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Lecture 13 9/30/05

Cellular Respiration: Cellular Respiration: Harvesting Chemical EnergyHarvesting Chemical Energy

Chapter 9

I. General Principles

Figure 9.12

Lecture OutlineLecture Outline

1. Regulation of Enzymes: competitive, allosteric, phosphorylation2. Equilibrium 3. Digestion vs Metabolism: catabolism and anabolism4. What is a metabolic pathway?5. Feedback regulation of pathways6. Catabolic pathways - stepping down the oxidation series of carbon7. Harvesting energy from redox reactions

- substrate level phosphorylation ATP– reducing equivalent carriers NADH + H+, FADH2

8. Example of a catabolic pathway: Fatty Acid Oxidation

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Reactions that proceed in a closed system–– Eventually reach equilibriumEventually reach equilibrium

Figure 8.7 A

(a) A closed hydroelectric system. Water flowing downhill turns a turbine that drives a generator providing electricity to a light bulb, but only until the system reaches equilibrium.

∆G < 0 ∆G = 0

Can doUsefulwork

CannotDo

work

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Living systems = Open System– Must have constant flow of materials in – Constant Energy Input

Figure 8.7

(b) An open hydroelectric system. Flowing water

keeps driving the generator because intake and outflow of water keep the system

from reaching equlibrium.

∆G < 0

Equilibrium to a livingsystem is called….

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Metabolism – totality of all chemical reactions of an organism

anabolismanabolism – energy utilizingutilizing reactionsuse energy carriers, build things

catabolismcatabolism – energy capturecapture reactionsoxidize substrates, produce energy carriers

digestiondigestionHydrolysis of polymers to monomersNo energy Harvested ! occurs “outside” the cell

Note: ∆G<0 6

∆G < 0

∆G < 0

∆G < 0

Metabolism: Metabolism: a series of favorable reactionsa series of favorable reactions

Figure 8.7

Metabolic Pathway: The product of each reaction becomes the reactant for a next, so

no reaction reaches equilibrium

Inputs

WasteProducts

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Metabolic PathwayEnzymes work in series

Each enzyme carries out one reactionReactions in series constitute a Pathway

Enzyme 3 promotes reaction C D

Enzyme 2 promotes reaction B C

Enzyme 1 promotes reaction A B

Enzyme 5 promotes reaction E FEnzyme 4 promotes reaction D E

Enzyme 6 promotes reaction F G

So as long as have A, G will be producedEach reaction is facilitated by a differentdifferent enzyme 8

Enzyme 1 Enzyme 2 Enzyme 3

A B C D

Reaction 1 Reaction 2 Reaction 3

Startingmolecule

Product

Chemistry of Life is organizedinto Metabolic Pathways

F F

FFA

A A

A

B

C D

E

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F F

F

F

FA

A A

A

“Plenty ofF

over Here, Shut it OFF!”

B

C D

E

FeedbackFeedback RegulationRegulation

Enzymes can be Enzymes can be regulatedregulated

AllostericAllosteric modulator ?modulator ? 10

Active siteavailable

Isoleucineused up bycell

Feedbackinhibition

Isoleucinebinds to allostericsite

Active site of enzyme 1 no longer binds threonine;pathway is switched off

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product(isoleucine)

Figure 8.21

thrthr

ileile

ProductProductOf Of

PathwayPathwayIsIs

AllostericAllostericRegulatorRegulator

Of Of First EnzymeFirst EnzymeIn PathwayIn Pathway

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For example, oxidation of glucose:C6H12O6 (glucose) + 6O2 6CO2 + 6H2O

∆G= -686 kcal/mol ∆H = -673 kcal/mol

T∆S= -13 kcal/mol

in the cell, this is done in >21 steps!

Capture the energy in small packetsCapture the energy in small packetsieie, 36 ATP units of 7.3 kcal, 36 ATP units of 7.3 kcal

Why so many steps in a pathway?

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15 gallons 15 gallons Of gasolineOf gasoline

ManySmall

Controlledreactions

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catabolic pathwaycatabolic pathway

Oxidize in discrete steps Step down the oxidation series of carbon

some activation stepoxidation step, with energy harvestreorganization stepoxidation step, another harvestetcyield product of pathway

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What is an What is an OXIDATION?OXIDATION?

For ionic species:Reduced means

“rich” in electrons

Oxidized means

“fewer” electrons

Fe++reduced Fe+++

oxidized

Oxidation: loss of e-Reduction : gain of e- Organic Reductions

X + 2e- + 2H+ XH2

Organic OxidationYH2 Y + 2e- +2H+

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Ease of Removing electrons

Oxidized“few” bonds to oxygen“many” bonds to hydrogen

Reduced = High enthalpy“few” bonds to oxygen“many” bonds to hydrogen

electronegativity

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Highly Highly reducedreduced

Highly oxidizedHighly oxidized

OXIDATION series of carbon

Hydrocarbon chain

Unsaturated hydrocarbon

Alcohol

Carbonyl

Carboxylic Acid

Carbon Dioxide

R-CH=CH2

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In Metabolism:

Highly reduced fully oxidized

CH3-CH2-CH2-(CH2)x-CH2-C-O + O2 H2O + CO2 + energy Fatty acid O (captured)

Partially reduced fully oxidized

+ O2 H2O + CO2+ energycarbohydrate (captured)

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R-CH2 -CH 3

“H-H” removed

“H- + H+” removed

“2e- + H+ + H+” removed”

R-CH2-CH2 -OH

R-CH2-C=OH

R-CH2-C=OOH

O=C=O

Catabolic PathwaysProgress down the Oxidation Series

Of Carbon

“adding O”

R-CH=CH2

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REDOX ReactionsREDOX Reactions

Oxidations always paired with reductions

If one thing gets oxidizedoxidized, another becomes reducedreduced

CH4

H

H

HH C O O O O OC H H

Methane(reducing

agent)

Oxygen(oxidizing

agent)

Carbon dioxide Water

+ 2O2 CO2 + Energy + 2 H2O

becomes oxidized

becomes reduced

Reactants Products

Figure 9.3

Change the degreeChange the degreeof electron sharing of electron sharing in covalent bondsin covalent bonds

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Carriers of Reducing Equivalents

NAD+ nicotinamide adenine dinucleotide

NADP+ nicotinamide adenine dinucleotide phosphate

FAD flavin adenine dinucleotide

NAD+ + H+ + 2e- -> NADH

NADP+ + H+ + 2e- -> NADPH

FAD + 2H+ + 2e- -> FADH2

CoEnzymes (CoFactors)

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Electrons from organic compoundsElectrons from organic compoundsAre usually first transferred to NADAre usually first transferred to NAD++, , a coenzymea coenzyme

NAD+

H

O

OO O–

OO O–

O

O

O

P

P

CH2

CH2

HO OHH

HHO OH

HO

H

H

N+

C NH2

HN

H

NH2

N

N

Nicotinamide(oxidized form)

NH2+ 2[H]

(from food)

Reduction of NAD+

Oxidation of NADH

2 e– + 2 H+

2 e– + H+

NADH

OH H

N

C +

Nicotinamide(reduced form)

N

Figure 9.4

H+

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2 e-2 H+

e- 1 e-1 H+

NAD+ to NADHCarries 2e- and 1 H+

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NADP+ looks like this:NADPNADP++ NADPHNADPH HH++

H+

2e-

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1 e-1 H+

1 e-1 H+

FAD to FADH2Carries 2e- and 2 H+

FAD looks like this:

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How harvest energy packets upon oxidation?How harvest energy packets upon oxidation?

-- high energy phosphate bondshigh energy phosphate bondsATP, GTPATP, GTP productionproductionsubstrate level substrate level phosphorylationphosphorylationless usual form of energy harvest less usual form of energy harvest

--Carriers of reducing equivalentsCarriers of reducing equivalentsOxidized form Oxidized form –– reduced formreduced form

NAD+ NADH + H+NAD+ NADH + H+FAD FADHFAD FADH22

--Can cash in reduced carriers for Can cash in reduced carriers for ATPATPoxidative oxidative phosphorylationphosphorylation 26

Carriers of Energy potentialCarriers of Energy potential

ATPATP – common energy currency “$$$”

High energy phosphate bondsHigh energy phosphate bonds

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Substrate Level PhosphorylationExample:

R- C -C-OH

= =O OR- C-Pi C

= =O O Pi =

O

“A” “B”

Oxidized to Carbon Dioxide

High EnergyCompound

Enzyme 1

R- C -OH

R- C-Pi + ADP-OH

=O“B”

ADP -Pi

(ATP)

O=

Oxidized to ACID

“C”

Energy of Oxidations “Captured”in the FORMATION of ATP

Enzyme 2

NAD+ NADH + H+

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How harvest energy packets upon oxidation?How harvest energy packets upon oxidation?

-- high energy phosphate bondshigh energy phosphate bondsATPATP, , GTPGTP productionproductionsubstrate level substrate level phosphorylationphosphorylationless usual form of energy harvest less usual form of energy harvest

--Carriers of reducing equivalentsCarriers of reducing equivalentsOxidized form Oxidized form –– reduced formreduced form

NAD+ NADH + H+NAD+ NADH + H+FAD FADHFAD FADH22

$$$

Poker chips

--Can cash in reduced carriers for Can cash in reduced carriers for ATPATPoxidative oxidative phosphorylationphosphorylation

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The Regeneration Energy CarriersThe Regeneration Energy Carriers

2e2e--

2H2H++

Captured in catabolismCaptured in catabolism

NADH + H+

Energy for cellular work(endergonic, energy-consuming processes)

Energy from catabolism(exergonic, energy yieldingprocesses)

NAD+

2e2e--

2H2H++

Cashed inCashed in

Energy carriers (ATP, NAD+, FAD) present in only minute amounts

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Let’s put it together

Step down oxidation seriesHarvest energy in discrete packets

Fatty Acid Oxidation Pathway

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Start of PathwayStart of Pathway

Priming Step (energy input)

CHCH33--CHCH22--RR--CHCH22--CHCH22--C=OC=O

OO--Fatty acidFatty acidATP ATP + + CoACoA--SHSH

CHCH33--CHCH22--RR--CHCH22--CHCH22--C=OC=O

SS--CoACoAFatty Fatty acylacyl CoACoAADPADP + + PPii

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Fatty Acid Oxidation (Fatty Acid Oxidation (ββ--oxidation) oxidation)

-steps down oxidationstates of carbon

-captures Reducing potential NADH + H+

FADH2

Saturatedhydrocarbon

unsaturatedhydrocarbon

2e-2 H+removed

alcoholKetone2e-

2 H+removed

Ester(acid)

PrimingStep

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Net Result of Fatty Acid Oxidation PathwayNet Result of Fatty Acid Oxidation Pathway

Fatty acid shortened by 2 carbon unit

2 carbon acid attached to CoA (acetyl CoA)

Oxidation of Carbon -CH2- to –C=Oto acid S CoA

Capture reducing equivalents2 NADH + H+

2 FADH234

Summary

• Digestion, Metabolism, Catabolism, Anabolism• Biochemical Pathway; feedback regulation• Catabolic Pathways

- Step down oxidation series of carbon- Harvest energy in discrete packets

• ATP, NADH + H+, FADH2• Fatty Acid Oxidation Pathway