After pyruvate is oxidized, the citric acid cycle completes the energy-yielding

oxidation of organic molecules.

Chapter 9, Section 3

Oxidation of Pyruvate to Acetyl CoA In the presence of O2, pyruvate enters the

mitochondrion (in eukaryotic cells) where the oxidation of glucose is completed.

Before the citric acid cycle can begin, pyruvate must be converted to acetyl Coenzyme A (acetyl CoA), which links glycolysis to the citric acid cycle.

This step is carried out by a multienzyme complex that catalyses three reactions.

Figure 9.10

Pyruvate

Transport protein

CYTOSOL

MITOCHONDRION

CO2 Coenzyme A

NAD + HNADH Acetyl CoA

1

2

3

The Citric Acid Cycle The citric acid cycle, also called the Krebs

cycle, completes the break down of pyruvate to CO2. If acetyl-CoA is NOT stored then it enters the

Citric Acid Cycle. The cycle oxidizes organic fuel derived from

pyruvate, generating 1 ATP, 3 NADH, and 1 FADH2 per turn.

Figure 9.12-1

1

Acetyl CoA

Citrate

Citricacidcycle

CoA-SH

Oxaloacetate

Step One: A Condensation Reaction 2-C acetyl-CoA + 4-C

molecule 6-C molecule+ CoA [which can be used over and over!].

Irreversible. Inhibited by large amounts

of ATP already present.

Figure 9.12-2

1

Acetyl CoA

CitrateIsocitrate

Citricacidcycle

H2O

2

CoA-SH

Oxaloacetate

Step Two: Isomerization Hydroxyl group repositioned. Water removed from one

carbon and then added to a different carbon.

RESULT: a change in position of an −H and −OH.

Molecule is still (6-C), but the –OH has moved!

Figure 9.12-3

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

Citricacidcycle

NADH+ H

NAD

H2O

3

2

CoA-SH

CO2

Oxaloacetate

Step Three: First Oxidation Molecule undergoes oxidative

decarboxylation--fancy talk for chopping off a carbon and losing a pair of e-’s in the process.

The pair of e- reduce NAD+ to NADH.

The chopped off C becomes CO2.

Now we have a 5-C molecule.

Figure 9.12-4

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Citricacidcycle

NADH

NADH

+ H

+ H

NAD

NAD

H2O

3

2

4

CoA-SH

CO2

CoA-SH

CO2

Oxaloacetate

Step Four: Second Oxidation The 5-C molecule

undergoes oxidative decarboxylation. Releases CO2. 2 more e- reduce

another NAD+ to NADH The 4-C fragment that

is left receives a CoA group.

Figure 9.12-5

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Succinate

Citricacidcycle

NADH

NADH

ATP

+ H

+ H

NAD

NAD

H2O

ADP

GTP GDP

P i

3

2

4

5

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Step Five: Substrate Level Phosphorylation CoA leaves the 4-C

molecule. Breaking of bond

releases energy. GDP + Pi GTP [just

substitute guanine for adenine in “A” TP]. GTP ATP

Remaining 4-C molecule.

Figure 9.12-6

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Succinate

Fumarate

Citricacidcycle

NADH

NADH

FADH2

ATP

+ H

+ H

NAD

NAD

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Step Six: Third Oxidation Four carbon molecule

is oxidized. FAD+ + FOUR e- and

TWO H+ FADH2. FAD+ is an integral

part of mitochondria membrane.

FADH2 can contribute e- to ETS.

Figure 9.12-8

NADH

1

Acetyl CoA

CitrateIsocitrate

-Ketoglutarate

SuccinylCoA

Succinate

Fumarate

Malate

Citricacidcycle

NAD

NADH

NADH

FADH2

ATP

+ H

+ H

+ H

NAD

NAD

H2O

H2O

ADP

GTP GDP

P i

FAD

3

2

4

5

6

7

8

CoA-SH

CO2

CoA-SH

CoA-SH

CO2

Oxaloacetate

Step Seven and Eight: Regeneration Water is added to

oxidized 4-C molecule.

After water is added, compound becomes oxidized again (2 e- are released) NAD+ + the 2 e-

NADH Ready to start again!

Figure 9.11

Pyruvate

NAD

NADH

+ HAcetyl CoA

CO2

CoA

CoA

CoA

2 CO2

ADP + P i

FADH2

FAD

ATP

3 NADH

3 NAD

Citricacidcycle

+ 3 H

Beating a dead horse!

Substrate Level

Phosphorylation

Oxidation

Glycolysis 2 ATP 2 NADH

Oxidation of Pyruvate

-------- 2NADH

Krebs 2 ATP 6 NADH + 2 FADH2

TOTAL 4 ATP 10 NADH + 2 FADH2

Concept 9.4: During oxidative phosphorylation, chemiosmosis couples electron transport to ATP synthesis

• Following glycolysis and the citric acid cycle, NADH and FADH2 account for most of the energy extracted from food.

• These two electron carriers donate electrons to the electron transport chain, which powers ATP synthesis via oxidative phosphorylation.

The Pathway of Electron Transport

The electron transport chain is in the inner membrane (cristae) of the mitochondrion.

Most of the chain’s components are proteins, which exist in multiprotein complexes.

The carriers alternate reduced and oxidized states as they accept and donate electrons.

Electrons drop in free energy as they go down the chain and are finally passed to O2, forming H2O.

Figure 9.13

NADH

FADH2

2 H + 1/2 O2

2 e

2 e

2 e

H2O

NAD

Multiproteincomplexes

(originally from NADH or FADH2)

III

III

IV

50

40

30

20

10

0

Fre

e e

ner

gy

(G)

rela

tiv

e to

O2 (

kcal

/mo

l)

FMN

FeS FeS

FAD

Q

Cyt b

Cyt c1

Cyt c

Cyt a

Cyt a3

FeS

Electrons are transferred from NADH or FADH2 to the electron transport chain.

Electrons are passed through a number of proteins including cytochromes (each with an iron atom) to oxygen.

The electron transport chain generates no ATP directly.

It breaks the large free-energy drop from food to O2 into smaller steps that release energy in manageable amounts.

Chemiosmosis: The Energy-Coupling Mechanism

Electron transfer in the electron transport chain causes proteins to pump H+ from the mitochondrial matrix to the intermembrane space.

H+ then moves back across the membrane, passing through the protein, ATP synthase.

ATP synthase uses the exergonic flow of H+ to drive phosphorylation of ADP.

This is an example of chemiosmosis, the use of energy in a H+ gradient to drive cellular work.

Figure 9.14

INTERMEMBRANE SPACE

Rotor

StatorH

Internalrod

Catalyticknob

ADP+P i ATP

MITOCHONDRIAL MATRIX

Figure 9.15

Proteincomplexof electroncarriers

(carrying electronsfrom food)

Electron transport chain

Oxidative phosphorylation

Chemiosmosis

ATPsynth-ase

I

II

III

IVQ

Cyt c

FADFADH2

NADH ADP P i

NAD

H

2 H + 1/2O2

H

HH

21

H

H2O

ATP

The energy stored in a H+ gradient across a membrane couples the redox reactions of the electron transport chain to ATP synthesis.

The H+ gradient is referred to as a proton-motive force, emphasizing its capacity to do work.

An Accounting of ATP Production by Cellular Respiration

During cellular respiration, most energy flows in this sequence: glucose NADH electron transport chain proton-motive force ATP

About 34% of the energy in a glucose molecule is transferred to ATP during cellular respiration, making about 32 ATP.

There are several reasons why the number of ATP is not known exactly.

Figure 9.16

Electron shuttlesspan membrane

MITOCHONDRION2 NADH

2 NADH 2 NADH 6 NADH

2 FADH2

2 FADH2

or

2 ATP 2 ATP about 26 or 28 ATP

Glycolysis

Glucose 2 Pyruvate

Pyruvate oxidation

2 Acetyl CoACitricacidcycle

Oxidativephosphorylation:electron transport

andchemiosmosis

CYTOSOL

Maximum per glucose:About

30 or 32 ATP