Enzymes and Enzymes and SubstratesSubstrates

Presenter:Presenter:

Mrs. KnopkeMrs. Knopke

FUHS Science Dept.FUHS Science Dept.

Laws of ThermodynamicsLaws of Thermodynamics

11stst law – Energy cannot be created or law – Energy cannot be created or Destroyed.Destroyed.

- it can be converted form one form to - it can be converted form one form to another. The sum of the energy id equal another. The sum of the energy id equal to the sum of the energy after the to the sum of the energy after the conversionconversion

22ndnd Law – Some usable energy dissipates Law – Some usable energy dissipates during transformation and is lost. Usually during transformation and is lost. Usually as heat, the amount of usable energy as heat, the amount of usable energy therefore decreases.therefore decreases.

Catabolic and Anabolic Catabolic and Anabolic ReactionsReactions

The energy-related reactions within The energy-related reactions within the cells generally involve the the cells generally involve the synthesis or the brekdown of synthesis or the brekdown of complex organic compoundscomplex organic compounds

ATP produced in catabolic reactions ATP produced in catabolic reactions provide the energy for Anabolic provide the energy for Anabolic reactions. Anabolic and Catabolic reactions. Anabolic and Catabolic reactions are therefore coupled.reactions are therefore coupled.

Anabolic ReactionsAnabolic Reactions Anabolic reactionsAnabolic reactions are those that are those that

synthesize compounds. Energy is synthesize compounds. Energy is required for these reactions.required for these reactions.

Atoms and molecules + Energy = larger moleculeAtoms and molecules + Energy = larger molecule

Energy

Catabolic ReactionsCatabolic Reactions

Reactions that break down molecules Reactions that break down molecules are called are called catabolic catabolic reactionsreactions. Energy is released when . Energy is released when molecules are broken down. molecules are broken down. 

Energy Energy

Coupled reactionsCoupled reactions

Anabolic Reactions consume Anabolic Reactions consume energyenergy

Catabolic Reactions Release Catabolic Reactions Release EnergyEnergy

Activation EnergyActivation EnergyIn either kind of reaction additional energy must be supplied to start the reaction. This energy is Activation Energy

What are enmzymes?What are enmzymes?

Substances that speed up chemical Substances that speed up chemical reactions are called reactions are called CatalystsCatalysts. . Organic catalysts are called Organic catalysts are called Enzymes.Enzymes.

Enzymes are specific for one particular Enzymes are specific for one particular reaction of group of related reactions.reaction of group of related reactions.

Many reactions cannot occur without Many reactions cannot occur without the correct enzymethe correct enzyme

Often named by adding an - Often named by adding an - asease

Induced-Fit TheoryInduced-Fit Theory

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

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

Enzymes are substrate Enzymes are substrate specificspecific

A A substratesubstrate is a reactant which binds to is a reactant which binds to an enzyme.an enzyme.

When a substrate or substrates binds to an When a substrate or substrates binds to an enzyme, the enzyme catalyzes the enzyme, the enzyme catalyzes the conversion of the substrate to the product.conversion of the substrate to the product.– Sucrase is an enzyme that binds to sucrose and Sucrase is an enzyme that binds to sucrose and

breaks the disaccharide into fructose and breaks the disaccharide into fructose and glucose.glucose.

The active site of an enzymes is typically a pocket or groove on the surface of the protein into which the substrate fits.The specificity of an enzyme is due to the fit between the active site and that of the substrate.As the substrate binds, the enzyme changes shape leading to a tighter induced fit, bringing chemical groups in position to catalyze the reaction.

The substrate molecule normally The substrate molecule normally does not fit exactly in the active does not fit exactly in the active site. This induces a change in the site. This induces a change in the enzymes conformation (shape) to enzymes conformation (shape) to make a closer fit.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 substrate. on certain bonds of the substrate. This lowers the amount of energy This lowers the amount of energy needed to break them.needed to break them.

The enzyme The enzyme does notdoes not form a form a chemical bond with the substrate. chemical bond with the substrate. After the reaction, the products are After the reaction, the products are released and the enzyme returns to released and the enzyme returns to its normal shape.its normal shape.

Because the enzyme does not form Because the enzyme does not form chemical bonds with the substrate, it chemical bonds with the substrate, it remains unchanged. As a result, the remains unchanged. As a result, the enzyme molecule can be reused. enzyme molecule can be reused. Only a small amount of enzyme is Only a small amount of enzyme is needed because they can be used needed because they can be used repeatedly.repeatedly.

Activation Energy and Activation Energy and Enzymes Enzymes

The amount of The amount of activation energyactivation energy that is that is required is considerably less when required is considerably less when enzyme is present.enzyme is present.

Conditions that Affect Conditions that Affect Enzymatic ReactionsEnzymatic Reactions

Rate of ReactionRate of Reaction Reactions with enzymes are up to 10 Reactions with enzymes are up to 10

billion times faster than those without billion times faster than those without enzymes. Enzymes typically react with enzymes. Enzymes typically react with between 1 and 10,000 molecules per between 1 and 10,000 molecules per second. second. 

Fast enzymes catalyze up to 500,000 Fast enzymes catalyze up to 500,000 molecules per second.molecules per second.

Substrate concentration, enzyme Substrate concentration, enzyme concentration, Temperature, and pH  concentration, Temperature, and pH  affect the rate of enzyme reactions.affect the rate of enzyme reactions.

Substrate Concentration Substrate Concentration

At lower concentrations, the active At lower concentrations, the active sites on most of the enzyme sites on most of the enzyme molecules are not filled because molecules are not filled because there is not much substrate.  Higher there is not much substrate.  Higher concentrations cause more collisions concentrations cause more collisions between the molecules.  With more between the molecules.  With more molecules and collisions, enzymes molecules and collisions, enzymes are more likely to encounter are more likely to encounter molecules of reactant.molecules of reactant.

Substrate ConcentrationSubstrate Concentration The maximum velocity The maximum velocity

of a reaction is reached of a reaction is reached when the active sites when the active sites are almost continuously are almost continuously filled. Increased filled. Increased substrate concentration substrate concentration after this point will not after this point will not increase the rate.  increase the rate.  Reaction rate therefore Reaction rate therefore increases as substrate increases as substrate concentration is concentration is increased but it levels increased but it levels off.off.

Enzyme Concentration Enzyme Concentration

If there is insufficient enzyme If there is insufficient enzyme present, the reaction will not present, the reaction will not proceed as fast as it otherwise proceed as fast as it otherwise would because all of the active would because all of the active sites are occupied with the sites are occupied with the reaction. Additional active sites reaction. Additional active sites could speed up the reaction.could speed up the reaction.

As the amount of enzyme is As the amount of enzyme is increased, the rate of reaction increased, the rate of reaction increases. If there are more enzyme increases. If there are more enzyme molecules than are needed, adding molecules than are needed, adding additional enzyme will not increase additional enzyme will not increase the rate. Reaction rate therefore the rate. Reaction rate therefore increases as enzyme concentration increases as enzyme concentration increases but then it levels off.increases but then it levels off.

Enzyme Concentration Enzyme Concentration

As the amount of As the amount of enzyme is increased, enzyme is increased, the rate of reaction the rate of reaction increases. If there are increases. If there are more enzyme more enzyme molecules than are molecules than are needed, adding needed, adding additional enzyme will additional enzyme will not increase the rate. not increase the rate. Reaction rate Reaction rate therefore increases as therefore increases as enzyme concentration enzyme concentration increases but then it increases but then it levels off.levels off.

TemperatureTemperatureeffect on enzymeseffect on enzymes

Higher temperature generally Higher temperature generally causes more collisions among the causes more collisions among the molecules and therefore increases molecules and therefore increases the rate of a reaction. More the rate of a reaction. More collisions increase the likelihood collisions increase the likelihood that substrate will collide with the that substrate will collide with the active site of the enzyme, thus active site of the enzyme, thus increasing the rate of an enzyme-increasing the rate of an enzyme-catalyzed reactioncatalyzed reaction..

Temperature Temperature

Above a certain Above a certain temperature, temperature, activity begins to activity begins to decline because the decline because the enzyme begins to enzyme begins to denaturedenature..

The rate of chemical The rate of chemical reactions therefore reactions therefore increases with increases with temperature but temperature but then decreases.then decreases.

pHpH

A change in pH can alter the A change in pH can alter the ionization of the ionization of the R groupsR groups of the of the amino acids. When the charges amino acids. When the charges on the amino acids change, on the amino acids change, hydrogen bonding within the hydrogen bonding within the protein molecule change and the protein molecule change and the molecule changes shape. The molecule changes shape. The new shape may not be effective.new shape may not be effective.

pH pH The diagram below shows The diagram below shows

that that pepsinpepsin functions best functions best in an acid environment. in an acid environment. This makes sense This makes sense because pepsin is an because pepsin is an enzyme that is normally enzyme that is normally found in the found in the stomachstomach where the pH is low due where the pH is low due to the presence of to the presence of hydrochloric acid. Trypsin hydrochloric acid. Trypsin is found in the duodenum, is found in the duodenum, and therefore, its and therefore, its optimum pH is in the optimum pH is in the neutral range to match neutral range to match the pH of the duodenum the pH of the duodenum

Metabolic Pathways Metabolic Pathways Metabolism refers to the chemical reactions Metabolism refers to the chemical reactions

that occur within cells. A hypothetical that occur within cells. A hypothetical metabolic pathway is shown below.metabolic pathway is shown below.

Reactions occur in a sequence and a specific enzyme catalyzes each step.

Intermediates can be used as starting points for other pathways. For example, "C" in the diagram above can be used to produce "D" but

can also be used to produce "F".

Cyclic Pathways Cyclic Pathways

Some metabolic pathways are cyclic. Some metabolic pathways are cyclic. The function of the cyclic pathway The function of the cyclic pathway below is to produce E from A. Several below is to produce E from A. Several intermediate steps are involved in intermediate steps are involved in the production of E.the production of E.

Some metabolic Some metabolic pathways are pathways are cyclic. The function cyclic. The function of the cyclic of the cyclic pathway below is pathway below is to produce E from to produce E from A. Several A. Several intermediate steps intermediate steps are involved in the are involved in the production of E.production of E.

Regulation of Enzyme Regulation of Enzyme ActivityActivity

Cells have built-in Cells have built-in control control mechanisms to mechanisms to regulate enzyme regulate enzyme concentration and concentration and activity.activity.

Regulation of Protein Regulation of Protein Synthesis (Genetic Synthesis (Genetic

Regulation)Regulation) Enzymes are proteins. You can Enzymes are proteins. You can

regulate them by making more or regulate them by making more or less of them as needed. The topic of less of them as needed. The topic of regulating protein synthesis is regulating protein synthesis is deferred to a later chapter.deferred to a later chapter.

Regulation of Enzymes Regulation of Enzymes Already ProducedAlready Produced

Competitive Competitive InhibitionInhibition

In competitive In competitive inhibition, a inhibition, a similar-shaped similar-shaped molecule competes molecule competes with the substrate with the substrate for active sites.for active sites.

Binding by some molecules, inhibitors, Binding by some molecules, inhibitors, prevent enzymes from catalyzing prevent enzymes from catalyzing reactions.reactions.– If binding involves covalent bonds, then If binding involves covalent bonds, then

inhibition is often irreversible.inhibition is often irreversible.– If binding is weak, inhibition may be If binding is weak, inhibition may be

reversible.reversible. If the inhibitor binds to the same site If the inhibitor binds to the same site

as the substrate, then it blocks as the substrate, then it blocks substrate binding via substrate binding via competitive competitive inhibitioninhibition..

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin CummingsFig. 6.17a, b

Noncompetitive Inhibition Noncompetitive Inhibition

Another form of Another form of inhibition involves inhibition involves an inhibitor that an inhibitor that binds to an binds to an allostericallosteric sitesite of an of an enzyme.  An enzyme.  An allosteric site is a allosteric site is a different location different location than the active than the active site. site.

Binding by the inhibitor causes the Binding by the inhibitor causes the enzyme to change shape, rendering enzyme to change shape, rendering the active site unreceptive at worst or the active site unreceptive at worst or less effective at catalyzing the less effective at catalyzing the reaction.reaction.

Reversible inhibition of enzymes is a Reversible inhibition of enzymes is a natural part of the regulation of natural part of the regulation of metabolism.metabolism.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.17c

Most allosterically regulated enzymes Most allosterically regulated enzymes are constructed of two or more are constructed of two or more polypeptide chains.polypeptide chains.

Each subunit has its own active site Each subunit has its own active site and allosteric sites are often located and allosteric sites are often located where subunits join.where subunits join.

The whole protein oscillates between The whole protein oscillates between two conformational shapes, one two conformational shapes, one active, one inactive.active, one inactive.

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.18a

Some allosteric regulators, activators, Some allosteric regulators, activators, stabilize the conformation that has a stabilize the conformation that has a functional active site.functional active site.

Other regulators, inhibitors, stabilize Other regulators, inhibitors, stabilize the conformation that lacks an active the conformation that lacks an active sitesite..

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.18b

In enzymes with multiple catalytic In enzymes with multiple catalytic subunits, binding by a substrate to subunits, binding by a substrate to one active site stabilizes favorable one active site stabilizes favorable conformational changes at all other conformational changes at all other subunits, a process called subunits, a process called cooperativitycooperativity..

This mechanism amplifies the This mechanism amplifies the response of enzymes to substrates, response of enzymes to substrates, priming the enzyme to accept priming the enzyme to accept additional substratesadditional substrates

Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.20

Noncompetitive Inhibition Noncompetitive Inhibition

The binding of an inhibitor to the The binding of an inhibitor to the allosteric site alters the shape of the allosteric site alters the shape of the enzyme, resulting in a distorted active enzyme, resulting in a distorted active site that does not function properly.site that does not function properly.

The binding of an inhibitor to an The binding of an inhibitor to an allosteric site is usually temporary.   allosteric site is usually temporary.   Poisons are inhibitors that bind Poisons are inhibitors that bind irreversibly. For example, penicillin irreversibly. For example, penicillin inhibits an enzyme needed by bacteria inhibits an enzyme needed by bacteria to build the cell wall.to build the cell wall.

Feedback InhibitionFeedback Inhibition Negative feedback inhibition is like a Negative feedback inhibition is like a

thermostat. When it is cold, the thermostat thermostat. When it is cold, the thermostat turns on a heater which produces heat. Heat turns on a heater which produces heat. Heat causes the thermostat to turn off the heater. causes the thermostat to turn off the heater. Heat has a negative effect on the thermostat; Heat has a negative effect on the thermostat; it feeds back to an earlier stage in the control it feeds back to an earlier stage in the control sequence as diagrammed below. sequence as diagrammed below. 

Feedback Inhibition Feedback Inhibition

Many enzymatic pathways are Many enzymatic pathways are regulated by feedback inhibition. As an regulated by feedback inhibition. As an enzyme's product accumulates, it turns enzyme's product accumulates, it turns off the enzyme just as heat causes a off the enzyme just as heat causes a thermostat to turn off the production of thermostat to turn off the production of heat.  The end product of the pathway heat.  The end product of the pathway binds to an allosteric site on the first binds to an allosteric site on the first enzyme in the pathway and shuts down enzyme in the pathway and shuts down the entire sequence.the entire sequence.

Feedback inhibition

Feedback inhibition occurs in most cells.

One of the common One of the common methods of methods of metabolic control is metabolic control is feedback feedback inhibitioninhibition in which in which a metabolic pathway a metabolic pathway is turned off by its is turned off by its end product.end product.

The end product acts The end product acts as an inhibitor of an as an inhibitor of an enzyme in the enzyme in the pathway.pathway.

When the product is When the product is abundant the abundant the pathway is turned pathway is turned off, when rare the off, when rare the pathway is active.pathway is active.Copyright © 2002 Pearson Education, Inc., publishing as Benjamin Cummings

Fig. 6.19

Function of Coenzymes:Function of Coenzymes: Many enzymes require nonprotein helpers, Many enzymes require nonprotein helpers,

cofactorscofactors, for catalytic activity., for catalytic activity.– They bind permanently to the enzyme or They bind permanently to the enzyme or

reversibly.reversibly.– Some inorganic cofactors include zinc, Some inorganic cofactors include zinc,

iron, and copper.iron, and copper. Organic cofactors, Organic cofactors, coenzymescoenzymes, include , include

vitamins or molecules derived from vitamins.vitamins or molecules derived from vitamins. The manners by which cofactors assist The manners by which cofactors assist

catalysis are diverse.catalysis are diverse.