Stages of Metabolism

Pyruvate Oxidation

Conversion to acetyl–CoA• Catalyzed by pyruvate

dehydrogenase• Decarboxylation - gives CO2

and aldehyde (uses thiamine pyrophosphate)

• Oxidation - gives acetyl group (uses FAD and NAD+ , makes NADH)

• Transfer to CoASH (uses lipoic acid)

pyruvate dehydrogenase

complex

O

C

CH3

C O–

O

NAD+ NADH + H +

CoASHCO2

C

CH3

S CoA

O

pyruvate acetyl–CoA

Citric Acid Cycle Overview

In the citric acid cycle,• Acetyl (2C) bonds to

oxaloacetate (4C) to form citrate (6C).

• Oxidation and decarboxylation reactions convert citrate to oxaloacetate.

• Oxaloacetate bonds with another acetyl to repeat the cycle.

The citric acid cycle (stage 3)• Operates under aerobic conditions only.• Oxidizes the two-carbon acetyl group in

acetyl CoA to 2CO2.

• Produces reduced coenzymes NADH and FADH2 and one ATP directly.

Citric Acid Cycle

Citric Acid CycleEntry from Acetyl–CoA

CH3COSCoAacetyl – CoAcitrateC CO2CH2 CH2CO2HOCO2CH2CO2C OCO2oxaloacetateCondensation reaction: very favorable

citratesynthase

Citric Acid CycleCitrate to Isocitrate

citrateisocitrateConversion of 3o alcohol into 2 o alcohol: Now able to be oxidized

aconitaseC C CH2HCO2HCO2HOCO2C C CH2HCO2HCO2OHCO2

Citric Acid CycleFirst Oxidation

isocitrateC CO2C OH CH2CO2HCO2Hα-ketoglutarateCH2CO2 C OCH2CO2OxidativedecarboxylationCO2NAD+NAD + H H+isocitratedehydrogenase

Key point: requires NAD+

Citric Acid CycleSecond Oxidation

α-ketoglutarateCH2CO2C OCH2CO2CO2NAD+NADH + H +α-ketoglutaratedehydrogenase

CoASHOxidative decarboxylation coupled to formation of an energized molecule

succinyl – CoACH2CH2SCoAC OCO2

Key point: requires NAD+

Citric Acid CycleSubstrate-Level Phosphorylation

succinyl – CoACH2CH2SCoAC OCO2succinyl–CoAsynthetase

CoASHGDP + P i GTPsuccinateCH2CO2CH2CO2Substrate-level phosphorylation.The energized succinate is used to drive the phosphorylation of GDP.

Citric Acid CycleThird Oxidation

succinatedehydrogenase

HCCO2CHCO2fumaratesuccinateCH2CO2CH2CO2FADFADH2Oxidation Insertion of doublebond as first steptowards regenerationof oxaloacetate

Citric Acid CycleHydration

fumaraseHCCO2CHCO2fumarateAddition of water to create a secondary alcohol.L- malateH2OCH2CO2CO2HOCH

Citric Acid CycleFourth Oxidation

CH2CO2C OCO2oxaloacetateNAD+NADH + H +malatedehydrogenase

CH2CO2CO2HOCH

Key point: requires NAD+

An acetyl group bonds with oxaloacetate to form citrate

Two decarboxylations remove two carbons as 2CO2

Four oxidations provide hydrogen for 3NADH and one FADH2.

A direct phosphorylation forms GTP (ATP).

acetyl-SCoA + 3NAD+ + FAD + GDP + Pi + 2H2O

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

One turn of the citric acid cycle produces:

2 CO2 1 GTP (1ATP)

3 NADH 1 HS-COA

1 FADH2

Overall Chemical Reaction for the Citric Acid Cycle

CH3COSCoAacetyl – CoA

CH2CO2C OCO2oxaloacetate

Energized acetyl group

citrateC CO2CH2 CH2CO2HOCO2 CH2CO2CO2isocitrateC CO2CHOHH

Conversion of 3 alcohol into 2 alcohol: Now able to be oxidized

o

o

α-ketoglutarateCH2CO2C OCH2CO2

NAD+

NADH + H +CO2

Oxidativedecarboxylation

SCoACH2CH2CO2COsuccinyl – CoA

NADH + H +

NAD+CoASH

CO2succinateCH2CO2CH2CO2

GTP GDP + P i

CoASH

Substrate-level phosphorylation.The energized succinate is used to drive the phosphorylation of GDP.

Oxidative decarboxylation coupled to formation of an energized molecule

L- malateCH2CO2HCO2 HOC HCCO2CHCO2fumarate

FADFADH2

Oxidation Insertion of double bond as first step towards regeneration of oxaloacetate

H2O

Addition of water to

create a 2 o alcohol.

NAD+

NADH + H +Oxidation of

2o alcohol

Condensation(very favorable reaction)

Fatty acids

Cholesterol

Amino acids

Glucose

HemeAmino acids

Amino acids

Citric Acid Cycle

Pyruvate(cytoplasm) NAD+NADH + H+CO2CoASH Pyruvate(mitochondria)

Amino acids

Amino acids

Regulation of Citric Acid Cycle

Increases when high levels of ADP or NAD+ activateisocitrate dehydrogenase andα-ketoglutarate dehydrogenase

The reaction rate forthe citric acid cycle

Decreases when high levels of ATP or NADH inhibit isocitrate dehydrogenase.

Formation of acetyl–CoA from pyruvate (catalyzed by pyruvate dehydrogenase) also activated by ADP and inhibited by ATP and NADH.

Decreases when high levels of NADH or succinyl–CoAinhibit α-ketoglutarate dehydrogenase.

MitochondrialStructure

FMN (Flavin mononucleotide)

FMN coenzyme• Contains flavin,

ribitol,and phosphate.

• Accepts 2H+ + 2e-

to form reduced coenzyme FMNH2.

Coenzyme Q (Q or CoQ)

Coenzyme Q (Q or CoQ) is

• A mobile electron carrier derived from quinone.

• Reduced when the keto groups accept 2H+ and 2e-

CytochromesCytochromes (cyt) are• Proteins containing

heme groups with iron ions.

Fe3+ + 1e- Fe2+

• Abbreviated as cyt a, cyt a3, cyt b, cyt c, and cyt c1.

Electron Transport Chain

Cytc1

2 NADH + 2 H+ + O2 2 NAD+ + 2 H2O 2 FADH2 + O2 2 FAD + 2 H2O

Chemiosmotic Model of Electron Transport

During electron flow Complexes I, III, and IV pump protons into the intermembrane space creating a proton gradient.

Protons pass through ATP synthase to return to the matrix.

The flow of protons through ATP synthase provides the energy for ATP synthesis (oxidative phosphorylation).

ATP Synthase

In ATP synthase• Protons flow back to

the matrix through a channel in the F0 complex.

• Proton flow provides the energy that drives ATP synthesis by the F1 complex

ATP from Electron TransportFrom NADH (Complex I) provides sufficient energy for 3ATPsNADH + 3ADP + 3Pi NAD+ + 3ATP

From FADH2 (Complex II) provides

sufficient energy for 2ATPsFADH2 + 2ADP + 3Pi FAD + 2ATP

Regulation of Electron Transport

The electron transport system is regulated by

Low levels of ADP, Pi, oxygen, and NADH that decrease electron transport activity.

High levels of ADP and NADH that activate electron transport.

ATP from GlycolysisReaction Pathway ATP for One Glucose

ATP from Glycolysis

Activation of glucose -2 ATP

Oxidation of 2 NADH (as FADH2) 4 ATP

Direct ADP phosphorylation (two triose) 4 ATP

6 ATP

Summary:

C6H12O6 2 pyruvate + 2H2O + 6 ATP glucose

ATP from Two PyruvatesUnder aerobic conditions• 2 pyruvate are oxidized to 2 acetyl CoA and 2 NADH.• 2 NADH enter electron transport to provide 6 ATP.

Summary:

2 Pyruvate 2 Acetyl CoA + 6 ATP

ATP from Citric Acid CycleReaction Pathway ATP (One Glucose)ATP from Citric Acid Cycle (2 acetyl-CoA)Oxidation of 2 isocitrate (2NADH) 6 ATPOxidation of 2 α-ketoglutarate (2NADH) 6 ATP2 Direct substrate phosphorylations (2GTP) 2 ATPOxidation of 2 succinate (2FADH2) 4 ATPOxidation of 2 malate (2NADH) 6 ATP

24 ATP

Summary: 2Acetyl CoA + 24 ADP + 24 Pi 4CO2 + 2H2O + 24 ATP + 2 CoASH

Overall ATP Production for one glucose

C6H12O6 + 6O2 + (36 – 38)ADP + (36 – 38) Pi glucose 6CO2 + 6H2O + (36 – 38) ATP

One glucose molecule undergoing complete oxidation provides:From glycolysis 6 – 8 ATPFrom 2 pyruvate 6 ATPFrom 2 acetyl CoA 24 ATP

36-38 ATP

ATP from Glucose

ATP Energy from Glucose

The complete oxidation of glucose yields

• 6 CO2

• 6 H2O

• 36-38 ATP