A catalyst Is a chemical agent that speeds up a

reaction without being consumed by the reaction

An enzyme is an organic catalyst Enzymes are proteins

Enzyme

Substrate(Reactants)

Allosteric Site

Active Site

Activator or Inhibitor

Enzymes can only work with certain substrates

The shape of the enzyme must match the shape of its substrate- The root of the enzyme’s name typically indicates the substrate which it acts upon e.g. ATPase, Amylase, Sucrase

When the substrate binds to the active site, the enzyme changes conformation (shape) to make a better fit.

Figure 8.16 (b)

Enzyme- substratecomplex

Figure 8.16

Substate

Active site

Enzyme

(a)

Sucrase

Sucrose + Sucrase Glucose + Fructose + Sucrase

http://www.youtube.com/watch?v=eMuyKN_VW3k

• The initial amount of energy needed to start a chemical reaction

The energy required to get the reactants ready to react

Fre

e en

ergy

Progress of the reaction

EA

Figure 8.14

A B

C D

Reactants

A

C D

B

Transition state

A B

C D

Products

activation energy EA

Enzymes lower the activation energy

How? Orienting substrates

correctly Putting stress on

substrate bonds Providing a

favorable environment

This increases the rate of the reaction.

Progress of the reaction

Products

Course of reaction without enzyme

Reactants

Course of reaction with enzyme

EA

withoutenzyme EA with

enzymeis lower

Fre

e en

ergy

Figure 8.15

http://www.youtube.com/watch?v=Dd1yi2aVoOc

(1) Increase the number of substrate molecules in solution (increase conc’n)

(2) Increase the number of enzymes in solution (increase conc’n)

(3) Increase the temperature of the solution (up to a certain point)

A reaction is said to be “saturated” when 100% of enzymes have their active sites filled with substrate.

Vmax is maximum velocity (speed) of rxn

If there are left-over reactants (substrates), then you could add more enzymes.

If there are no more left-over reactants (substrates), then adding more enzymes will not increase the rate.

Change the pH so it is above or below its optimal value.

This changes the enzyme’s conformation (shape) causing it to become dysfunctional

E.g. Add H2SO4(aq) (Sulfuric Acid)

Above a certain temperature, enzymes’ activity starts to decline because the enzyme begins to denature (unravel)

Reactions occur in a sequence and specific enzymes catalyze each step

Z U

V

WX

Y

(a) Z + Y U U V W W X + Y

(b) Initial Reactant: Z End product: E (desired product)

Figure 8.19 (b) Competitive inhibition

A competitiveinhibitor mimics the

substrate, competingfor the active site.

Competitiveinhibitor

A substrate canbind normally to the

active site of anenzyme.

Substrate

Active site

Enzyme

(a) Normal binding

Figure 8.19

A noncompetitiveinhibitor binds to the

enzyme away fromthe active site, altering

the conformation ofthe enzyme so that its

active site no longerfunctions.

Noncompetitive inhibitor

(c) Noncompetitive inhibition

When one activator or inhibitor bind to an allosteric site, it will have an effect on all the subunits of an enzyme

Stabilized inactiveform

Allosteric activaterstabilizes active fromAllosteric enyzme

with four subunitsActive site

(one of four)

Regulatorysite (oneof four)

Active formActivator

Stabilized active form

Allosteric activaterstabilizes active form

InhibitorInactive formNon-functionalactivesite

(a) Allosteric activators and inhibitors. In the cell, activators and inhibitors dissociate when at low concentrations. The enzyme can then oscillate again.

Oscillation

Figure 8.20

http://www.youtube.com/watch?v=d5fDEUhjo-M

• The end product of a metabolic pathway shuts down the pathway

Active siteavailable

Isoleucineused up bycell

Feedbackinhibition

Isoleucine binds to allosteric site

Active site of enzyme 1 no longer binds threonine;pathway is switched off

Initial substrate(threonine)

Threoninein active site

Enzyme 1(threoninedeaminase)

Intermediate A

Intermediate B

Intermediate C

Intermediate D

Enzyme 2

Enzyme 3

Enzyme 4

Enzyme 5

End product(isoleucine)

Figure 8.21

http://www.youtube.com/watch?v=c8FcZVJS1Q8