1308 Energy

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Energy and Metabolism

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The Energy of Life

The living cell generates thousands of

different reactions

Metabolism

Is the totality of an organisms chemical

reactions

Arises from interactions between

molecules

An organisms metabolism transforms matter

and energy, subject to the laws of

thermodynamics

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Metabolic Pathways

Biochemical pathways are the organizational units of metabolism

Metabolism is the total of all chemical reactions carried out by anorganism

A metabolic pathway has many steps that begin with a specificmolecule and end with a product, each catalyzed by a specific enzyme

Reactions that join small molecules together to form larger, morecomplex molecules are called anabolic.

Reactions that break large molecules down into smaller subunits arecalled catabolic.

Enzyme 1 Enzyme 2 Enzyme 3

A B C D

Reaction 1 Reaction 2 Reaction 3

Starting

molecule

Product

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Metabolic Pathway A sequence of chemical reactions, where the product of

one reaction serves as a substrate for the next, is called ametabolic pathway or biochemical pathway

Most metabolic pathways take place in specific regions ofthe cell.

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Bioenergetics

Bioenergetics is the study of how organismsmanage their energy resources via

metabolic pathways

Catabolic pathways release energy bybreaking down complex molecules into

simpler compounds

Anabolic pathways consume energy to build

complex molecules from simpler ones

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Energy

Energy is the capacity to do work or ability tocause change. Any change in the universe

requires energy. Energy comes in 2 forms:

Potential energy is stored energy. Nochange is currently taking place

Kinetic energy is currently causing

change. This always involves some type

of motion.

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Form s of Energy

Kinetic energy is theenergy associated withmotion

Potential energy

Is stored in the

location of matter Includes chemical

energy stored inmolecular structure

Energy can be convertedfrom one form to another

On the platform, a diver

has more potential energy.

Diving converts potential

energy to kinetic energy.

Climbing up converts kinetic

energy of muscle movement

to potential energy.

In the water, a diver has

less potential energy.

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Laws of Energy Transformation

Thermodynamics is the study of energychanges.

Two fundamental laws govern all energy

changes in the universe. These 2 laws aresimply called the first and second laws of

thermodynamics:

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The First Law o f Thermodynam ics

According to the first law of thermodynamics

Energy cannot be created or destroyed

Energy can be transferred and transformed

For example, the chemical (potential) energy

in food will be converted to the kinetic

energy of the cheetahs movement

Chemical

energy

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Second Law of Thermodynamics The disorder (entropy) in the universe is continuously increasing.

Energy transformations proceed spontaneously to convert matter from a

more ordered, less stable form, to a less ordered, more stable form Spontaneous changes that do not require outside energy increase the

entropy, or disorder, of the universe

For a process to occur without energy input, it must increase the entropy of

the universe

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During each conversion, some of the energy dissipates into theenvironment as heat.

During every energy transfer or transformation, some energy isunusable, often lost as heat

Heat is defined as the measure of the random motion of molecules

Living cells unavoidably convert organized forms of energy to heat

According to the second law of thermodynamics, every energytransfer or transformation increases the entropy (disorder) of theuniverse

Second Law of Thermodynamics

For example, disorder is added to the cheetahssurroundings in the form of heat and

the small molecules that are the by-products of metabolism.

Heat co2

H2O

+

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B iolog ical Order and Diso rder

Cells create ordered structures from lessordered materials

Organisms also replace ordered forms of

matter and energy with less ordered forms

The evolution of more complex organisms does

not violate the second law of thermodynamics

Entropy (disorder) may decrease in an

organism, but the universes total entropyincreases

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B iolog ical Order and Diso rder

Living systems

Increase the entropy of the universe Use energy to maintain order

A living systems free energy is energy that can

do work under cellular conditions Organisms live at the expense of free energy

50m

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Free Energy

Free energyis the portion of a systems energy that is able

to perform work when temperature and pressure is uniformthroughout the system, as in a living cell

Free energy also refers to the amount of energy actuallyavailable to break and subsequently form other chemicalbonds

Gibbs free energy (G) in a cell, the amount of energycontained in a molecules chemical bonds (T&P constant)

Change in free energy -G

Endergonic - any reaction that requires an input ofenergy

Exergonic - any reaction that releases free energy

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Exergonic reactions Reactants have more free energy than the products

Involve a net release of energy and/or an increase inentropy

Occur spontaneously (without a net input of energy)

Reactants

Products

Energy

Progress of the reaction

Amount of

energy

released

(G

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Endergonic Reactions

Reactants have less free energy than the products

Involve a net input of energy and/or a decrease in

entropy

Do not occur spontaneously

Energy

Products

Amount of

energy

released

(G>0)

Reactants


Freeenergy

(b) Endergonic reaction: energy required

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Reactant Reactant

Product

Product

ExergonicEndergonic

Energy isreleased.

Energymust be

supplied.Energy

supplied

Energ

y

released

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Equ i libr ium and Metabo l ism

Reactions in a closed system eventually reachequilibrium and then do no work

Cells are not in equilibrium; they are open

systems experiencing a constant flow ofmaterials

A catabolic pathway in a cell releases free

energy in a series of reactions

Closed and open hydroelectric systems can

serve as analogies

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An Analogy For Cellular Respiration

Glucose Catabolism

(c) A multistep open hydroelectric system.Cellular respiration isanalogous to this system: Glucoce is brocken down in a series

of exergonic reactions that power the work of the cell. The product

of each reaction becomes the reactant for the next, so no reaction

reaches equilibrium.

G< 0

G< 0

G< 0

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Energy Coupling

Living organisms have the ability to coupleexergonic and endergonic reactions:

Energy released by exergonic reactions is

captured and used to make ATP from ADP

and Pi

ATP can be broken back down to ADP and

Pi, releasing energy to power the cells

endergonic reactions.

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The Structure and Hydrolysis of ATP

ATP (adenosine triphosphate) Is the cells energy shuttle

Provides energy for cellular functions

O O O O CH2

H

OH OH

H

N

H H

O

NC

HC

N CC

N

NH2Adenine

RibosePhosphate groups

O

O O

O

O

O

-

- - -

CH

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Cellular Work

A cell does three main kinds of work Mechanical

Transport

Chemical Energy coupling is a key feature in the way cells

manage their energy resources to do this work

ATP powers cellular work by coupling exergonic

reactions to endergonic reactions

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Energy Coupling - ATP / ADP Cycle

Releasing the third phosphate from ATP to make ADP generates

energy (exergonic):

Linking the phosphates together requires energy - so making ATP from

ADP and a third phosphate requires energy (endergonic),

Catabolic pathways drive the regeneration of ATP from ADP and

phosphate

ATP synthesis from

ADP + P irequires energy

ATP

ADP + P i

Energy for cellular work

(endergonic, energy-

consuming processes)

Energy from catabolism

(exergonic, energy yielding

processes)

ATP hydrolysis toADP + P iyields energy

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How ATP Performs Work

ATP drives endergonic reactions byphosphorylation, transferring a phosphate

group to some other molecule, such as a

reactant

The recipient molecule is now phosphorylated

The three types of cellular work (mechanical,

transport, and chemical) are powered by the

hydrolysis of ATP

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How ATP Performs Work ATP drives endergonic reactions by phosphorylation, transferring a

phosphate to other molecules - hydrolysis of ATP

(c) Chemical work: ATP phosphorylates key reactants

P

Membrane

protein

Motor protein

P i

Protein moved

(a) Mechanical work: ATP phosphorylates motor proteins

ATP

(b) Transport work: ATP phosphorylates transport proteins

Solute

P P i

transportedSolute

Glu GluNH3

NH2P i

P i

+ +

Reactants: Glutamic acid

and ammoniaProduct (glutamine)

made

ADP+

P

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Activation Energy

All reactions, both endergonic and exergonic, require an input of energy

to get started. This energy is called activation energy

The activation energy, EA Is the initial amount of energy needed to start a chemical reaction

Activation energy is needed to bring the reactants close together and

weaken existing bonds to initiate a chemical reaction.

Is often supplied in the form of heat from the surroundings in a system.

Copyright The McGraw-Hill Companies, Inc. Permission required for reproduction or display.

Freeenergy


G < O

EAA B

C D

Reactants

A

C D

B

Transition state

A B

C D

Products

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Reaction Rates

In most cases, molecules do not have enough

kinetic energy to reach the transition state whenthey collide.

Therefore, most collisions are non-productive, and

the reaction proceeds very slowly if at all.

What can be done to speed up these reactions?

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Increasing Reaction Rates

Add Energy (Heat) - molecules move faster so they collide

more frequently and with greater force. Add a catalysta catalyst reduces the energy needed to

reach the activation state, without being changed itself.

Proteins that function as catalysts are called enzymes.

Reactant

Product

CatalyzedUncatalyzed

Product

Reactant

Activationenergy

Activationenergy

Ene

rgy

supplied

Energy

released

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Activation Energy and Catalysis

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Enzymes Lower the EABarrier

An enzyme catalyzes reactions by lowering

the EAbarrier


Products

Course of

reaction

without

enzyme

Reactants

Course ofreaction

with enzyme

EA

withoutenzyme

EA with

enzyme

is lower

Gis unaffectedby enzyme

Free

energy

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Substrate Specificity of Enzymes Almost all enzymes are globular proteins with one or more active sites on their

surface.

The substrate is the reactant an enzyme acts on Reactants bind to the active siteto form an enzyme-substrate complex.

The 3-D shape of the active site and the substrates must match, like a lock andkey

Binding of the substrates causes the enzyme to adjust its shape slightly, leadingto a better induced fit.

Induced fit of a substrate brings chemical groups of the active site into positions

that enhance their ability to catalyze the chemical reaction When this happens, the substrates are brought close together and existing

bonds are stressed. This reduces the amount of energy needed to reach thetransition state.

Substate

Active site

Enzyme

Enzyme- substrate

complex

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The Catalytic Cycle Of An Enzyme

Substrates

Products

Enzyme

Enzyme-substrate

complex

1 Substrates enter active site; enzyme

changes shape so its active site

embraces the substrates (induced fit).

2 Substrates held in

active site by weak

interactions, such as

hydrogen bonds and

ionic bonds.

3 Active site (and R groups of

its amino acids) can lower EA

and speed up a reaction by

acting as a template for

substrate orientation,

stressing the substrate bonds

and stabilizing the

transition state,

providing a favorablemicroenvironment,

participating directly in the

catalytic reaction.

4 Substrates are

Converted intoProducts.

5 Products are

Released.

6Active site

Is available for

two new substrate

Mole.

Figure 8.17

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1 The substrate, sucrose, consists

of glucose and fructose bonded together.

Bond

Enzyme

Active site

The substrate binds to the enzyme,

forming an enzyme-substrate

complex.

2

H2O

The binding of the substrate

and enzyme places stress onthe glucose-fructose bond,

and the bond breaks.3

Glucose Fructose

Products are

released, and the

enzyme is free to

bind other

substrates.

4

The Catalytic Cycle Of An Enzyme

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Factors Affecting Enzyme Activity

Temperature - rate of an enzyme-catalyzed

reaction increases with temperature, but only

up to an optimum temperature.

pH - ionic interactions also hold enzymes

together.

Inhibitors and Activators

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Effects o f Temperatu re and pH

Each enzyme has an optimal temperature in

which it can function

Optimal temperature for

enzyme of thermophilic

Rateofreaction

0 20 40 80 100Temperature (C)

(a) Optimal temperature for two enzymes

Optimal temperature for

typical human enzyme

(heat-tolerant)bacteria

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Effects o f Temperatu re and pH

Each enzyme has an optimal pH in which

it can function

Figure 8.18

Rateofreaction

(b) Optimal pH for two enzymes

Optimal pH for pepsin

(stomach enzyme)Optimal pH

for trypsin

(intestinalenzyme)

10 2 3 4 5 6 7 8 9

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Inhibitor - substance that binds to an enzyme

and decreases its activityfeedback

Competitive inhibitors - compete with the

substrate for the same active site

Noncompetitive inhibitors - bind to the

enzyme in a location other than the active

site

Allosteric sites - specific binding sites

acting as on/off switches

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Enzyme Inh ibi tors

Competitive inhibitors bind to the active site of an enzyme,

competing with the substrate

(b) Competitive inhibition

A competitiveinhibitor mimics the

substrate, competing

for the active site.

Competitive

inhibitor

A substrate can

bind normally to the

active site of an

enzyme.

Substrate

Active site

Enzyme

(a) Normal binding

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Enzyme Inh ibi tors

Noncompetitive inhibitors bind to anotherpart of an enzyme, changing the function

A noncompetitive

inhibitor binds to the

enzyme away from

the active site, altering

the conformation of

the enzyme so that its

active site no longer

functions.

Noncompetitive inhibitor

(c) Noncompetitive inhibition

R l ti Of E A ti it H l

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Regulation Of Enzyme Activity Helps

Control Metabolism

Chemical chaos would result if a cells metabolicpathways were not tightly regulated

To regulate metabolic pathways, the cell switches

on or off the genes that encode specific enzymes

Allosteric regulation is the term used to describe

any case in which a proteins function at one site is

affected by binding of a regulatory molecule at

another site Enzymes change shape when regulatory

molecules bind to specific sites, affecting function

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A l los ter ic Act ivation and Inh ibi t ion

Most allosterically regulated enzymes aremade from polypeptide subunits

Each enzyme has active and inactive forms

The binding of an activator stabilizes the activeform of the enzyme

The binding of an inhibitor stabilizes theinactive form of the enzyme

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Allosteric Regulation of Enzymes Allosteric regulation may either inhibit or stimulate an

enzymes activity

Stabilized inactiveform

Allosteric activater

stabilizes active fromAllosteric enyzme

with four subunitsActive site

(one of four)

Regulatory

site (oneof four)

Active form

Activator

Stabilized active form

Allosteric activater

stabilizes inactive form

InhibitorInactive formNon-

functional

active

site

(a) Allosteric activators and inhibitors.In the cell, activators and inhibitors

dissociate when at low concentrations. The enzyme can then oscillate again.

Oscillation

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Cooperativity

Is a form of allosteric regulation that can amplify

enzyme activity

Binding of one substrate molecule to

active site of one subunit locks

all subunits in active conformation.

Substrate

Inactive form Stabilized active form

(b)Cooperativity: another type of allosteric activation.Note that the

inactive form shown on the left oscillates back and forth with the active

form when the active form is not stabilized by substrate.

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Activators - substances that bind to allosteric

sites and keep the enzymes in their active

configurations - increases enzyme activity

Cofactors - chemical components that

facilitate enzyme activity

Coenzymes - organic molecules that

function as cofactors

R l ti f Bi h i l P th

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Regulation of Biochemical Pathways

Biochemical pathways must be coordinated

and regulated to operate efficiently.

Advantageous for cell to temporarily shut

down biochemical pathways when their

products are not needed

Feedback Inhibition

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Feedback Inh ibi t ion

In feedback inhibition theend product of a metabolic

pathway shuts down the

pathway

When the cell producesincreasing quantities of a

particular product, it

automatically inhibits its

ability to produce more

Active siteavailable

Isoleucine

used up by

cell

Feedback

inhibition

Isoleucine

binds toallosteric

site

Active site of

enzyme 1 no

longer binds

threonine;

pathway is

switched off

Initial substrate

(threonine)

Threonine

in active site

Enzyme 1

(threonine

deaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product

(isoleucine)

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1308 Energy