carbons from Acetyl CoA are fully oxidized to CO25. TCA intermediates are precursors for other compounds.

6.

4 pairs of electrons are transferred to oxidize Acetyl CoA to CO2.7. Shuttles transfer reducing power from to the cytosol to the

mitochondrial matrix


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Documents

L17_1B BioChemistry 2014