Bioenergetic Lecture

8/10/2019 Bioenergetic Lecture

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PRASETYASTUTIDEPARTMENT OFBIOCHEMISTRY

GADJAH MADA UNIVERSITY


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BIOENERGETIKA


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Bioenergetics, or biochemical thermodynamics,is the study of the energy changes accompanying

biochemical reactions.

Biologic systems are essentially isothermic

and use chemical energy to power living processes. Death from starvation occurs when available

energy reserves are depleted, and

marasmus malnutrition are associated withenergy imbalance .


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Thyroid hormones control the rate of energyrelease (metabolic rate), and disease results

when they malfunction.

Excess storage of surplus energy causesobesity, one of the most common diseases

of Western society.


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Gibbs change in free energy (G) is thatportion of the total energy change in a

system that is available for doing workie,the useful energy, also known as the

chemical potential

FREE ENERGY IS THE USEFUL ENERGYIN A SYSTEM


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The first law of thermodynamics

The total energy of a system, including itssurroundings, remains constant

Within the total system energy is

Neither lost nor gained during any change.

Energy may be

- transferred from one part of the system toanother or

- transformed into another form of energy.


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In living system,

Chemical energy may be transformed into

- heat- electrical,

- radiant,

- mechanical energy


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The second law of thermodinamics :

The total entropy of a system must increase ifa process is to occur spontaneously

Entropy : is the extent disorder of the systemand becomes maximum as equilibrium isapproached


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Under condition of constant temperature andpressure ----

The relationship between the free energychange (G) of a reacting system and thechange in entropy (S)

G = H TS

H : the change in entalpy (heat)

T : absolute temperature


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In biochemical reactions,

H E (the total change in internal energy )

--G = E - T S

If G is negative- the reaction proceeds spontaneously with loss

of free energy

- ie, it is exergonic


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If G is positive

The reaction proceeds only if free energy can

be gainedIe, It is endergonic

If G is zeroThe system is at equilibrium

When the consentration of reactan is 1.0 mol/L--is the standard free energy change (G o)


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G o: The standard free energy change atstandard state (pH of 7.0)

(G o)= -RT ln Keq

R : gas constant T : absolute temperature

G may be larger / smaller than G o

Depending on the concentrations of the various

reactants, including the solvent , various ion &protein.


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ENDERGONIC PROCESSES PROCEED BYCOUPLING TO EXERGONIC PROCESSES

The vital processeseg, synthetic reactions,muscular contraction, nerve impulse conduction,and active transportobtain energy by chemical

linkage, or coupling,to oxidative reactions.

The terms exergonic and endergonic rather thanthe normal chemical


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terms exothermic and endothermic are used

to indicate that a process is accompanied by loss

or gain, respectively, of free energy in any form,not necessarily as heat.

In practice, an endergonic process cannot exist

independently but must be a component of a

coupled exergonic-endergonic system where the

overall net change is exergonic.


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The exergonic reactions are termed catabolism

(generally, the breakdown or oxidation of fuel

molecules),the synthetic reactions are termed anabolism.

The combined catabolic and anabolic processes

constitute metabolism.


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Coupling ofanexergonic

to anendergonicreaction.


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An alternative method of coupling an exergonic

to an endergonic process is to synthesize a

compound of high-energy potential in theexergonic reaction and to incorporate this new

compound into the endergonic reaction,thus

effecting a transference of free energy fromthe exergonic to the endergonic pathway


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The biologic advantage of this mechanism is that

The compound of high potential energy, unlike I

E , to serve as a transducer of energy from aexergonic reactions to an endergonic reactions orprocesses (biosyntheses, muscular contraction,nervous excitation, and active transport).

In the living cell,

The principal high-energy intermediate or carrier

compound (designated

E) is

Adenosine triphosphate (ATP).


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Transfer of freeenergy from anexergonic

to an endergonicreaction via ahigh-energy

intermediate compound (E ).


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HIGH-ENERGY PHOSPHATES PLAY A CENTRAL ROLEIN ENERGY CAPTURE AND TRANSFER

to maintain living processes, all organismsmust obtain supplies of free energy from

their environment.

Autotrophic organisms utilize simple exergonicprocesses; eg,

the energy of sunlight (green plants),

The reaction Fe2+ Fe3+ (some bacteria).


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heterotrophic organisms obtain free energy by

- coupling their metabolism to the breakdown of

complex organic molecules in their environment.-

In all these organisms, ATP plays a central role

in the transference of free energy from theexergonic to the endergonic


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ATP is a nucleoside triphosphate containing

- adenine,

- ribose, and three phosphate groups.

In its reactions in the cell, it functions as the

Mg2+ complex


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Standard free energy of hydrolysis of someorganophosphates of biochemical mportance.


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Low-energy phosphates, exemplified by theester hosphates found in the intermediates of

glycolysis, have G0 values smaller than thatof ATP,

high-energy phosphates the value is higherthan that of ATP. (eg, the 1-phosphate of1,3-bisphosphoglycerate), enolphosphates (eg,phosphoenolpyruvate),and phosphoguanidines

(eg, creatine phosphate, arginine phosphate).


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Other high-energy compounds are

- thiol esters involving coenzyme A (eg, acetylCoA),

- Acyl carrier protein,- amino acid esters involved in protein synthesis,

- S-adenosylmethionine (active methionine),

- UDPGlc (uridine diphosphate glucose), and- PRPP (5-phosphoribosyl-1-pyrophosphate).


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Structure ofATP, ADP, andAMP showing

the position andthe number ofhigh-energyphosphates

(P ).

ATP contains twohigh energyphosphate groups

ADP contains one,

AMP is of the low-energy type,since it is anormal ester


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HIGH-ENERGY PHOSPHATES ACT AS THEENERGY CURRENCY OF THE CELL

ATP is able to act as a donor of high-energyphosphate

ADP can accept high-energy phosphate to

form ATP from those compounds above ATPin the table.

In effect, an ATP/ADP cycle connectsthose processes that generate P to those

processes that utilize P, continuously consuming and regenerating ATP.


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This occurs at a very rapid rate, since thetotal ATP/ADP pool is extremely small andsufficient to maintain an active tissue for

only a few seconds.

There are three major sources of P takingpart in energy conservation or energy

capture:(1) Oxidative phosphorylation:The greatest quantitative source of P in

aerobic organisms.Free energy comes from respiratory chain

oxidation using molecular O2 withinmitochondria


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(2) Glycolysis: A net formation of two P results

from the formation of lactate from one molecule

of glucose, generated in two reactions catalyzedby - phosphoglycerate kinase and

- pyruvate kinase,

(3) The citric acid cycle: One P is generated

directlyin the cycle at the succinyl thiokinase

step


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Role of ATP/ADP cycle intransfer of high-energyphosphate.


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Phosphagens act as storage forms of high-energy phosphate and include

- creatine phosphate, occurring in vertebrateskeletal muscle, heart, spermatozoa, and brain;and

- arginine phosphate, occurring in invertebratemuscle.

When ATP is rapidly being utilized as a source ofenergy for muscular contraction, phosphagens

permit its concentrations to be maintained,


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Transfer of high-energy phosphate between

ATP and creatine.


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ATP Allows the Coupling ofThermodynamicallyUnfavorable Reactions to Favorable Ones

The phosphorylation of glucose to glucose 6-phosphate,

the first reaction of glycolysis is highly

endergonic and cannot proceed underphysiologic conditions.

(1) Glucose+Pi Glucose 6- phosphate+ H2O

( G0 = +13.8 kJ/ mol)


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To take place, the reaction must be coupledwith anothermore exergonicreaction suchas the hydrolysis of the terminal phosphate ofATP.

(2) ATPADP+Pi (G0 = 30.5 kJ /mol)

When (1) and (2) are coupled in a reactioncatalyzed by hexokinase, phosphorylation of

glucose readily proceeds in a highly exergonicreaction that under physiologic conditions isirreversible.


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Adenylyl Kinase (Myokinase)Interconverts Adenine Nucleotides

This enzyme is present in most cells. Itcatalyzes the following reaction:

ATP + AMP


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This allows:

(1) High-energy phosphate in ADP to be used in thesynthesis of ATP.

(2) AMP, formed as a consequence of severalactivating reactions involving ATP, to be recoveredby rephosphorylation to ADP.

(3) AMP to increase in concentration when ATP

becomes depleted and act as a metabolic

(allosteric) signal to increase the rate of catabolicreactions, which in turn lead to the generation ofmore ATP


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Phosphate cycles and interchange of adenine nucleotides.

When ATP Forms AMP Inor anic


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When ATP Forms AMP, InorganicPyrophosphate (PPi) Is Produced

ATP + CoA- SH + RCOOH -AMP PPi +

RCO-SCoA

Catalyzed bt Acyl- CoA synthetase


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This reaction is accompanied by loss of freeenergy as heat, the activation reaction will go to

the right; and is further aided by the hydrolyticsplitting of PPi, catalyzed by inorganicpyrophosphatase, G0 of 27.6 kJ/mol.

activations via the pyrophosphate pathway

result in the loss of two P rather than one P asoccurs when ADP and Pi are formed.

PPi + H2O -----2Pi by inorganicpyrophosphatase


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Other Nucleoside Triphosphates Participate inthe Transfer of High-Energy Phosphate

the enzyme nucleoside diphosphate kinase, UTP, GTP, and CTP can be synthesized from

their diphosphates,

ATP + UDP ADP + UTP


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THANK YOU


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Bioenergetic Lecture