bioenergetics. 03sep2009(d)

8/14/2019 BIOENERGETICS. 03SEP2009(d)

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BIOENERGETICSBIOENERGETICS

ByBy

MUHAMMAD BILAL AZMIMUHAMMAD BILAL AZMI

M. Sc, (M.S), (M.B.A).M. Sc, (M.S), (M.B.A).


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For a better understanding ofFor a better understanding ofbiological oxidation, it is worthwhile tobiological oxidation, it is worthwhile to

have a basic knowledge ofhave a basic knowledge of

bioenergetics and the role of high-bioenergetics and the role of high-energy compounds in biologicalenergy compounds in biological

processprocess

BIOLOGICAL OXIDATION


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Bioenergetics:Bioenergetics:

Bioenergetics orBioenergetics or biochemicalbiochemical

thermodynamicsthermodynamics deals with the study ofdeals with the study ofenergy changes (transfer and utilization) inenergy changes (transfer and utilization) in

biochemical reactions. The reactions arebiochemical reactions. The reactions are

broadly classified as exergonic (energybroadly classified as exergonic (energy

releasing) and endergonic (energyreleasing) and endergonic (energyconsuming). Bioenergetics is concerned withconsuming). Bioenergetics is concerned with

the initial and final states of energythe initial and final states of energy

component of the reactants and not thecomponent of the reactants and not the

mechanism of chemical reactions.mechanism of chemical reactions.


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Free Energy:Free Energy:The energy actualThe energy actual available to do workavailable to do work

(utilizable) is known as free energy. Changes(utilizable) is known as free energy. Changesin the free energy (G) are valuable inin the free energy (G) are valuable inpredicting the feasibility of chemicalpredicting the feasibility of chemicalreactions. The reactions can occurreactions. The reactions can occur

spontaneously if they are accompanied byspontaneously if they are accompanied bydecrease in free energy.decrease in free energy. During a chemical reaction, heat may beDuring a chemical reaction, heat may be

released or absorbed.released or absorbed. Enthalpy (H)Enthalpy (H) is ais a

measure of the change in heat content of themeasure of the change in heat content of thereactants, compared to products.reactants, compared to products.


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Entropy (S)Entropy (S) represents a change in therepresents a change in the

randomness or disorder of reactants and products.randomness or disorder of reactants and products.

Entropy attains a maximum as the reactionEntropy attains a maximum as the reactionapproaches equilibrium. The relation between theapproaches equilibrium. The relation between the

changes of free energy (G), enthalpy (H) andchanges of free energy (G), enthalpy (H) and

entropy (S) is expressed asentropy (S) is expressed as

G= H- T( S)G= H- T( S)T= absolute temperature in kelvinT= absolute temperature in kelvin

The termThe term standard free energystandard free energy represented byrepresented by

GGoo is often used. It indicates the free energyis often used. It indicates the free energy

change when the reactants or products are at achange when the reactants or products are at aconcentration of 1 mol/l at pH 7.0.concentration of 1 mol/l at pH 7.0.


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Negative and Positive G:Negative and Positive G: If free energy change (G) is representedIf free energy change (G) is represented

by a negative sign, there is a loss of freeby a negative sign, there is a loss of freeenergy. The reaction is said to beenergy. The reaction is said to beexergonicexergonic, and proceeds spontaneously., and proceeds spontaneously.On the other hand, a positive G indicatesOn the other hand, a positive G indicates

that energy must be supplied to thethat energy must be supplied to thereactants. The reactions can not proceedreactants. The reactions can not proceedspontaneously and isspontaneously and is endergonicendergonic inincharacter.character.

The hydrolysis of ATP is a classicalThe hydrolysis of ATP is a classicalexample of exergonic reaction.example of exergonic reaction.

ATP + HATP + H22O ADP + PiO ADP + Pi

(G(Goo = 7.3 Cal/mol)= 7.3 Cal/mol)


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The reversal of the reaction (ADP + Pi ATP) isThe reversal of the reaction (ADP + Pi ATP) is

endergonic and occurs only when there is a supplyendergonic and occurs only when there is a supply

of energy of at least 7.3 Cal/mol (Gof energy of at least 7.3 Cal/mol (Goo

is positive).is positive).

The free energy change becomes zero (G = 0)The free energy change becomes zero (G = 0)

when a reaction is at equilibrium. At a constantwhen a reaction is at equilibrium. At a constant

temperature and pressure, G is dependent on thetemperature and pressure, G is dependent on theactual concentration of reactant and products. Foractual concentration of reactant and products. For

the conversion of reactant A to product B (A B),the conversion of reactant A to product B (A B),

the following mathematical relation can be derived.the following mathematical relation can be derived.

G = GG = Goo ++ RT lnRT ln [ B ][ B ]

[ A ][ A ]


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where Gwhere Goo == Standard free energy changeStandard free energy change

RR == Gas constant (1.987 Cal/mol)Gas constant (1.987 Cal/mol)

TT == Absolute temperature (273 +Absolute temperature (273 + ooC)C)

lnln == Natural logarithmNatural logarithm

[B][B] == Concentration of productConcentration of product

[A][A] == Concentration of reactant.Concentration of reactant.


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GGoo is related to equilibriumis related to equilibrium

constant (Kconstant (Keqeq)) When a reaction A B is atWhen a reaction A B is at

equilibrium (eq), the free energyequilibrium (eq), the free energy

change is zero. The above equationchange is zero. The above equationmay be written as;may be written as;

G = 0 = GG = 0 = Goo + RT+ RT [ B ] eq.[ B ] eq.

[ A ] eq.[ A ] eq.Hence GHence Goo = RT ln K= RT ln K

eqeq..


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G is an additive value for pathways:G is an additive value for pathways:

Biochemical pathways often involve a series ofBiochemical pathways often involve a series of

reactions. For such reactions, free energy changereactions. For such reactions, free energy changeis an additive value. The sum of G is crucial inis an additive value. The sum of G is crucial in

determining whether a particular pathway willdetermining whether a particular pathway will

proceed or not. As long as the sum of Gproceed or not. As long as the sum of Gss ofof

individual reactions is negative, the pathway canindividual reactions is negative, the pathway canoperate. This happens despite the fact that someoperate. This happens despite the fact that some

of the individual reactions may have positive G.of the individual reactions may have positive G.


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High Energy Compounds:High Energy Compounds: Certain compounds are encountered in the biologicalCertain compounds are encountered in the biological

system which, on hydrolysis, yield energy. The termsystem which, on hydrolysis, yield energy. The term

high-energy compounds orhigh-energy compounds or energy rich compoundsenergy rich compoundsis usually applied to substances which possessis usually applied to substances which possesssufficient free energy to liberatesufficient free energy to liberate atatleast 7 Cal/molleast 7 Cal/molat pH 7.0. Certain other compounds which liberateat pH 7.0. Certain other compounds which liberateless than 7.0 Cal/mol (lower than ATP hydrolysis toless than 7.0 Cal/mol (lower than ATP hydrolysis to

ADP + Pi) are referred to as low-energy compounds.ADP + Pi) are referred to as low-energy compounds. All the high-energy compounds when hydrolysedAll the high-energy compounds when hydrolysed

liberate more enery than that of ATP. These includeliberate more enery than that of ATP. These includephosphoenol pyruvate, 1, 3-phosphoenol pyruvate, 1, 3-bisphosphoglycerate, phosphocreatinebisphosphoglycerate, phosphocreatine etc. Mostetc. Most

of the high-energy compounds contain phosphateof the high-energy compounds contain phosphategroup (exception acetyl CoA) hence they are calledgroup (exception acetyl CoA) hence they are calledhigh-energy phosphate compound.high-energy phosphate compound.


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Classification of High-EnergyClassification of High-Energy

Compounds:Compounds:

There are at least 5 groups of high-energyThere are at least 5 groups of high-energycompounds.compounds.

Pyrophosphates e.g. ATP.Pyrophosphates e.g. ATP.

Acyl phosphates e.g. 1,3-Acyl phosphates e.g. 1,3-

bisphosphoglycerate.bisphosphoglycerate.

Enol phosphates e.g. phsophenolpyruvate.Enol phosphates e.g. phsophenolpyruvate.

Thioesters e.g. acetyl CoA.Thioesters e.g. acetyl CoA.

Phosphoguanidines e.g. phosphocreatine.Phosphoguanidines e.g. phosphocreatine.


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High Energy Bonds:High Energy Bonds: The high energy compounds possess acid anhydride bondsThe high energy compounds possess acid anhydride bonds

(mostly phsophoanhydride bonds) which are formed by the(mostly phsophoanhydride bonds) which are formed by thecondensation of two acidic groups or related compounds.condensation of two acidic groups or related compounds.

These bonds are referred to as high energy bonds, since theThese bonds are referred to as high energy bonds, since thefree energy is liberated when these bonds are hydrolysed.free energy is liberated when these bonds are hydrolysed.Lipmann suggested use of theLipmann suggested use of the symbol ~ to representsymbol ~ to representhigh-energy bondhigh-energy bond.. For instance, ATP is written asFor instance, ATP is written asAMP~P~P.AMP~P~P.

ATP- The Most Important High-Energy Compound:ATP- The Most Important High-Energy Compound: Adenosine triphosphate (ATP) is a unique and the mostAdenosine triphosphate (ATP) is a unique and the most

important high-energy molecule in the living cells. It consistimportant high-energy molecule in the living cells. It consistof an adenine, a ribose and a triphosphate moiety . ATP is aof an adenine, a ribose and a triphosphate moiety . ATP is ahigh-energy compound due to the presence of twohigh-energy compound due to the presence of two

phsophanhydride bonds in the triphosphate unit. ATP servesphsophanhydride bonds in the triphosphate unit. ATP servesas theas the energy currency of the cellenergy currency of the cellas is evident from theas is evident from theATP-ADP cycle.ATP-ADP cycle.


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ATP-ADP Cycle:ATP-ADP Cycle:The hydrolysis of ATP is associated with theThe hydrolysis of ATP is associated with the

release of large amount of energy.release of large amount of energy. ATP + H2O ADP + Pi + 7.3 cal.ATP + H2O ADP + Pi + 7.3 cal.The energy liberated is utilized for variousThe energy liberated is utilized for various

processes like muscle contraction, activeprocesses like muscle contraction, active

transport etc. ATP can also act as a donor oftransport etc. ATP can also act as a donor ofhigh-energy phosphate to low-energyhigh-energy phosphate to low-energycompounds, to make them energy rich. On thecompounds, to make them energy rich. On theother hand, ADP can accept high-energyother hand, ADP can accept high-energy

phosphate from the compounds possessingphosphate from the compounds possessinghigher free energy content to form ATP.higher free energy content to form ATP.


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Fig. 11.1 Structure ofATP.

ATP serves as animmediately availableenergy currency of the cellwhich is constantly beingutilized and regenerated.This is represented by ATP-ADP cycle, the fundamentalbasis of energy exchange

reactions in living system(Fig.11.2). The turnover ofATP is very high.ATP acts as an energy link

between the catabolism( degradation of molecules)and anabolism (synthesis)in the biological system.


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Fig. 11. 2 ATP-ADP cyclealong with sources andutilization of ATP (Note:that P does not exist infree form, but is only

transferred).

Synthesis of ATP:ATP can be synthesized in

two ways:

OxidativePhosphorylation:

This is the major source ofATP in aerobic organism. It

is linked with themitochondrial electrontransport chain (detailsdescribed later).


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Substrate Level Phosphorylation:Substrate Level Phosphorylation:

ATP may be directly synthesized during substrateATP may be directly synthesized during substrateoxidation in the metabolism. The high-energyoxidation in the metabolism. The high-energy

compounds such as phosphoenolpyruvate and 1,3-compounds such as phosphoenolpyruvate and 1,3-bisphosphoglycerate (intermediates of glycolysis) andbisphosphoglycerate (intermediates of glycolysis) andsuccinyl CoA (of citric acid cycle) can transfer high-succinyl CoA (of citric acid cycle) can transfer high-energy phosphate to ultimately produce ATP.energy phosphate to ultimately produce ATP.

Storage Forms of High-Energy Phosphates:Storage Forms of High-Energy Phosphates: Phosphocreatine (creatine phosphate) stored inPhosphocreatine (creatine phosphate) stored in

vertebrate muscle and brain is an energy richvertebrate muscle and brain is an energy richcompound. In invertebrates, phosphoargininecompound. In invertebrates, phosphoarginine(arginine phosphates) replaces phsophocreatine.(arginine phosphates) replaces phsophocreatine.


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BIOLOGICAL OXIDATION:BIOLOGICAL OXIDATION:

Oxidation is defined as the loss of electrons and reductionOxidation is defined as the loss of electrons and reduction

as the gain of electrons. This may be illustrated by theas the gain of electrons. This may be illustrated by the

interconversion of ferrousion (Feinterconversion of ferrousion (Fe2+2+ ) to ferric ion (Fe) to ferric ion (Fe3+3+ ))

The electron lost in the oxidation is acceptedby an acceptor which is said to be reduced.Thus the oxidation-reduction is a tightlycoupled process.


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The general principle of oxidation-reduction isThe general principle of oxidation-reduction is

applicable to biological systems also. The oxidation ofapplicable to biological systems also. The oxidation of

NADH to NAD+ coupled with the reduction of FMN toNADH to NAD+ coupled with the reduction of FMN to

FMNH2 is illustrated.FMNH2 is illustrated.

In the above illustration, there are two redoxpairs NADH/HAD+ and FMN/FMNH2. the redoxpairs differ in their tendency to lose or gain

electrons.


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Redox Potential (ERedox Potential (Eoo))

The oxidation reduction potential or, simply, redoxThe oxidation reduction potential or, simply, redoxpotential, is a quantitative measure of the tendency ofpotential, is a quantitative measure of the tendency of

a redox pair to lose or gain electrons. The redox pairsa redox pair to lose or gain electrons. The redox pairsare assigned specific standard redox potential (Eare assigned specific standard redox potential (Eoo

volts) at pH 7.0 and 25volts) at pH 7.0 and 25ooC.C. The more negative redox potential represents aThe more negative redox potential represents a

greater tendency (of reductant) to lose electrons.greater tendency (of reductant) to lose electrons.

On the other hand, a more positive redox potentialOn the other hand, a more positive redox potentialindicates a greater tendency (of oxidant) to acceptindicates a greater tendency (of oxidant) to acceptelectrons. The electrons flow from a redox pair withelectrons. The electrons flow from a redox pair withmore negative Emore negative Eoo to another redox pair with moreto another redox pair with more

positive Epositive Eoo. The redox potential (E. The redox potential (Eoo) is directly related) is directly relatedto the change in the free energy (Gto the change in the free energy (Goo).).


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ELECTRON TRANSPORT CHAIN:ELECTRON TRANSPORT CHAIN:

The energy-rich carbohydrates (particularl glucose),The energy-rich carbohydrates (particularl glucose),

fatty acids and amino acids undergo a series offatty acids and amino acids undergo a series of

metabolic reactions and finally, get oxidized to COmetabolic reactions and finally, get oxidized to CO22 andand

HH22O. The reducing equivalents from various metabolicO. The reducing equivalents from various metabolic

intermediates are transferred to coenzymes NAD+ andintermediates are transferred to coenzymes NAD+ and

FAD to produce, respectively, NADH and FADH2. TheFAD to produce, respectively, NADH and FADH2. The

latter two reduced coenzymes pass through thelatter two reduced coenzymes pass through theelectron transport chain (ETC) or respiratory chain andelectron transport chain (ETC) or respiratory chain and

finally, reduce oxygen to water. The passage offinally, reduce oxygen to water. The passage of

electrons through the ETC is associated with the losselectrons through the ETC is associated with the loss

of free energy. A part of this free energy is utilized toof free energy. A part of this free energy is utilized to

generate ATP from ADP and Pi (Fig. 11.3).generate ATP from ADP and Pi (Fig. 11.3).


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An overview of the ETC is depicted in Fig.11.4.


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Mitochondria the power houses of Cell:Mitochondria the power houses of Cell:The mitochondria are the centres for metabolicThe mitochondria are the centres for metabolic

oxidative reactions to generate reducedoxidative reactions to generate reduced

coenzymes (NADH) and FADH2) which, in turn arecoenzymes (NADH) and FADH2) which, in turn areutilized in ETC to liberate energy in the form ofutilized in ETC to liberate energy in the form ofATP. For this reason, mitochondrion isATP. For this reason, mitochondrion isappropriately regarded as theappropriately regarded as thepower house ofpower house ofthe cellthe cell..

Mitochondrial Organization:Mitochondrial Organization:

The mitochondrion consists of five distinct parts.The mitochondrion consists of five distinct parts.These are the outer membrane, the innerThese are the outer membrane, the inner

membrane, the intermembrane space, the cristaemembrane, the intermembrane space, the cristaeand the matrix (Fig. 11.5).and the matrix (Fig. 11.5).


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Inner Mitochondrial Member:Inner Mitochondrial Member: The electron transport chain and ATP synthesizingThe electron transport chain and ATP synthesizing

system are located on the inner mitochondrialsystem are located on the inner mitochondrialmembrane which is a specialized structure, rich inmembrane which is a specialized structure, rich in

proteins. It is impermeable to ions (Hproteins. It is impermeable to ions (H++, K, K++ NANA++) and) andsmall molecules ADP, ATP). This membrane is highlysmall molecules ADP, ATP). This membrane is highlyfolded to form cristae. The surface area of innerfolded to form cristae. The surface area of innermitochondrial membrane is greatly increased due tomitochondrial membrane is greatly increased due tocristae. The inner surface of the inner mitochondrialcristae. The inner surface of the inner mitochondrialmembrane possesses specialized particles (that lookmembrane possesses specialized particles (that looklike lollipops), the phosphorylating subunits whichlike lollipops), the phosphorylating subunits whichare the centres for ATP production.are the centres for ATP production.

Mitochondrial Matrix:Mitochondrial Matrix: The interior ground substance forms the matrix ofThe interior ground substance forms the matrix of

mitochondria. It is rich in the enzymes responsiblemitochondria. It is rich in the enzymes responsiblefor the citric acid cycle, -oxidation of fatty acidsfor the citric acid cycle, -oxidation of fatty acidsand oxidation of amino acid.and oxidation of amino acid.


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Structural Organization of Respiratory Chain:Structural Organization of Respiratory Chain: The inner mitochondrial membrane can be disruptedThe inner mitochondrial membrane can be disrupted

into five distinct respiratory or enzyme complexes,into five distinct respiratory or enzyme complexes,

denoted as complex I, II, III, IV and V (Fig. 11.6). Thedenoted as complex I, II, III, IV and V (Fig. 11.6). Thecomplexes I-V are carriers of electrons while complex Vcomplexes I-V are carriers of electrons while complex Vis responsible for ATP synthesis. Besides these enzymeis responsible for ATP synthesis. Besides these enzymecomplexes, there are certain mobile electron carriers incomplexes, there are certain mobile electron carriers inthe respiratory chain.the respiratory chain.

Components and Reactions of the ElectronComponents and Reactions of the ElectronTransport Chain:Transport Chain: There are five distinct carriers that participate in theThere are five distinct carriers that participate in the

electron transport chain (ETC). These carriers areelectron transport chain (ETC). These carriers aresequentially arranged (Fig. 11.7) and are responsible forsequentially arranged (Fig. 11.7) and are responsible for

the transfer of electrons from a given substrate tothe transfer of electrons from a given substrate toultimately combine with proton and oxygen to formultimately combine with proton and oxygen to formwater.water.


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Nicotinamide Nucieotides:Nicotinamide Nucieotides:

Of the two coenzymes NAD+ and NADP+Of the two coenzymes NAD+ and NADP+derived from the vitamin niacin, NAD+ isderived from the vitamin niacin, NAD+ is

more actively involved in the ETC. NAD+ ismore actively involved in the ETC. NAD+ is

reduced to NADH + H+ by dehydrogenasesreduced to NADH + H+ by dehydrogenases

with the removal of two hydrogen atoms fromwith the removal of two hydrogen atoms fromthe substrate (AHthe substrate (AH22).The substrates include).The substrates include

glyceraldehydre-3 phsophate, pyruvate,glyceraldehydre-3 phsophate, pyruvate,

isocitrate, -ketoglutarate and malate.isocitrate, -ketoglutarate and malate.


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AH2 + NAD+ == A + NADH + H+

NADPH + H+ produced by NADP+ dependentdehydrogenase is not usually a substrate for ETC.NADPH is more effectively utilized for anabolicreactions (e.g. fatty acid synthesis, cholesterolsynthesis).


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Flavoproteins:Flavoproteins:

The enzyme NADH dehydrogenase (NADH-coenzyme QThe enzyme NADH dehydrogenase (NADH-coenzyme Q

reductase) is a flavoprotein with FMN as the prostheticreductase) is a flavoprotein with FMN as the prosthetic

group. The coenzyme FMN accepts two elelectrons andgroup. The coenzyme FMN accepts two elelectrons anda proton to form FMNHa proton to form FMNH22. NADH dehydrogenase is a. NADH dehydrogenase is a

complex enzyme closely associated with non-heme ironcomplex enzyme closely associated with non-heme iron

proteins (NHI) or iron-sulfur protein (FeS).proteins (NHI) or iron-sulfur protein (FeS).

NADH + HNADH + H

++

FMN NAD+ FMNHFMN NAD+ FMNH22 Succinate dehydrogenase (succinate-coenzyme QSuccinate dehydrogenase (succinate-coenzyme Q

reductase) is an enzyme found in the innerreductase) is an enzyme found in the inner

mitochondrial membrane. It is also a flavoprotein withmitochondrial membrane. It is also a flavoprotein with

FAD as the coenzyme. This can accept two hydrogenFAD as the coenzyme. This can accept two hydrogen

atoms (2H+ + 2eatoms (2H+ + 2e--) from succinate.) from succinate.

Succinate + FAD Fumarate + FADHSuccinate + FAD Fumarate + FADH22


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Iron-Sulfur Proteins:Iron-Sulfur Proteins:

The iron-sulfur (FeS) proteins exist in the oxidizedThe iron-sulfur (FeS) proteins exist in the oxidized(Fe(Fe3+3+ ) or reduced (Fe) or reduced (Fe2+2+ ) state. About half a dozen) state. About half a dozen

FeS proteins connected with respiratory chainFeS proteins connected with respiratory chain

have been identified. However, the mechanism ofhave been identified. However, the mechanism of

action of iron-sulfur proteins in the ETC is notaction of iron-sulfur proteins in the ETC is notclearly understood.clearly understood.

One FeS participates in the transfer of electronsOne FeS participates in the transfer of electrons

from FMN to coenzyme Q. other FeS proteinsfrom FMN to coenzyme Q. other FeS proteins

associated with cytochrome b and cytochrome C1associated with cytochrome b and cytochrome C1

participate in the transport of electrons.participate in the transport of electrons.

C Q


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Coenzyme Q:Coenzyme Q: Coenzyme Q is also known as ubiquinone since it isCoenzyme Q is also known as ubiquinone since it is

ubiquitous in living system. It is a quinone derivativeubiquitous in living system. It is a quinone derivativewith a variable isoprenoid side chain. The mammalianwith a variable isoprenoid side chain. The mammalian

tissues possess a quinone with 10 isoprenoid unitstissues possess a quinone with 10 isoprenoid unitswhich is known as coenzyme Qwhich is known as coenzyme Q1010 (CoQ(CoQ1010 ).).


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Coenzyme Q is a lipophilic electron carrier. It canCoenzyme Q is a lipophilic electron carrier. It can

accept electrons from FMNHaccept electrons from FMNH22 produced in the ETCproduced in the ETC

by NADH dehydrogenase of FADH2 producedby NADH dehydrogenase of FADH2 producedoutside ETC (e.g. succinate dehydrogenase, acyloutside ETC (e.g. succinate dehydrogenase, acyl

CoA dehydrogenase).CoA dehydrogenase).

Coenzyme Q is not found in mycobacteria. VitaminCoenzyme Q is not found in mycobacteria. VitaminK performs similar function as coenzyme Q in theseK performs similar function as coenzyme Q in these

organisms. Coenzyme Q has no known vitaminorganisms. Coenzyme Q has no known vitamin

precursor in animals. It is directly synthesized inprecursor in animals. It is directly synthesized in

the body. (Refer cholesterol biosyntehsis).the body. (Refer cholesterol biosyntehsis).

C t hC t h


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Cytochromes:Cytochromes: The cytochromes are conjugated proteins containing hemeThe cytochromes are conjugated proteins containing heme

group of cytochromes differ from that found in the structuregroup of cytochromes differ from that found in the structureof hemoglobin and myoglobin. The iron of heme inof hemoglobin and myoglobin. The iron of heme in

cytochromes is alternately oxidized (Fe3+) and reducedcytochromes is alternately oxidized (Fe3+) and reduced(Fe2+), which is essential for the transport of electrons in the(Fe2+), which is essential for the transport of electrons in theETC. This is in contrast to the heme iron of hemoglobin andETC. This is in contrast to the heme iron of hemoglobin andmyoglobin which remains in the ferrous (Fe2+) state.myoglobin which remains in the ferrous (Fe2+) state.

Three cytochromes were initially discovered from theThree cytochromes were initially discovered from themammalian mitochondria. They were designated asmammalian mitochondria. They were designated as

cytochrome a b and c depending on the type of hemecytochrome a b and c depending on the type of hemepresent and the respective absorption spectrum. Additionalpresent and the respective absorption spectrum. Additionalcytochromes such as c1, ba, b2, a3 etc. were discoveredcytochromes such as c1, ba, b2, a3 etc. were discoveredlater.later.

The electrons are transported from coenzyme Q toThe electrons are transported from coenzyme Q to

cytochromes (in the order), b, c1, c, and a3. The property ofcytochromes (in the order), b, c1, c, and a3. The property ofreversible oxidation reduction of heme iron Fereversible oxidation reduction of heme iron Fe2+2+ ====FeFe3+3+ present in cytochomes allows them to function aspresent in cytochomes allows them to function aseffective carries of electron in ETC.effective carries of electron in ETC.


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Cytochrome c (mol wt. 13,000)is a small proteinCytochrome c (mol wt. 13,000)is a small proteincontaining 104 amino acids and a heme group. It iscontaining 104 amino acids and a heme group. It isa central member of etc with an intermediatea central member of etc with an intermediate

redox potential. It is rather loosely bound to innerredox potential. It is rather loosely bound to innermitochondrial membrane and can be easilymitochondrial membrane and can be easilyextracted .extracted .

Cytochrome a and a3 : the term cytochromeCytochrome a and a3 : the term cytochromeoxidase is frequently used to collectively representoxidase is frequently used to collectively represent

cytochrome a and a3 which is the terminalcytochrome a and a3 which is the terminalcomponent of ETC. cytochrome oxidase is thecomponent of ETC. cytochrome oxidase is theonlyelectron carrier,the heme iron of which canonlyelectron carrier,the heme iron of which candirectly react with molecular oxygen beside hemedirectly react with molecular oxygen beside heme(with iron), this oxidase also contains copper that(with iron), this oxidase also contains copper thatundergoes oxidation-reduction (Cu2+ == Cu+)undergoes oxidation-reduction (Cu2+ == Cu+)during the transports of electrons.during the transports of electrons.

In the final stage of ETC, the transported electrons,In the final stage of ETC, the transported electrons,the free protons and the molecular oxygenthe free protons and the molecular oxygencombine to produce water.combine to produce water.


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Oxidative PhosphorylationOxidative Phosphorylation The transport of electrons though the ETC isThe transport of electrons though the ETC is

linked with the release of free energy. Thelinked with the release of free energy. Theprocess of synthesizing ATP from ADP and piprocess of synthesizing ATP from ADP and picoupled with the electron transport chain iscoupled with the electron transport chain isknown as oxidative phosphorylation. Theknown as oxidative phosphorylation. Thecomplex V (see fig 11.6 ) of the linercomplex V (see fig 11.6 ) of the linermitochondrial membrane in the site of oxidativemitochondrial membrane in the site of oxidativephosphorylation.phosphorylation.

P. O Ratio;P. O Ratio; The P : O ratio refer to the number of inorganicThe P : O ratio refer to the number of inorganic

phosphate molecules utilized for ATPphosphate molecules utilized for ATPgeneration for every atom of oxygen consumed.generation for every atom of oxygen consumed.

More appropriately, P : O ratio represents theMore appropriately, P : O ratio represents thenumber of molecules of ATP synthesized pernumber of molecules of ATP synthesized perpair of electrons carried through ETC.pair of electrons carried through ETC.


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The mitochondrial oxidation of NADH with a PThe mitochondrial oxidation of NADH with a P: O ratio of 3 can be represented by the: O ratio of 3 can be represented by thefollowing equation :following equation :

NADH + H +NADH + H + 11 O2 + 3ADP 3Pi NAD+ + 3ATP 4H2OO2 + 3ADP 3Pi NAD+ + 3ATP 4H2O

22

P : O ratio of 2 is assigned to the oxidation ofP : O ratio of 2 is assigned to the oxidation of

FADH2. (Note : Although yet to be provedFADH2. (Note : Although yet to be provedbeyond doubt, some workers suggest a P : Obeyond doubt, some workers suggest a P : Oratio of 2.5 for NADH + H+, and 1.5 forratio of 2.5 for NADH + H+, and 1.5 forFADH2, based on the proton translocation.FADH2, based on the proton translocation.


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Sites of Oxidative Phosphorylation in ETC:Sites of Oxidative Phosphorylation in ETC:

There are three reactions in the ETC that areThere are three reactions in the ETC that areexergonic to result in the syntehsisi of 3 ATPexergonic to result in the syntehsisi of 3 ATP

molecules. The three sites of ATP formation inmolecules. The three sites of ATP formation inETC areETC are

Oxidation of FMNH2 by coenzyme Q.Oxidation of FMNH2 by coenzyme Q.

Oxidation of cytochrome b by cytochrome c1.Oxidation of cytochrome b by cytochrome c1.

Cytochrome oxidase reaction.Cytochrome oxidase reaction. Each one of the above reactions represents aEach one of the above reactions represents a

coupling site for ATP production. There are onlycoupling site for ATP production. There are onlytwo coupling sites foe oxidation of FADH2 (P : Otwo coupling sites foe oxidation of FADH2 (P : Oratio 2), since the first site is bypassed.ratio 2), since the first site is bypassed.


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Energetics of Oxidative Phsophorylation:Energetics of Oxidative Phsophorylation: The transport of ellerons from redox pairThe transport of ellerons from redox pair

NAD+/NADH (Eo= 0.32) to finally the redox pair NAD+/NADH (Eo= 0.32) to finally the redox pair

O2 H2O (Eo = + 0.82) may be simplified andO2 H2O (Eo = + 0.82) may be simplified andrepresented in the following equation.represented in the following equation.

O2 + NADH + H+ H2O + NAD+ O2 + NADH + H+ H2O + NAD+

The redox potential difference between these twoThe redox potential difference between these tworedox pairs is 1.14 V, which is equivalent to anredox pairs is 1.14 V, which is equivalent to anenergy 52 Cal/mol.energy 52 Cal/mol.

Three ATP are synthesized in the ETC when NADH isThree ATP are synthesized in the ETC when NADH isoxidized which equals to 21.9 Cal (each ATP = 7.3oxidized which equals to 21.9 Cal (each ATP = 7.3Cal.). The efficiency of energy conservation isCal.). The efficiency of energy conservation iscalculated ascalculated as


39/39

21.9 x 10021.9 x 100 = 42%= 42%

5252

Therefore, when NADH is oxidized, Therefore, when NADH is oxidized,

about 42% of energy is trapped in theabout 42% of energy is trapped in the

form of 3 ATP and the remaining isform of 3 ATP and the remaining is

lost as heat. The heat liberation is notlost as heat. The heat liberation is nota wasteful process, since it allows ETCa wasteful process, since it allows ETC

to go on continuously to generateto go on continuously to generate

ATP. Further, this heat is necessary toATP. Further, this heat is necessary tomaintain body temperature.maintain body temperature.

bioenergetics. 03sep2009(d)

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