chapter 15 part 3 inhibition

8/17/2019 Chapter 15 Part 3 Inhibition

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Chapter 15

Biocatalysis (Part 3)



Chapter review…

biocatalysisbiocatalysis

roperties and

mechanisms of

actions

roperties

of enzymes

Mechanism

of enzyme

action

Activation

energy

Models

Lock

and key

2 hypotheses

Induced

fit

Cofactors

Types &

f(x) of

cofactors

Inhibition

reversible irreversible

Competitive Non

competitive

Factors

affecting

enzymatic

reaction

Classification



LEARNING OUTCOMES

• Explain the roles and types of inhibitors.

• Explain inhibition of enzyme reaction :

- reversible inhibition : competitive and

non competitive inhibitor

- irreversible inhibition : cyanide



What is an inhibitor?

• A substance that interacts with the enzyme to

prevent it from working in the normal manner,

hence stopping or slowing a specific

enzymatic reaction.



inhibition

reversible irreversible

Competitive Non competitive

2 types inhibitors- cyanide



INHIBITION

The rate of enzyme-controlled

reactions may be decreased by the

presence of inhibitors. There are two

type of inhibitors:

• reversible inhibitors

• non reversible inhibitors



Reversible Inhibitors

Inhibitor bind to enzyme by weak bonds

The effect of this type of inhibitor is temporary

Causes no permanent damage to the enzyme



• The association of the inhibitor and enzyme isa loose one

• It can easily be removed• Removal of the inhibitor restores the activity

of the enzyme to normal

• There are two types: competitive inhibitorsand non competitive inhibitors

Reversible Inhibitors



Competitive Inhibitors

• It competes with the substrate for the active sites

• The inhibitor may have similar structure withsubstrate

• While it remains bound to the active site, it preventsother substrate molecules from occupying that siteand so decreases the rate of the reaction

Reversible InhibitorsReversible Inhibitors



The substrate continues to use any unaffected enzyme

Substrate and inhibitor are in direct competition

The greater concentration of substrate, the greater theirchance of finding the active sites – more substratemolecules than inhibitor molecules are around to gain entryto active site





• If the concentration of the substrate is increased, lessinhibition occurs

Competitive Inhibitors Reversible InhibitorsReversible Inhibitors



Inhibitor absent – The substrate

attaches to the active site of the

enzyme in the normal way.Reaction takes place as normal.

Inhibitor present – The inhibitor

prevents the normal enzyme-

substrate complex being formed.The reaction rate is reduced.

Succinic acid

Succinic

dehydrogenase

Malonic acid

Succinic dehidrogenase (enzyme)

Succinic acid fumaric acid

Malonic acid (inhibitor)



Not attached to the active site but elsewhere on the

enzyme molecule (allosteric site).

They alter the shape of the enzyme - rendering the activesite unreceptive to substrate OR

Leaving enzyme less effective at catalyzing the conversionof substrate to product.

Non competitive InhibitorsNon competitive Inhibitors




Non competitiveNon competitive InhibitorsInhibitors




Inhibitors and substrate are not competing for the sameInhibitors and substrate are not competing for the same

sitessites

An increase in substrate concentration will not thereforeAn increase in substrate concentration will not thereforereduce the effect of the inhibitor.reduce the effect of the inhibitor.

Allosteric site Allosteric site – – specific receptor site on some part of thespecific receptor site on some part of theenzyme molecule remote from the active site.enzyme molecule remote from the active site.





• E.g. – i) some poison absorbed from environment act by

inhibiting enzymes• Pesticides DDT and parathion are inhibitor of key

enzymes in nervous system

– ii) Many antibiotic are inhibitor of specific enzyme inbacteria• Penicillin block the active site of enzyme that many

bacteria use to make their cell walls





A Competitive inhibitor mimic

the substrate competing for the

active site

A Noncompetitive inhibitor bind to

the enzyme away from the active site,

altering the conformation of the

enzyme so that its active site nolonger function

Inhibition Of enzyme activity



Inhibitors leave the enzymeInhibitors leave the enzyme permanently damaged

Enzyme unable to carry out its catalytic function

Inhibitor attaches to the enzyme by covalent bonds

Irreversible InhibitorsIrreversible Inhibitors



• Heavy metal ions such as mercury (Hg 2+) and silver (Ag+ ) cause disulphide bonds to break!

• These bonds help to maintain the shape of the enzymemolecule

• Once broken the enzyme molecule

s structure becomes

irreversibly altered with the permanent loss of itscatalytic properties.




Example :

inhibition of cytochrome oxidase complex

by cyanide




• Cyanide is a poison chemical compound which can

inhibit the normal activity of cytochrome oxidase (Cytochrome oxidase is one of a superfamily of proteins which act as the

terminal enzymes of respiratory chains) – and enzyme in

mitochondria




• Cyanide bind tightly to the iron atom of the

enzyme cytochrome c oxidase – inactivate

the enzyme unable to transport e-

• Blocks the passage of electron transportchain, STOP the ATP production!



Summary

chapter 15 part 3 inhibition

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