cmb lect 5 2011 colour 2 slides per page

8/14/2019 CMB Lect 5 2011 Colour 2 Slides Per Page

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Lecture 5

Enzymes and the nature of enzyme

catalysis

BIOSCI 101: Cellular and Molecular Biology

Enzymes and catalysis

- anabolic reactions - large molecules synthesisedfrom small ones

- catabolic reactions - break down of largemolecules yielding energy and raw materials



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Energy-yielding processes arecoupled with energy-consuming

ones

BIOSCI 101: Cellular and Molecular BiologyCampbell 8.11

ATP is an energy-rich compound

BIOSCI 101: Cellular and Molecular BiologyCampbell 8.8b


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ATP drives cellular processes

Transport protein Solute

ATP P iADP

P P i

Solute transported

Vesicle Cytoskeletal track


P iADPATP

Motor protei n Protein andvesicle moved

Enzymes and catalysis (contd)

- most chemical reactions require energy to getstarted (need energy to break chemical bonds)

-activation energy is the energy which must be


put into the system in order for a reaction tocommence ie. the energy molecules need to reactwith each other


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An energy profile of a chemicalreaction

A B

C D

Transition state

ReactantsFreeenergy

EAA B

C D

A B


Products

Progress of the reaction

G OC D


Enzymes as catalysts

- a catalyst is a substance that increases the rateof a reaction by lowering the activation energyrequired by the interacting molecules

- catalysts are recovered after the reactionunchanged and may be used again


- ce s use enzymes as cata ysts to spee upchemical reactions because at 37oC, mostreactions would go very slowly


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Enzymes lower the activationenergy

Course ofreactionwithout

EA

enzymeenzyme EA with

enzymeis lower

Course ofreactionwith enzyme

Reactants

G is unaffectedby enzyme

reeenergy


Products

Progress of the reaction


- have 1, 2, 3, 4 structure

- the 3D shape of the enzyme controls its activity



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- non-protein low molecular weight substances.

a) prosthetic groups - organic molecules tightlybound to the protein (eg. haeme)

b) coenzymes - small organic molecules, non-


covalently bound to protein (eg. many vitamins)

c) metal ions -inorganic ions that are essential forenzyme activity eg. Cu2+, Mg2+


Types of enzymes

- names of many enzymes end in -ase and oftendescribe the reaction catalysed eg isomerasescatalyse intramolecular rearrangements

- some important enzymes in molecular biology:

- ribonuclease and deoxyribonucleasehydrolyse RNA and DNA


- an po ymerase syn es seand DNA

- DNA ligase and DNA gyrase andtopoisomerase are involved in DNA replication


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T es of enz mes contd

- restriction enzymes hydrolyse phosphodiesterbonds in DNA at defined sequences

eg. Bam H1 - from Bacillus amyloliquefaciensGGATCC (sticky ends)

Eco RV - from Eschericia coli strain R 5th


isolate) GATATC (blunt ends)

- recognition sequences are palindromic(sequence on top strand is same as reverse onbottom strand)

DNA sequences contain

restriction enzyme sites



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Enzyme-substrate interactions

E + S --------> ES ---------> E + P

Sucrase


Sucrose

(C12H22O11)

Glucose

(C6H12O6)

The catalytic cycle of an enzyme

Substrates enter active site.Substrates are held

in active site by weakinteractions.

12

Substrates

Enzyme-substratecomplex Act ive s ite can

lower EA and speedup a reaction.

Act ivesite is

availablefor two new

substratemolecules.

3

6


Enzyme

Products

Products arereleased.

Substrates areconverted toproducts.

45


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Enzymes and catalysis (contd) Enzyme-substrate interactions (contd)

a) the enzyme and substrate combine at the active

- the bonds holding the enzyme and substratetogether are weak (non-covalent)

- the interaction is very specific due to theconfiguration of the active site of the enzyme(involves only a few amino acids)

-


(lock and key vs induced fit)

- the enzyme exerts catalytic activity by bringingreacting molecules close together

- once reaction has occurred, E + P dissociate, Erecycled for next time

Induced fit between an enzymeand its substrate



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3 amino acids are essential inthe active site of chymotrypsin



-

b) the union between enzyme and substrate isvery brief

- enzymes are very efficient. eg catalasecatalyses degradation of peroxide at a rate of


. x mo ecu es m n


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-

c) factors affecting reaction rates

(i) temperature, pH and ionic strength (affect2 3 and 4 structure)

- enzymes have an optimum temperature


- each enzyme also has a favoured pH for

operation called pH optimum

Environmental

Optimal temperature fortypical human enzyme (37C)

Optimal temperature for

enzyme of thermophilic

(heat-tolerant)bacteria (77C)

Rateofreaction

actorsaffectingenzymes

Temperature (C)

(a) Optimal temperature fo r tw o enzymes

ction

120100806040200

Optimal pH for pepsin(stomachenzyme)

Optimal pH for trypsin(intestinal

enzyme)


Rateofre

a

0 1 2 3 4 5 6 7 8 9 10

pH

(b) Optimal pH for tw o enzymes


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Enzymes and catalysis (contd)c factors affectin reaction rates contd

(ii) enzyme and substrate concentrations

- enzyme concentration is directlyproportional to reaction rate (the moreenzyme the faster the reaction)

- substrate concentration can also be critical


in cells because the available enzyme canbecome saturated

- different enzymes have differing affinitiesfor their substrates

Enzyme and substrateconcentration affect reaction rates



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Regulation of enzyme activity

a) by activation (conversion from an inactiveto an active form)

eg. chymotrypsinogen ---> chymotrypsin viaaction of trypsin in duodenum

b) by sequestration (enzymes may be locked


into certain organelles inside the cell) eg.

lysosomes


Regulation of enzyme activity (contd)

c) allosteric enzymes and feedback inhibition

- the function of an allosteric enzyme is affectedby the binding of a regulatory molecule

- these enzymes can exist in two distinct spatialconformations, one active, the other inactive

- many metabolic pathways are regulated byfeed-back inhibition of allosteric enzymes


d) multienzyme complexes

- related enzymes close together, substratespassed directly on from one reaction to the nexteg DNA replication enzymes, mitochondria andchloroplasts


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Allostericregulation

BIOSCI 101: Cellular and Molecular BiologyCampbell 8.19a

inhibition



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Test question

Which of the following statements about enzymes isINCORRECT?.

their three-dimensional structure2. the substrate binds to the enzyme at the

active site3. enzymes increase the rate of chemical

reaction by lowering activation energybarriers


4. enzyme function is influenced by

temperature but not by pH5. some enzymes require a cofactor for

catalytic activity

cmb lect 5 2011 colour 2 slides per page

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