Enzymes!!!

Enzymes• Enzymes speed up metabolic reactions by

lowering energy barriers

• A catalysto Is a chemical agent that speeds up a reaction without

being consumed by the reaction

• An enzymeo Is a catalytic protein

E + S ES E + P

Every chemical reaction between molecules

involves both bond breaking and bond forming

1. True 2. False

Every chemical reaction between molecules involves both bond breaking and bond forming

•Hydrolysis is an example of a chemical reaction

Figure 8.13

H2O

H

H

H

H

HO

OH

OH

OH

O

O OO OHH H H

H

H

H

CH2OH CH2OH

OHCH2OH

Sucrase

HOHO

OH OH

CH2OHH

CH2OH

H

CH2OH

H

O

Sucrose Glucose Fructose

C12H22O11 C6H12O6 C6H12O6

+HOH H

Activation Energy (EA)• The activation energy, EA

o Is the initial amount of energy needed to start a chemical reactiono Is often supplied in the form of heat from the surroundings in a system

• An enzyme catalyzes reactionso By lowering the EA barrier

Exergonic ReactionsF

ree

ener

gy

Progress of the reaction

∆G < O

EA

Figure 8.14

A B

C D

Reactants

A

C D

B

Transition state

A B

C D

Products

Progress of the reaction

Products

Course of reaction without enzyme

Reactants

Course of reaction with enzyme

EA

withoutenzyme

EA with enzymeis lower

∆G is unaffected by enzymeF

ree

ener

gy

Figure 8.15

Substrate Specificity of Enzymes• The substrate

o Is the reactant an enzyme acts on

• The enzymeo Binds to its substrate, forming an

enzyme-substrate complex

• The active siteo Is the region on the enzyme where

the substrate binds

Figure 8.16

Substrate

Active site

Enzyme

(a)

Induced fit of a substrate

o Brings chemical groups of the active site into positions that enhance their ability to catalyze the chemical reaction

Figure 8.16 (b)

Enzyme- substratecomplex

Enzyme & Substrate fit like a lock & key (Shape specific)

pH or temperature can change the active site shape on any enzyme

Active site is where the reactants bind to the enzyme

Substrates

Products

Enzyme

Enzyme-substratecomplex

1 Substrates enter active site; enzymechanges shape so its active siteembraces the substrates (induced fit).

2 Substrates held inactive site by weakinteractions, such ashydrogen bonds andionic bonds.

3 Active site (and R groups ofits amino acids) can lower EA

and speed up a reaction by• acting as a template for substrate orientation,• stressing the substrates and stabilizing the transition state,• providing a favorable microenvironment,• participating directly in the catalytic reaction.

4 Substrates are Converted intoProducts.

5 Products areReleased.

6 Active siteIs available fortwo new substrateMole.

Figure 8.17

The active site can lower an EA barrier by:

o Orienting substrates correctlyo Straining substrate bondso Providing a favorable microenvironmento Covalently bonding to the substrate

The activity of an enzyme

o Is affected by general environmental factorso Temperatureo pH

Figure 8.18

Optimal temperature for enzyme of thermophilic

Rat

e o

f re

actio

n

0 20 40 80 100Temperature (Cº)

(a) Optimal temperature for two enzymes

Optimal temperature fortypical human enzyme

(heat-tolerant) bacteria

Figure 8.18

Rat

e o

f re

actio

n

(b) Optimal pH for two enzymes

Optimal pH for pepsin (stomach enzyme)

Optimal pHfor trypsin(intestinalenzyme)

10 2 3 4 5 6 7 8 9

Enzyme cofactors• Cofactors

o Are non-protein enzyme helpers e.g. zinc, iron, copper atoms

• Coenzymeso Are organic cofactors e.g. vitamins

• Ex Vitamin C is used in at least 8 enzymatic reactions (to make collagen)

Enzyme Inhibition

Enzyme Inhibitors• Competitive

inhibitors

o Bind to the active site of an enzyme, competing with the substrate

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

Michaelis-Menten Equation-V = Vmax [S] / Km + [S]

V = velocity (rate of reaction)Vmax = when all of the enzyme molecules contain bound substrate (saturated),

maximal velocity [S] = concentration of the substrate Km = concentration of the substrate needed to give half maximal velocity (1/2

Vmax), a measure of affinity, enzyme for substrate

                                                                                                    

Enzyme Inhibitors• Noncompetitive inhibitors

o Bind to another part of an enzyme, changing the function

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

Enzyme Regulation• Regulation of enzyme activity helps control

metabolism

• A cell’s metabolic pathwayso Must be tightly regulated

Allosteric regulationo Is the term used to describe any case in which a protein’s function at one site

is affected by binding of a regulatory molecule at another

Activator Inhibitor

Stabilized inactiveform

Allosteric activaterstabilizes active fromAllosteric enyzme

with four subunitsActive site

(one of four)

Regulatorysite (oneof four)

Active formActivator

Stabilized active form

Allosteric inhibitorsstabilizes inactive 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

Cooperativityo Is a form of allosteric regulation that can amplify enzyme activity

Figure 8.20

Binding of one substrate molecule toactive site of one subunit locks all subunits in active conformation.

Substrate

Inactive form Stabilized active form

(b) Cooperativity: another type of allosteric activation. Note that the inactive form shown on the left oscillates back and forth with the active form when the active form is not stabilized by substrate.

Feedback inhibitiono 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

How to Name Enzymes

• Change the ending on the name of the substrate to –ase

• Sucrose (substrate) – Sucrase (enzyme)

• Lipid – Lipase• Protein – Protease• DNA - DNAse