EnzymesEnzymes

What are enzymes?

Enzymes are biological catalysts that can change the rate of chemical

reactions.

R Hardy 2007R Hardy 2007

Enzymes as organic catalystsEnzymes as organic catalysts

• The lowering of The lowering of activation energy.activation energy.

• Mechanism of Mechanism of enzyme action: enzyme action: lock and key lock and key hypothesis.hypothesis.

• The properties of The properties of enzymes.enzymes.

The lowering of activation The lowering of activation energyenergy• Enzymes can accelerate reaction rate by Enzymes can accelerate reaction rate by

lowering activation energy of the substrate. lowering activation energy of the substrate. The amount of energy needs to bring one The amount of energy needs to bring one mole of a substance to the activated state at mole of a substance to the activated state at a given temperature termed a given temperature termed activation activation energyenergy..

Characteristics of enzymes:Characteristics of enzymes:

• protein in nature• remain unchanged at the end of reaction• denatured by high temperature• work best at a particular pH medium• specific in nature• Catalytic function of enzymes• Specific active site of enzyme for specific

substrate molecule to combine with.

Examples of enzymatic reactionsExamples of enzymatic reactions

• Sucrose + water glucose + fructose

• Fats + water fatty acids + glycerol

Lock and key mechanismLock and key mechanism

• The substrate acts as ________. (lock / key)

• The enzyme acts as _________.

• Enzyme + Substrate [Enzyme-substrate-complex]

Enzyme + Product

Lock and key hypothesisLock and key hypothesis

• The enzyme molecule has a specific region The enzyme molecule has a specific region termed active site to combine with the substrate termed active site to combine with the substrate molecules, , substrate molecules which have a molecules, , substrate molecules which have a shape fitting the active site bind to the enzyme shape fitting the active site bind to the enzyme (seem a lock and key) in the enzyme-substrate (seem a lock and key) in the enzyme-substrate complex. The substrate is activated and reactions complex. The substrate is activated and reactions occur readily between substrate molecules occur readily between substrate molecules changing them into the product molecule which changing them into the product molecule which no longer fit into the active site and escape into no longer fit into the active site and escape into the surrounded medium.the surrounded medium.

The model of lock and key The model of lock and key hypothesishypothesis

Factors affecting enzymatic Factors affecting enzymatic reactions:reactions:• Temperature

• pH

• substrate concentration

• enzyme concentration

• Inhibitors

• The presence of cofactor

• The presence of inhibitor

Factors affecting enzymatic Factors affecting enzymatic activity activity

temperature pH value Substrate concentration

enzymeconcentration

cofactors inhibitors

TemperatureTemperature

• At high temperature, heat can denature At high temperature, heat can denature the protein structure of enzyme and then the protein structure of enzyme and then change the active site , hence it will change the active site , hence it will prevent the substrate from binding on the prevent the substrate from binding on the enzyme and decrease the reaction rate.enzyme and decrease the reaction rate.

• And the low temperature will curb the And the low temperature will curb the activity of the enzyme.activity of the enzyme.

Effect of temperature on the rate Effect of temperature on the rate of an enzymatic reactionof an enzymatic reaction

pH valuepH value

• Extreme pH change can affect the Extreme pH change can affect the ionic state of functional groups ionic state of functional groups within the active site.within the active site.

The enzyme activity in different The enzyme activity in different pH valuepH value

Graph to show the effect of pH Graph to show the effect of pH on the rate of reaction of four on the rate of reaction of four different enzymesdifferent enzymes

pHpH77

Substrate concentrateSubstrate concentrate• Increasing the substrate concentration at Increasing the substrate concentration at

the suitable enzyme concentration, the the suitable enzyme concentration, the reaction rate will be increased because of reaction rate will be increased because of more enzyme-substrate complex formed.more enzyme-substrate complex formed.

• Over the saturation point, further Over the saturation point, further increasing the substrate concentration has increasing the substrate concentration has no effect on the reaction rate.no effect on the reaction rate.

• The merely way to increase the reaction The merely way to increase the reaction rate in this time is to raise the enzyme rate in this time is to raise the enzyme concentration. concentration.

The rate of reaction vs the The rate of reaction vs the substrate concentrationsubstrate concentration

Graph to show the effect of Graph to show the effect of substrate concentration on the substrate concentration on the rate of an enzyme-controlled rate of an enzyme-controlled reactionreaction

Enzyme concentrationEnzyme concentration• Increasing the enzyme concentration at Increasing the enzyme concentration at

the suitable substrate concentration, the the suitable substrate concentration, the reaction rate will be increased because of reaction rate will be increased because of more enzyme-substrate complex formed.more enzyme-substrate complex formed.

• Over the saturation point, further Over the saturation point, further increasing the enzyme concentration has increasing the enzyme concentration has no effect on the reaction rate.no effect on the reaction rate.

• The merely way to increase the reaction The merely way to increase the reaction rate in this time is to raise the substrate rate in this time is to raise the substrate concentration. concentration.

Graph to show the effect of Graph to show the effect of enzyme concentration on the rate enzyme concentration on the rate of an enzyme-controlled reactionof an enzyme-controlled reaction

Cofactor 1Cofactor 1

• The non-protein component of an enzyme which present for enzyme activity.

• The enzyme-cofactor complex is termed holoenzyme.

• The enzyme portion without cofactor is termed apoenzyme.

Cofactor 2Cofactor 2

• The cofactors may be simple inorganic ions or complex organic molecules.

• The cofactors can keep unchanged or be regenerated at the end of the reaction.

• The are stable at relatively high temperature.

• There are several kinds of cofactors.

The kinds of cofactorsThe kinds of cofactors

• Enzyme activator.

• Coenzyme.

• Prosthetic group.

Enzymeactivator

Coenzyme Prostheticgroup

Cofactors

Enzyme activatorEnzyme activator

• It is an inorganic ion which certainly bound to an enzyme or an substrate to increase the chance to an enzyme-substrate complex forming.

• e.g. copper, iron, calcium etc.

Coenzyme Coenzyme

• It is an organic non-protein molecule firmly associated with the enzyme and is essential for enzyme activity.

• It acts a carrier for transferring chemical groups or atoms from one enzyme to another.

• Many coenzymes are synthesized from vitamins.

• e.g. ATP, NADP etc.

Prosthetic groupProsthetic group

• It is an organic non-protein group which is tightly bound to the enzyme.

• It is part of enzyme and is essential for enzyme function.

• It can act as carriers of atoms or electrons while transferring one compound to another in an overall metabolic pathway.

• e.g. FAD, biotin etc.

InhibitorInhibitor

• They are substances interact with an enzyme and lead to a loss in a catalytic activity of enzyme.

• There are reversible inhibitor and irreversible inhibitor.

Competitiveinhibitors

Non-competitiveinhibitors

Reversibleinhibitor

irreversibleinhibitor

Inhibitor

Reversible inhibitorReversible inhibitor

• The activity of enzyme is restored when the inhibitor is removed.

• There are including competitive inhibitors and non-competitive inhibitors.

Types of inhibitorsTypes of inhibitors

• Competitive inhibitors:

• compete with the substrate for the active sites of enzyme molecules

• Non-competitive inhibitors:

• example: cyanide is a respiratory inhibitor

• they attach to the enzyme and alter its shape so that the active site no longer fits the substrate

Competitive inhibitorsCompetitive inhibitors

• Substance with a similar molecular structure as the substrate, competing with the substrate for the active site on the enzyme.

• The degree of inhibition rely on the relative concentration of substrate and inhibitor.

• Competitive inhibition can be overcome by increasing the concentration of substrate due to more chance to react the active site.

Competitive inhibitionCompetitive inhibition

Non-competitive inhibitorsNon-competitive inhibitors

• The inhibitor has no structural similarity to the substrate, it forms an enzyme-inhibitor complex in the next part or site of the enzyme other than the active site.

Non-competitive inhibitionNon-competitive inhibition

Irreversible inhibitorIrreversible inhibitor

• The inhibitor permanently attaches to the active site excluding the substrate molecule.

• The degree of inhibition depends completely on the inhibitor concentration and cannot be reversed by increasing substrate concentration.

The biological importance of The biological importance of inhibitorsinhibitors• They can affect the catalytic function of

enzymes.

• They can be the end product in some metabolic pathways.