Enzymes

Forms of EnergyEnergy is the capacity to cause changeEnergy exists in various forms, some of which can

perform workKinetic energy is energy associated with motion

Heat (thermal energy) is kinetic energy associated with random movement of atoms or molecules

Potential energy is energy that matter possesses because of its location or structureChemical energy is potential energy available for

release in a chemical reactionEnergy can be converted from one form to another

The free-energy change of a reaction tells us whether the reaction occurs spontaneouslyBiologists want to know which reactions occur

spontaneously and which require input of energyTo do so, they need to determine energy changes

that occur in chemical reactionsA living system’s free energy is energy that can

do work when temperature and pressure are uniform, as in a living cell

Free EnergyThe change in free energy (∆G) during a process

is related to the change in enthalpy, or change in total energy (∆H), and change in entropy (T∆S):

∆G = ∆H - T∆S• Only processes with a negative ∆G are

spontaneous• Spontaneous processes can be harnessed to

perform work

Free Energy, Stability, and EquilibriumFree energy is a measure of a system’s instability,

its tendency to change to a more stable stateDuring a spontaneous change, free energy

decreases and the stability of a system increasesEquilibrium is a state of maximum stabilityA process is spontaneous and can perform work

only when it is moving toward equilibriumThe concept of free energy can be applied to the

chemistry of life’s processes

Exergonic and Endergonic Reactions in MetabolismAn exergonic reaction proceeds with a net

release of free energy and is spontaneousAn endergonic reaction absorbs free energy from

its surroundings and is nonspontaneous

LE 8-6a

Reactants

EnergyProducts

Progress of the reaction

Amount ofenergyreleased(G < 0)

Free

ene

rgy

Exergonic reaction: energy released

LE 8-6b

ReactantsEnergy

Products

Progress of the reaction

Amount ofenergyrequired(G > 0)

Free

ene

rgy

Endergonic reaction: energy required

Catalysts• So what’s a cell got to do to reduce

activation energy?– get help! … chemical help… ENZYMES

G

Call in the ENZYMES!

Enzymes vocabularysubstrate

• reactant which binds to enzyme• enzyme-substrate complex: temporary association

product • end result of reaction

active site • enzyme’s catalytic site; substrate fits into active site

substrate

enzyme

productsactive site

Enzymes speed up metabolic reactions by lowering energy barriersA catalyst is a chemical agent that speeds up a

reaction without being consumed by the reactionAn enzyme is a catalytic proteinHydrolysis of sucrose by the enzyme sucrase is

an example of an enzyme-catalyzed reaction

LE 8-13

SucroseC12H22O11

GlucoseC6H12O6

FructoseC6H12O6

The Activation Energy BarrierEvery chemical reaction between molecules

involves bond breaking and bond formingThe initial energy needed to start a chemical

reaction is called the free energy of activation, or activation energy (EA)

Activation energy is often supplied in the form of heat from the surroundings

LE 8-14

Transition state

C D

A B

EA

Products

C D

A B

G < O

Progress of the reaction

Reactants

C D

A B

Free

ene

rgy

How Enzymes Lower the EA BarrierEnzymes catalyze reactions by lowering the EA

barrierEnzymes do not affect the change in free-energy

(∆G); instead, they hasten reactions that would occur eventually

LE 8-15

Course ofreactionwithoutenzyme

EA

without enzyme

G is unaffectedby enzyme

Progress of the reaction

Free

ene

rgy

EA withenzymeis lower

Course ofreactionwith enzyme

Reactants

Products

Substrate Specificity of EnzymesThe reactant that an enzyme acts on is called the

enzyme’s substrate The enzyme binds to its substrate, forming an

enzyme-substrate complexThe active site is the region on the enzyme where

the substrate bindsInduced fit of a substrate brings chemical groups

of the active site into positions that enhance their ability to catalyze the reaction

Catalysis in the Enzyme’s Active SiteIn an enzymatic reaction, the substrate

binds to the active siteThe active site can lower an EA barrier by

Orienting substrates correctlyStraining substrate bondsProviding a favorable microenvironmentCovalently bonding to the substrate

LE 8-17

Enzyme-substratecomplex

Substrates

Enzyme

Products

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

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

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.

Substrates areconverted intoproducts.

Products arereleased.

Activesite is

availablefor two new

substratemolecules.

Effects of Local Conditions on Enzyme ActivityAn enzyme’s activity can be affected by:

General environmental factors, such as temperature and pH

Chemicals that specifically influence the enzyme

Each enzyme has an optimal temperature in which it can function

Each enzyme has an optimal pH in which it can function

CofactorsCofactors are nonprotein enzyme helpersCoenzymes are organic cofactors

Enzyme InhibitorsCompetitive inhibitors bind to the active site of

an enzyme, competing with the substrateNoncompetitive inhibitors bind to another part of

an enzyme, causing the enzyme to change shape and making the active site less effective

LE 8-19Substrate

Active site

Enzyme

Competitiveinhibitor

Normal binding

Competitive inhibition

Noncompetitive inhibitor

Noncompetitive inhibition

A substrate canbind normally to the

active site of anenzyme.

A competitiveinhibitor mimics the

substrate, competingfor the active site.

A noncompetitiveinhibitor binds to the

enzyme away from theactive site, altering the

conformation of theenzyme so that its

active site no longerfunctions.

Regulation of enzyme activity helps control metabolismChemical chaos would result if a cell’s metabolic

pathways were not tightly regulatedTo regulate metabolic pathways, the cell switches

on or off the genes that encode specific enzymes

Allosteric Regulation of EnzymesAllosteric regulation is the term used to describe

cases where a protein’s function at one site is affected by binding of a regulatory molecule at another site

Allosteric regulation may either inhibit or stimulate an enzyme’s activity

Most allosterically regulated enzymes are made from polypeptide subunits

Each enzyme has active and inactive formsThe binding of an activator stabilizes the active form

of the enzymeThe binding of an inhibitor stabilizes the inactive form

of the enzyme

LE 8-20a

Allosteric enzymewith four subunits

Regulatorysite (oneof four) Active form

ActivatorStabilized active form

Active site(one of four)

Allosteric activatorstabilizes active form.

Non-functionalactive site

Inactive form Inhibitor Stabilized inactive form

Allosteric inhibitorstabilizes inactive form.

Oscillation

Allosteric activators and inhibitors

Cooperativity is a form of allosteric regulation that can amplify enzyme activity

In cooperativity, binding by a substrate to one active site stabilizes favorable conformational changes at all other subunits

LE 8-20b

Substrate

Binding of one substrate molecule toactive site of one subunit locks allsubunits in active conformation.

Cooperativity another type of allosteric activation

Stabilized active formInactive form

Feedback InhibitionIn feedback inhibition, the end product of a

metabolic pathway shuts down the pathwayFeedback inhibition prevents a cell from

wasting chemical resources by synthesizing more product than is needed

LE 8-21

Active siteavailable

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Enzyme 2

Intermediate A

Isoleucineused up bycell

Feedbackinhibition Active site of

enzyme 1 can’tbindtheoninepathway off

Isoleucinebinds toallostericsite

Enzyme 3

Intermediate B

Enzyme 4

Intermediate C

Enzyme 5

Intermediate D

End product(isoleucine)

Specific Localization of Enzymes Within the CellStructures within the cell help bring order to

metabolic pathwaysSome enzymes act as structural components of

membranesSome enzymes reside in specific organelles, such

as enzymes for cellular respiration being located in mitochondria