An Introduction to Metabolism

A.P. Biology

Chapter 8

Mr. Knowles

Liberty High School

Why do you eat?What is the purpose of a

food chain?

Answer: Energy and Organic Building Blocks

Bioenergetics• Bioenergetics- how energy

behaves and changes in living systems (food chains).

• Metabolism = Anabolism (require energy to make new bonds) + Catabolism (release energy from bonds)

Energy-The Stuff of Life!• Energy – the capacity to do work.

• Two States of Energy:

–Kinetic Energy- energy of motion.

–Potential Energy- stored energy.

Potential Kinetic Potential

Forms of energy?• Chemical• Mechanical• Electrical• Light• HEAT• Nuclear• Biology – mostly potential chemical and kinetic

chemical; light; heat (from chemical bonds)

• Some organisms – Convert chemical energy to light, as in

bioluminescence

Figure 8.1

Measuring Energy in Biology• In terms of heat changes

(thermodynamics).• Unit of heat in biology – calorie.• 1.0 calorie = amount of heat req’d to

raise 1.0g of H20, 1° C.• In biology, 1.0 kilocalorie (kcal) =

1,000.0 calories (cal)• 1,000 cal = 1.0 kcal. = 1.0 Calorie

• Energy can be converted from one form to anotherOn the platform, a diverhas more potential energy.

Diving converts potentialenergy to kinetic energy.

Climbing up converts kinetic

energy of muscle movement

to potential energy.

In the water, a diver has less potential energy.

Figure 8.2

Photosynthesis • Conversion of unusable light energy

into usable chemical energy.

C6H12O6

Redox Reactions

PhotosynthesisLight

6 CO2 + 12 H2O 6 O2 + 6 H2O

C6H12O6

Stores 686 kcal/mol

Photosynthesis is a Redox Reaction

6 CO2 + 12 H2O 6 O2 + 6 H2O

C6H12O6

H

Cellular Respiration

C6H12O6 + 6 O2 6 CO2 + 6 H2O +

ENERGY (ATP)

686 kcal/mole glucose

Cellular Respiration is a Redox Reaction

C6H12O6 + 6 O2 6 CO2 + 6 H2O +

ENERGY (ATP) 686 kcal/mole glucoseH

First Law of Thermodynamics• Energy cannot be created nor

destroyed, but it can be converted from one form to another.

• Ex. Food chains

An example of energy conversion

Figure 8.3 

First law of thermodynamics: Energy can be transferred or transformed but Neither created nor destroyed. For example, the chemical (potential) energy in food will be converted to the kinetic energy of the cheetah’s movement in (b).

(a)

Chemicalenergy

First Law of ThermodynamicsProducers (Light Energy Potential

Chemical)

1st Order Consumer or Herbivore (Potential Chemical Potential, Kinetic Chemical)

2nd Order Consumer or Carnivore (Potential Chemical Potential, Kinetic Chemical)

HEAT

HEAT

The Second Law of Thermodynamics• According to the second law of thermodynamics

– Spontaneous changes that do not require outside energy increase the entropy, or disorder, of the universe

Figure 8.3 

Second law of thermodynamics: Every energy transfer or transformation increasesthe disorder (entropy) of the universe. For example, disorder is added to the cheetah’ssurroundings in the form of heat and the small molecules that are the by-productsof metabolism.

(b)

Heat co2

H2O

+

Biological Order and Disorder• Living systems

– Increase the entropy of the universe– Use energy to maintain order

50µm

Figure 8.4

2nd Law of Thermodynamics• As energy is transformed from one

state to another, some of the energy is lost as heat (random molecular motion) and the ability to do work decreases.

• Disorder (Entropy) in the universe is increasing.

• Order Disorder

Show me the 2nd Law of Thermodynamics in

Action

Video: Predators: How They Hunt, Cheetahs

Free-Energy Change, G

• A living system’s free energy

– Is energy that can do work under cellular conditions

• At maximum stability– The system is at equilibrium

Chemical reaction. In a cell, a sugar molecule is broken down into simpler molecules.

.

Diffusion. Molecules in a drop of dye diffuse until they are randomly dispersed.

Gravitational motion. Objectsmove spontaneously from ahigher altitude to a lower one.

• More free energy (higher G)• Less stable• Greater work capacity

• Less free energy (lower G)• More stable• Less work capacity

In a spontaneous change • The free energy of the system decreases (∆G<0) • The system becomes more stable• The released free energy can be harnessed to do work

(a) (b) (c)

Figure 8.5 

Chemical Bonds have Potential Energy

• When bonds are broken, some of the energy is lost as heat and is not available to do work (form new chemical bonds).

• Free Energy (G) – the amt. of energy in a bond available to do work.

• In a molecule, the amt. of Free Energy (G) = the amt. of energy in chemical bond (Enthalpy, H) – the energy lost as disorder (Entropy, S). G = H - TS

Free Energy of Chemical Rxns.

• ΔG = difference in bond energies between reactants and products of a chemical reaction.

• Δ G = ΔH – TΔS• If ΔG is negative (products contain less

energy than reactants) = Exergonic Reactions. Ex. Cellular Respiration

• If ΔG is positive (products contain more energy than reactants) = Endergonic Reactions. Ex. Photosynthesis

C6H12O6 + O2

CO2 + H2OFre

e E

ner

gy

G

Rxn Time

Cellular Respiration

HEAT

CO2 + H2OFre

e E

ner

gy

G

Rxn Time

Photosynthesis

C6H12O6 + O2

Light Energy Supplied

Exergonic and Endergonic Reactions in Metabolism

• An exergonic reaction– Proceeds with a net release of free energy and is

spontaneous

Figure 8.6

Reactants

Products

Energy

Progress of the reaction

Amount ofenergyreleased (∆G <0)

Fre

e e

ne

rgy

(a) Exergonic reaction: energy released

An endergonic reaction:

– Is one that absorbs free energy from its surroundings and is nonspontaneous

Figure 8.6

Energy

Products

Amount ofenergyreleased (∆G>0)

Reactants

Progress of the reaction

Fre

e e

ne

rgy

(b) Endergonic reaction: energy required

Equilibrium and Metabolism• Reactions in a closed system

– Eventually 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

• Cells in our body– Experience a constant flow of materials in and

out, preventing metabolic pathways from reaching equilibrium

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

An analogy for cellular respiration

Figure 8.7 (c) A multistep open hydroelectric system. Cellular respiration is analogous 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

Concept 8.3: ATP powers cellular work by coupling exergonic reactions to endergonic reactions

• 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

The Structure and Hydrolysis of ATP• ATP (adenosine triphosphate)

– Is the cell’s energy shuttle– Provides energy for cellular functions

Figure 8.8

O O O O CH2

H

OH OH

H

N

H H

O

NC

HC

NC

C

N

NH2Adenine

RibosePhosphate groups

O

O O

O

O

O

-

- - -

CH

• Energy is released from ATP– When the terminal phosphate bond is broken

Figure 8.9

P

Adenosine triphosphate (ATP)

H2O

+ Energy

Inorganic phosphate Adenosine diphosphate (ADP)

PP

P PP i

ATP hydrolysis:

Can be coupled to other reactionsEndergonic reaction: ∆G is positive, reaction is not spontaneous

∆G = +3.4 kcal/molGlu Glu

∆G = + 7.3 kcal/molATP H2O+

+ NH3

ADP +

NH2

Glutamicacid

Ammonia Glutamine

Exergonic reaction: ∆ G is negative, reaction is spontaneous

P

Coupled reactions: Overall ∆G is negative; together, reactions are spontaneous ∆G = –3.9 kcal/molFigure 8.10

How ATP Performs Work

• ATP drives endergonic reactions:– By phosphorylation, transferring a phosphate to

other molecules

The three types of cellular work– Are powered by the hydrolysis of ATP

(c) Chemical work: ATP phosphorylates key reactants

P

Membraneprotein

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

GluGlu

NH3

NH2

P i

P i

+ +

Reactants: Glutamic acid and ammonia

Product (glutamine)made

ADP+

P

Figure 8.11

The Regeneration of ATP• Catabolic pathways

– Drive the regeneration of ATP from ADP and phosphate

ATP synthesis from ADP + P i requires energy

ATP

ADP + P i

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

Energy from catabolism(exergonic, energy yieldingprocesses)

ATP hydrolysis to ADP + P i yields energy

Figure 8.12

If all Exergonic Reactions happen spontaneously…

…then how come all of them haven’t already

happened?

Activation Energy!

We need catalysts!

Biological Catalysts Are Enzymes

Activation EnergyE

ner

gy

C6H12O6 +O2

CO2 + H20

Reaction Time

EnergyActivation

Energy

Enzymes are Proteins• Organic catalysts - increase the

rate of chemical reactions in cells.

• Hold reactant molecules close together for reaction to occur- uses an active site.

• The active site is used to bind the reactant molecules-substrate.

The Lock and Key Model

Enzyme Binding

The active site:

– Is the region on the enzyme where the substrate binds.

Figure 8.16

Substate

Active site

Enzyme

(a)

• Induced fit of a substrate:– Brings chemical groups of the active site into

positions that enhance their ability to catalyze the chemical reaction

Figure 8.16 (b)

Enzyme- substratecomplex

The catalytic cycle of an enzyme

Substrates

Products

Enzyme

Enzyme-substratecomplex

1 Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).

2 Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.

3 Active site (and R groups ofits amino acids) can lower EA

and speed up a reaction by• acting as a template for substrate orientation,• stressing the substrates and stabilizing the transition state,• providing a favorable microenvironment,• participating directly in the catalytic reaction.

4 Substrates are Converted intoProducts.

5 Products areReleased.

6 Active siteIs available fortwo new substrateMole.

Figure 8.17

Conditions that Affect Protein ShapeCan disrupt H bonds by:

• High Temperature

• pH Changes (Acidic or Basic)

• Salt (or Ion) Concentration

• Binding of Regulatory Molecules

Disrupting the 2°, 3°, 4° structure is called denaturation.

Enzymes• Enzymes have a temperature

optimum.• Too Cold - H bonds and van der

Waals forces aren’t flexible enough to allow the induced fit for catalysis.

• Too Hot - they are too weak to maintain the enzymes shape and break apart.

Rat

e of

Rea

ctio

n

Rxn Temperature, °C

3735 3933 434131

Most Human Enzyme Temp. Optimums

Male Reprod. Enzymes

Siamese Cats and Pigment Enzymes

3 wks old

4 yrs. old

Show me a harp seal!

Video: Discovery-Blue Planet: Frozen Seas

Pigment Enzymes in Harp Seal Pups

Show me an application!

Video: Discovery-The Science of the Sexes

Rat

e of

Rea

ctio

n

Rxn pH

54 63 72

Most Human Enzyme pH Optimums

8

TrypsinPepsin

Who cares about optimum conditions for enzymes?

Inhibitors and Activators• Inhibitors – are molecules (usually proteins)

that bind to an enzyme and decrease its activity.

• Two Kinds:– A. Competitive Inhibitors – compete for

the same active site as the substrate; displaces some of the substrate.

– B. Noncompetitive Inhibitors – bind to the enzyme in some other location other than its active site., changing its shape.

Enzyme Inhibitors• Competitive inhibitors

– Bind to the active site of an enzyme, competing with the substrate

Figure 8.19 (b) Competitive inhibition

A competitiveinhibitor mimics the

substrate, competingfor the active site.

Competitiveinhibitor

A substrate canbind normally to the

active site of anenzyme.

Substrate

Active site

Enzyme

(a) Normal binding

Competitive Inhibition

Competitive Inhibition

Noncompetitive inhibitors– Bind to another part of an enzyme, changing the

function

Figure 8.19

A noncompetitiveinhibitor binds to the

enzyme away fromthe active site, altering

the conformation ofthe enzyme so that its

active site no longerfunctions.

Noncompetitive inhibitor

(c) Noncompetitive inhibition

Noncompetitive Inhibition

Noncompetitive Inhibitors• These inhibitors bind to a site other

than the active site called the allosteric site (“other form”).

• Chemical “on/off” switches.• Called Allosteric Inhibitor.• If a molecule binds to an allosteric site

and keeps the enzyme in its active shape – Allosteric Activator.

Enzymes change shape when regulatory molecules bind to specific sites, affecting function

Stabilized inactiveform

Allosteric activaterstabilizes active fromAllosteric enyzme

with four subunitsActive site

(one of four)

Regulatorysite (oneof four)

Active formActivator

Stabilized active form

Allosteric activaterstabilizes active form

InhibitorInactive formNon-functionalactivesite

(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again.

Oscillation

Figure 8.20

Another Look at Allosteric Inhibitors

Allosteric Activation

Things that help Enyzmes• Molecules that bind to the active site and HELP

the enzyme’s function – Cofactor.• Often metal ions, vitamins, etc.• If cofactor is a nonprotein organic molecule-

Coenzyme.• Coenzymes often serve as electron acceptors to

help break bonds.• Coenzymes then transfer the electrons to other

compounds. (Remember NAD+ and FAD+?

Enzymes Work with Co-Enzymes

Captures H+

An Example of a Biochemical Pathway

Feedback Inhibition

E1

E1

Substrate A Substrate B

E2

E2

Substrate C

Substrate CAllosteric Inhibitor

Feedback inhibitionActive siteavailable

Isoleucineused up bycell

Feedbackinhibition

Isoleucine binds to allosteric site

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