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Enzymes

A catalyst is a chemical agent that speeds up a reaction without being consumed by the reaction.An enzyme is a catalytic protein.Hydrolysis of sucrose by the enzyme sucrase is an Hydrolysis of sucrose by the enzyme sucrase is an example of an enzyme-catalyzed reaction

Substrate(sucrose)

Enzyme

Glucose

Enzyme(sucrose)

Fructose

Every chemical reaction between molecules involves bond breaking and bond forming.The initial energy needed to start a chemical reaction is called the free energy of activation, or activation energy (EA).gy ( A)Activation energy is often supplied in the form of heat from the surroundings.

Transition state

C D

A B

EA

C D

A B

ener

gy

Products

C D

A B

ΔG < O

Progress of the reaction

ReactantsFree

e

Enzymes catalyze reactions by lowering the EAbarrier.Enzymes do not affect the change in free-energy (∆G); instead, they hasten reactions that would occur eventually.y

Animation: How Enzymes WorkAnimation: How Enzymes Work

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Course ofreactionwithoutenzyme

EAwithout enzyme

ener

gy

EA withenzymeis lower

C f

Reactants

ΔG is unaffectedby enzyme

Progress of the reaction

Free

e Course ofreactionwith enzyme

Products

The reactant that an enzyme acts on is called the enzyme’s substrate.The enzyme binds to its substrate, forming an enzyme-substrate complex.The active site is the region on the enzyme g ywhere the substrate binds.Induced fit of a substrate brings chemical groups of the active site into positions that enhance their ability to catalyze the reaction.

Substrate

Active site

Enzyme Enzyme-substratecomplex

In an enzymatic reaction, the substrate binds to the active site.The active site can lower an EA barrier by• Orienting substrates correctly.

b b d• Straining substrate bonds.• Providing a favorable microenvironment.• Covalently bonding to the substrate.

Enzyme-substratecomplex

Substrates

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 EAand speed up a reaction by• acting as a template forsubstrate orientation,

• stressing the substratesand stabilizing thetransition state,

• providing a favorablemicroenvironment,Active

Enzyme

Products

• participating directly in thecatalytic reaction.

Substrates areconverted intoproducts.

Products arereleased.

site isavailable

for two newsubstrate

molecules.

Each enzyme has an optimal temperature and pH in which it can f ti

Optimal temperature fortypical human enzyme

Optimal temperature forenzyme of thermophilic

(heat-tolerantbacteria)

0 20 40 60 80 100function.An enzyme’s activity can also be affected by chemicals that specifically influence the enzyme.

Temperature (°C)Optimal temperature for two enzymes

0 20 40 60 80 100

Optimal pH for pepsin(stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

pHOptimal pH for two enzymes

0 1 2 3 4 5 6 7 8 9 10

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Cofactors are nonprotein enzyme helpers.Coenzymes are organic cofactors.

Competitive inhibitors bind to the active site of an enzyme, competing with the substrate.N titi

Substrate

Active site

Enzyme

Competitiveinhibitor

Normal binding

A substrate canbind normally to the

active site of anenzyme.

A competitiveinhibitor mimics the

substrate, competingfor the active site.

Noncompetitive inhibitors bind to another part of an enzyme, causing the enzyme to change shape and making the active site less effective.

Competitive inhibition

Noncompetitive inhibitorNoncompetitive inhibition

A noncompetitiveinhibitor binds to the

enzyme away from theactive site, altering the

conformation of theenzyme so that its

active site no longerfunctions.

Chemical chaos would result if a cell’s metabolic pathways were not tightly regulated.To regulate metabolic pathways, the cell switches on or off the genes that encode specific enzymes.enzymes.

Allosteric 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

multiple polypeptide subunits.• Each enzyme has active and inactive forms.• The binding of an activator stabilizes the active form

of the enzyme.• The binding of an inhibitor stabilizes the inactive form

of the enzyme.

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. Binding of one substrate molecule to

active site of one subunit locks all

Substrate

active site of one subunit locks allsubunits in active conformation.

Cooperativity another type of allosteric activation

Stabilized active formInactive form

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In feedback inhibition, the end product of a metabolic pathway shuts down the pathway.Feedback inhibition prevents a cell from wasting chemical resources by synthesizing more product than is neededthan is needed.

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’tbindth i Intermediate Btheoninepathway off

Isoleucinebinds toallostericsite

Enzyme 3

Enzyme 4

Intermediate C

Enzyme 5

Intermediate D

End product(isoleucine)

Structures within the cell help bring order to metabolic pathways.Some enzymes act as structural components of membranes.Some enzymes reside in specific organelles such Some enzymes reside in specific organelles, such as enzymes for cellular respiration being located in mitochondria. Mitochondria,

sites of cellular respiration

1 µm