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Chapter 6

Metabolism: Energy and

Enzymes

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Energy: ability to do work or causechange required for all life processes

originates from sun

Forms of Energy

kinetic energy: energy of motion

potential energy: energy of

position; stored energy

6.1 Cells andEnergy Flow

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Fig. 6.1 Flow of energy

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Two Laws ofThermodynamicsenergy flows and does not cycle

First law of thermodynamics (law

of conservation of energy): energy

cannot be created or destroyed, but

it can be changed from one form toanother


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First law of thermodynamics, cont.energy transfers can be mechanical

(kinetic), heat (kinetic), light (kinetic)

sound (kinetic), electrical (kinetic orpotential), chemical (potential), etc.


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Second law of thermodynamics:energy cannot change forms without

a loss of useable energyheat flows from warmer to cooler

objects

PEF < PEIenergy transformations in closed

systems result in increased entropy

(disorder)


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Cells and EntropyEntropy: relative amount of

disorganizationas energy transformations occur,

energy is usually lost in the form of

heat

because of this loss of energy, living

things require an outside source of

energy (the sun) to maintain order


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Fig. 6.2a Entropy and glucose

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Fig. 6.2b Entropy anddiffusion

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Metabolism: all chemical reactionsin a cell (organism)some reactions are spontaneous,

some require energy inputHow can we tell which it will be?

Free energy: amount of energy

available to do work after a chemicalreaction; (G

6.2 Metabolic Reactions

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(G=(H-T(S (G= change in free energy

(H= change in heat (enthalpy)

- (H: heat released (exothermic) +(H: heat required (endothermic)

T= temperature (K)

(S= change in entropy


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exergonic reactions (G

energy released (usually as heat)

products have less free energy thanreactants (PEF < PEI)

spontaneous; favored


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endergonic reactions (G

energy absorbed (required)

products have more free energy thanreactants (PEF > PEI)

not favored


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in a reversible reaction, one direction isfavored (- (G) and the other is not (+(G)

example: A B forward is favored as indicated byarrows

if [B] is high enough, reaction will still

proceed backwards spontaneously


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An exergonic process

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ATP: Energy forCellsmany reactions are endergonic; ATP

(adenosine triphosphate) provides

the energy to drive themStructure:adenine ribose three phosphate groupsATPJADP + Pi + 7.3 kcal/mol


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ATPcycle overview

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Fig. 6.3 The ATPcycle

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The ATPcycle

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The ATPcycle

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The ATPcycle

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The ATPcycle

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The ATPcycle

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The ATPcycle

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ATP: Energy forCells, cont.Coupled Reactionsenergy released by breakdown of ATP

drives endergonic reactions, i.e., it isused to do work

Function of ATPchemical work (synthesis) transport work (membrane pumps)mechanical work (movement)


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Coupled reactions

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Metabolic pathway: series of linkedreactionsallows for better control, organization,

and for pathways to overlapenzymes are large, globular proteins

that catalyze a reaction without beingaffected by the reaction carry out reactions at high speed and

low temperature effective in small quantities

reusable

6.3 MetabolicPathways/Enzymes

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Energy of Activationeven in spontaneous (-(G)

reactions, energy is required to start

the reaction (activation energy, Ea)enzymes lower activation energy

without changing (G of the reaction

by bringing reactants (substrates)together


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Fig. 6.5 Energy of activation

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Energy of Activation, cont.exergonic reactions push and pull

endergonic reactions

works on Le Chateliers principle

A B C D(G (G (G

A B C DShifts equilibrium to the

right, reaction proceeds

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Enzyme-Substrate Complexactive site: part of enzyme that

binds with substrate(s)

old model: lock and keynew model: induced fit (active site

changes shape slightly to achieve

optimum fit)every reaction in a pathway requires

a specific enzyme, usually named

for its substrate and ending in ase


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Fig. 6.6 Enzymatic action

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Fig. 6.7 Induced fit model

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

active site

enzyme

Enzymatic action

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active site

enzyme enzyme-substratecomplex

substrate

Enzymatic action

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active site

enzyme enzyme-substratecomplex

substrate

enzyme

products

Enzymatic action

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active site

enzyme

active site

enzymeenzyme-substratecomplex

substrate

enzyme

products

Enzymatic action

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active site

enzyme

active site


substrate

enzyme

products

enzyme-substratecomplex

substrates

Enzymatic action

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active site

enzyme

active site


substrate

enzyme

products

enzyme-substratecomplex

substrates

enzyme

product

Enzymatic action

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Table 6.1

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Factors Affecting Enzymatic SpeedSubstrate Concentration rate will increase with increasing

substrate concentration until activesites are almost continually occupiedby substrates

Enzyme Concentration rate will increase with increasing

enzyme concentrationcells regulate enzyme concentration by

controlling gene expression


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Factors Affecting Enz. Speed, cont.Temperature and pH rate increases as temperature

increases until optimum temp. reached as temp. increases, molecular motion

increases as motion increases, more effective

collisions occurpast optimum temp., bonds become

weaker, shape changes, enzyme

denatures


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Fig. 6.9a Temp. effect on rate

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Fig. 6.9b,cTemp. effect on rate

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Fig. 6.9 Temp. effect on enzymes

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Factors Affecting Enz. Speed, cont.Temperature and pH, cont. rate increases

excess H+

or OH-

will cause ions tobind with enzymeenzyme chemically alteredstructure changes and enzyme wont

work (denatured)all enzymes have an optimal pH; for

most it is 6-8

why homeostasis of pH is important


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Fig. 6.10 Effect of pH on rate

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Factors Affecting Enz. Speed, cont.Metabolic Considerationsenzymes may require cofactors: low

MW, non-protein substances that allowenzyme to function often inorganic ions (copper, zinc,

iron) often organic molecules, then calledcoenzymes

often in active site to attract substrate

or contribute to reaction


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Factors Affecting Enz. Speed, cont.Metabolic Considerations, cont.vitamins are required in trace amounts

for synthesis of coenzymes relatively small organic molecules become part of coenzymes structure deficiency leads to decrease in

enzymatic activity


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Factors Affecting Enz. Speed, cont.Metabolic Considerations, cont.phosphorylation is used to activate

some enzymes enzymes called kinases do the

phosphorylating (and thereforeactivating)


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Factors Affecting Enz. Speed, cont.Metabolic Considerations, cont.enzyme inhibition helps conserve raw

materials and energy after enoughproduct is made usually reversible; enzyme not

damaged many poisons are irreversible

inhibitors (cyanide, penicillin,mercury, lead); enzyme permanently

disabled or damaged


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Factors Affecting Enz. Speed, cont.Metabolic Considerations, cont.enzyme inhibition, cont. c

ompetitive

inhibition

: inhibitorbinds enzymes active site,preventing substrate from entering

noncompetitive inhibition: inhibitor

binds to an allosteric site, changingthe shape of the enzyme and itsactive site


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Inhibition

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Allosteric Regulation

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Enzyme

Active Site

Substrate

Allosteric site

Allosteric Effector

Allosteric Interactions

All i I i

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Enzyme

Substrate


All t i I t ti

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Enzyme

Substrate


All t i I t ti

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Enzyme

Substrate


Substrate does not fit properly in active site.

All t i I t ti

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Enzyme

Substrate


Allosteric effector binds at the allosteric site

All t i I t ti

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Enzyme

Substrate


Active site changes shape so that the

substrate binds properly.

All t i I t ti

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Enzyme


Reaction is catalyzed.

All t i I t ti

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Enzyme

Product


Product is released. Effector and enzyme

can work again.

6 3 M t b li P th /E

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Factors Affecting Enz. Speed, cont.Metabolic Considerations, cont.enzyme inhibition, cont.

binding to an allosteric site can alsobe used to promote binding ofsubstrate to the active site

feedbackinhibition: final product of

a metabolic pathway inhibits anearlier reaction in the pathway(negative feedback)


F db k i hibiti

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Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5

Feedbackinhibition

F db k i hibiti

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Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5

E1active site

Feedbackinhibition

Feedback i hibitio

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Pathway. E1 has two sites: the active site wherereactantA binds and an allosteric site where end

product F binds.

Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5

E1active siteallosteric

site

Feedbackinhibition

Feedback inhibition

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product F binds.

Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5


siteE1

allosteric

site

first

reactant

A

Feedbackinhibition

Feedback inhibition

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product F binds.

Active Pathway. Reactant A binds to the activesite ofE1; therefore, the pathway is active and the

end product is produced.

Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5


siteE1

allosteric

site

first

reactant

A

Bend

product

F

C D E

E1 E2 E3 E4 E5

A

Feedbackinhibition

Feedback inhibition

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product F binds.



Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5


siteE1

allosteric

site

first

reactant

A

Bend

product

F

C D E

E1 E2 E3 E4 E5

A

E1active site

end

product

F

Feedbackinhibition

Feedback inhibition

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product F binds.



InhibitedPathway. When there is sufficient end product F,some binds to the allosteric site ofE1. Now a change of shape

prevents reactantA from binding to the active site ofE1, and

the end product is no longer produced.

Bend

product

F

C D Efirst

reactant

A

E1 E2 E3 E4 E5


siteE1

allosteric

site

first

reactant

A

Bend

product

F

C D E

E1 E2 E3 E4 E5

A

E1active site

end

product

F

E1

first

reactant

A

X

Feedbackinhibition

6 4 Redox and Flow of Energy

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Oxidation-reduction (redox)reactions involve a partial orcomplete transfer of electrons from

one molecule to another results in lower potential energysome of the energy can be harnessed

to make ATPoxidation and reduction always happen

at the same timeoccur in photosynthesis and cellular

respiration

6.4 Redox and Flow ofEnergy


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One molecule gives up electrons:

OxidizedOne molecule gains electrons:

Reduced



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LEO the lion goes GER!

Lose

Electrons (or H atoms)

Oxidation

Gain

Electrons (or H atoms)

Reduction


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Photosynthesis

6CO2+ 6H

2O + energy C

6H

12O

6+ 6O

2

What is oxidized and what isreduced?


Oxidation

Reduction


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Photosynthesis, cont.NADP+ carries electrons and a

hydrogen ion from water to carbon

dioxideNADP+ + 2e- + H+ NADPH



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Cellular Respiration

C6

H12

O6

+ 6O2 6CO

2

+ 6H2

O + energy

What is oxidized and what isreduced?


Oxidation

Reduction


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Cellular Respiration, cont.NAD+ carries electrons and a

hydrogen ion from water to carbon

dioxideNAD+ + 2e- + H+ NADH



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Electron Transport Chainused to make ATP in photosynthesis, chloroplasts use

solar energy to generate ATP in cellular respiration, mitochondria useglucose energy to generate ATP

electron transport chain is a series of

membrane-bound carriers that passelectrons from one to anotherelectrons enter with high E, leave with

low E; energy is released


Fig 6 12 ETC

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Fig. 6.12 ETC

Harnessing chemical energy

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Harnessing chemical energy

food

NADH

electron

transport

chain

oxygen


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ATP ProductionThe Story of Peter Mitchell the sodium-potassium pump moves

ions against a gradient and requiresATP

wild man Peter Mitchell wanted to see

if he could make the pump runbackwards


Na+/K+ Pump

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Na/K Pump

Na

+

K+

Na+

K+

ATP

ADP + PI

Mitchells Experiment

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Mitchell s Experiment

K+

Na+ADP + P

I

ATP

By setting up very high concentration gradients, he made

the pump run backwards and make ATP!


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ATP Production, cont.The Story of Peter Mitchell, cont.Mitchell discovered chemiosmoticcoupling using a hydrogen ion concentration

gradient to make ATP electron transport chain pumps

hydrogen ions to one side ofmembrane

this proton-motive force is used to

make ATP


Fig 6 13 Chemiosmosis

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Fig. 6.13 Chemiosmosis

Chemiosmosis

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Chemiosmosis

6.4 Redox and Flow of Energy

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ATP Production, cont.The Story of Peter Mitchell, cont.Proton-Motive Force separation of protons and electrons

results in a charge difference, V it also results in a pH difference, pHV+pH=proton-motive force protons want to move across

membrane, but the only way isthrough ATP synthase

flow of ions coupled to ATP synthesis


ATP synthase

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ATP synthase

ATP synthesis

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ATP synthesis

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