ENZYMES

• 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

1. An Enzyme

Enzyme

Substrate(Reactants)

Allosteric Site

Active Site

Inhibitor

2. Enzyme Specificity

• 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

3. Induced Fit Model

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

• Interactions between chemical groups on substrate and those of the amino acids as well as the shape of the active site cause the induced fit

Figure 8.16 (b)

Enzyme- substratecomplex

Figure 8.16

Substate

Active site

Enzyme

(a)

Sucrose Glucose + FructoseSucrase

Sucrose + Sucrase Glucose + Fructose + Sucrase

Activation Energy• The initial amount of energy needed to start a

chemical reaction (i.e. break the bonds)

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

What do enzymes do?• 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

5. Increasing the Rate of Reaction

(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)

6. Saturation• 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

Question 7.

• 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.

8. Stopping a Reaction

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

• This changes the enzyme’s conformation (shape) making it lose it dysfunctional

• e.g. add H2SO4(aq) – Sulfuric Acid

• E.g. Add H2SO4(aq) (Sulfuric Acid)

9. Effect of Increasing Temperature

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

Metabolic Pathways

• Reactions occur in a sequence and specific enzymes catalyze each step

10. Cyclic Metabolic PathwaysZ U

V

WX

Y

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

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

11. Competitive vs. Non-competitive Inhibition

• If the inhibitor attaches by covalent bonds, it is usually irreversible (usually (c) is)

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

• If the inhibitor attaches by weak bonds, it is usually reversible.

12. Allosteric Regulation:Activation and Inhibition

– When one activator or inhibitor bind to an allosteric site, and have an effect on all four subunits of an enzyme

– Bonds are non-covalent, so they are reversible

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

Feedback Inhibition• The end product of a metabolic pathway shuts

down the pathway• Prevents the cell from wasting chemical resources

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