EnzymesEnzymes

L. SchefflerL. Scheffler

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EnzymesEnzymesEnzymes are Enzymes are catalysts. catalysts. They increase They increase the speed of a chemical reaction without the speed of a chemical reaction without themselves undergoing any permanent themselves undergoing any permanent chemical change. chemical change.

Enzymes are neither used up in the Enzymes are neither used up in the reaction, nor do they appear as reaction reaction, nor do they appear as reaction products. products.

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EnzymesEnzymesEnzymes are Enzymes are proteinprotein molecules that catalyze molecules that catalyze biochemical reactionsbiochemical reactionsThe substance on which the enzyme acts is The substance on which the enzyme acts is known as the known as the substratesubstrate

3Models of some proteins – tertiary and quaternary structure.

Discovery of EnzymesDiscovery of Enzymes1825 Jon Jakob Berzelius 1825 Jon Jakob Berzelius discovered the catalytic effect of discovered the catalytic effect of enzymes.enzymes.

1926 James Sumner isolated the 1926 James Sumner isolated the first enzyme in pure form.first enzyme in pure form.

1947 Northrup and Stanley together 1947 Northrup and Stanley together with Sumner were awarded the with Sumner were awarded the Nobel prize for the isolation of the Nobel prize for the isolation of the enzyme enzyme pepsinpepsin..

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Berzelius

Sumner

NorthrupStanley

Enzyme CharacteristicsEnzyme Characteristics

High molecular weight High molecular weight proteinsproteins with masses with masses ranging from 10,000 to as ranging from 10,000 to as much as 2,000,000 grams much as 2,000,000 grams per moleper moleSubstrate specific catalystsHighly efficientHighly efficient, increasing , increasing reaction rates by a factor as reaction rates by a factor as high as 10high as 1088

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Enzyme NomenclatureEnzyme Nomenclature

The earliest enzymes that were The earliest enzymes that were discovered have common names: discovered have common names:

i.e. Pepsin, Renin, Trypsin, Pancreatini.e. Pepsin, Renin, Trypsin, Pancreatin

The enzyme name for most other The enzyme name for most other enzymes ends in enzymes ends in “ase”“ase”

The enzyme name indicates the The enzyme name indicates the substrate substrate acted uponacted upon and the and the type of reaction that it type of reaction that it catalyzescatalyzes

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Enzyme NamesEnzyme Names

Examples of Enzyme NamesExamples of Enzyme Names

Glutamic Oxaloacetic Transaminase (GOT)Glutamic Oxaloacetic Transaminase (GOT)

L-aspartate: 2-oxoglutarate L-aspartate: 2-oxoglutarate aminotransferase. aminotransferase.

Enzyme names tend to be long and Enzyme names tend to be long and complicated. They are often abbreviated complicated. They are often abbreviated with acronymswith acronyms

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Types of Enzyme Types of Enzyme SpecificitySpecificity

Enzyme specificity may be characterized asEnzyme specificity may be characterized as

1.1. Absolute:Absolute: The enzyme catalyzes on one The enzyme catalyzes on one reactionreaction

2.2. Group SpecificGroup Specific:: The enzyme acts only on The enzyme acts only on molecules having a particular functional groupmolecules having a particular functional group

3.3. Linkage SpecificLinkage Specific: The enzyme acts on a : The enzyme acts on a particular type of chemical bondparticular type of chemical bond

4.4. Stereo-chemical SpecificStereo-chemical Specific: The enzyme acts : The enzyme acts on a particular stereo or optical isomeron a particular stereo or optical isomer

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Enzyme SpecificityEnzyme Specificity

The action of an enzyme depends The action of an enzyme depends primarily on the tertiary and quaternary primarily on the tertiary and quaternary structure of the protein that constitutes the structure of the protein that constitutes the enzyme.enzyme.The part of the enzyme structure that acts The part of the enzyme structure that acts on the substrate is called the on the substrate is called the active site.active site.The The active siteactive site is a groove or pocket in is a groove or pocket in the enzyme structure where the substrate the enzyme structure where the substrate can bind.can bind.

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CofactorsCofactors

CofactorsCofactors are other compounds or ions are other compounds or ions that enzymes require before their catalytic that enzymes require before their catalytic activity can occur.activity can occur.

The protein portion of the enzyme is The protein portion of the enzyme is referred to as the referred to as the apoenzyme.apoenzyme.

The enzyme plus the cofactor is known as The enzyme plus the cofactor is known as a a holoenzyme.holoenzyme.

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CofactorsCofactorsCofactorsCofactors may be one of three types may be one of three types

1.1. CoenzymeCoenzyme:: A non protein organic A non protein organic substance that is loosely attached to the substance that is loosely attached to the enzymeenzyme

2.2. Prosthetic GroupProsthetic Group:: A non protein organic A non protein organic substance that is firmly attached to the substance that is firmly attached to the enzymeenzyme

3.3. Metal ion activatorsMetal ion activators:: K K++, Fe, Fe2+2+, Fe, Fe3+3+, , CuCu2+2+, Co, Co2+2+, Zn, Zn2+2+, Mn, Mn2+2+, Mg, Mg2+2+, Ca, Ca2+2+, or , or MoMo2+2+, ,

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Types of CofactorsTypes of Cofactors

Enzymes have varying degrees of specificity.Enzymes have varying degrees of specificity.

One cofactor may serve many different One cofactor may serve many different enzymes.enzymes.

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Enzymes and CofactorsEnzymes and Cofactors

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Enzyme MechanismsEnzyme Mechanisms Enzymes Enzymes lower the activation energylower the activation energy for reactions for reactions

and and shorten the pathshorten the path from reactants to products from reactants to products

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Enzyme MechanismsEnzyme Mechanisms The basic enzyme reaction can be The basic enzyme reaction can be

represented as follows:represented as follows:

E + S ES E + PEnzyme Substrate Enzyme substrate Enzyme Product(s) complex

The enzyme binds with the substrate to form the Enzyme-Substrate Complex. Then the substrate is released as the product(s).

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Enzyme MechanismsEnzyme Mechanisms

Diagram of Diagram of the action the action of the of the enzyme enzyme sucrase on sucrase on sucrose.sucrose.

E+SES E+P

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Enzyme MechanicsEnzyme Mechanics

An enzyme-substrate complex forms when An enzyme-substrate complex forms when the enzyme’s the enzyme’s active siteactive site binds with the binds with the substrate like a key fitting a lock. substrate like a key fitting a lock. 

The shape of the enzyme must match the The shape of the enzyme must match the shape of the substrate. shape of the substrate.

Enzymes are therefore very specific; they Enzymes are therefore very specific; they will only function correctly if the shape of will only function correctly if the shape of the substrate matches the active site.the substrate matches the active site.

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Induced Fit TheoryInduced Fit Theory

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Induced Fit TheoryInduced Fit Theory

The substrate molecule normally The substrate molecule normally does not fit exactly in the active does not fit exactly in the active site. site. This induces a change in the This induces a change in the enzymes conformation (shape) enzymes conformation (shape) to make a closer fit.to make a closer fit.In reactions that involve breaking In reactions that involve breaking bonds, the inexact fit puts stress bonds, the inexact fit puts stress on certain bonds of the on certain bonds of the substrate. substrate. This lowers the amount of energy This lowers the amount of energy needed to break them.needed to break them.

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Induced Fit TheoryInduced Fit Theory

The enzyme The enzyme does not does not actually actually form a chemical bond with the form a chemical bond with the substrate. After the reaction, the substrate. After the reaction, the products are released and the products are released and the enzyme returns to its normal enzyme returns to its normal shape.shape.Because the enzyme does not Because the enzyme does not form chemical bonds with the form chemical bonds with the substrate, it remains unchanged. substrate, it remains unchanged. The enzyme molecule can be The enzyme molecule can be reused repeatedlyreused repeatedlyOnly a small amount of enzyme is Only a small amount of enzyme is neededneeded

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Enzymes and Reaction Enzymes and Reaction RatesRates

Factors that influence reaction rates of Factors that influence reaction rates of Enzyme catalyzed reactions includeEnzyme catalyzed reactions include

1.1. Enzyme and substrate Enzyme and substrate concentrationsconcentrations

2.2. TemperatureTemperature

3.3. pHpH

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Enzymes and Reaction Enzymes and Reaction RatesRates

At At low concentrationslow concentrations, , an increase in substrate an increase in substrate concentration increases concentration increases the rate because there the rate because there are many active sites are many active sites available to be occupiedavailable to be occupied

At At high substratehigh substrate concentrationsconcentrations the the reaction rate levels off reaction rate levels off because most of the because most of the active sites are occupiedactive sites are occupied

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Substrate concentrationSubstrate concentrationThe The maximum maximum velocityvelocity of a reaction of a reaction is reached when the is reached when the active sites are active sites are almost continuously almost continuously filled. filled. Increased substrate Increased substrate concentration after concentration after this point will not this point will not increase the rate. increase the rate. VVmaxmax is the is the maximum maximum reaction ratereaction rate

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Substrate concentrationSubstrate concentrationVVmaxmax is the maximum is the maximum reaction ratereaction rateThe The Michaelis-Michaelis-Menton constant , Menton constant , KKmm is the substrate is the substrate concentration when concentration when the rate isthe rate is ½ V ½ Vmaxmax

KKmm for a particular for a particular enzyme with a enzyme with a particular substrate particular substrate is always the sameis always the same

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Effect of TemperatureEffect of TemperatureHigher temperature Higher temperature increases the number increases the number of effective collisions of effective collisions and therefore and therefore increases the rate of a increases the rate of a reaction. reaction. Above a certain Above a certain temperature, the rate temperature, the rate begins to decline begins to decline because the enzyme because the enzyme protein begins to protein begins to denaturedenature

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Effect of pHEffect of pHEach enzyme has an Each enzyme has an optimal pH at which it is optimal pH at which it is most efficientmost efficientA change in pH can alter A change in pH can alter the ionization of the R the ionization of the R groups of the amino acids. groups of the amino acids. When the charges on the When the charges on the amino acids change, amino acids change, hydrogen bonding within hydrogen bonding within the protein molecule the protein molecule change and the molecule change and the molecule changes shape. changes shape. The new shape may not be The new shape may not be effective.effective.Pepsin is most efficient at

pH2.5-3 while Trypsin is efficient at a much higher pH 26

InhibitorsInhibitors

Enzyme inhibitorsEnzyme inhibitors are substances are substances which alter the catalytic action of the which alter the catalytic action of the enzyme and consequently slow down or enzyme and consequently slow down or stop catalysis.stop catalysis.There are three common types of There are three common types of enzyme inhibition enzyme inhibition

1.1. Competitive inhibitors Competitive inhibitors 2.2. Non-competitive inhibitorsNon-competitive inhibitors3.3. Substrate inhibition.Substrate inhibition.

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Competitive InhibitorsCompetitive Inhibitors

Competitive inhibitionCompetitive inhibition occurs when the occurs when the substrate and a substrate and a substance resembling substance resembling the substrate are both the substrate are both added to the enzyme. added to the enzyme.

The inhibitor blocks the The inhibitor blocks the active site on the active site on the enzyme stopping its enzyme stopping its catalytic actioncatalytic action

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Non-competitive InhibitorsNon-competitive Inhibitors

Non-competitive Non-competitive inhibitors deactivate inhibitors deactivate the active site of the the active site of the enzyme.enzyme.

They alter the They alter the enzyme so that it enzyme so that it can no longer bind to can no longer bind to the substrate the substrate

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Effect of inhibitors on the Effect of inhibitors on the Reaction RateReaction Rate

For non-competitive For non-competitive inhibitors inhibitors Vmax Vmax is is lower but lower but KKmm is the is the

same.same.

For competitive For competitive inhibitors, inhibitors, VVmaxmax is the is the same but same but KKmm is is

increased.increased.

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Substrate InhibitorsSubstrate Inhibitors

Substrate inhibition occurs Substrate inhibition occurs when excessive amounts when excessive amounts of substrate are present. of substrate are present. Additional substrate Additional substrate sometimes interferes with sometimes interferes with the ability of substrate the ability of substrate molecules to find active molecules to find active sites on enzymessites on enzymesIn these cases the reaction In these cases the reaction velocity decreases after velocity decreases after the maximum velocity has the maximum velocity has been reached. been reached.

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