bio.lesson document 11

8/6/2019 bio.lesson document 11

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BIO 203 Biochemistry Iby

Seyhun YURDUGL, Ph.D.

Lecture 10

Enzyme inhibition


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Content Outline

Types of inhibition

Competitive inhibition

Non-competitive inhibition Uncompetitive inhibition

Feedback inhibition

Allosteric inhibition Irreversible inhibition


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Inhibition of enzymes

important for several reasons

serves as a major control mechanism inbiological systems

providing a means of regulating metabolic

pathways.


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Other reasons for inhibition of

enzymes

Also many drugs are used;

which act by inhibiting specific enzymes inbrain or body tissues,

so it is essential to understand the

mechanism of enzyme inhibition. Also used as tools to study the mechanismof enzyme action.


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Types of inhibition

Basically there are two types of inhibitors:

1. reversible, binds to the enzyme, but not

permanently so inhibition can be transient. 2. irreversible, binds permanently so

inhibition is complete.


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Types of inhibition

Reversible inhibitors:

can normally be removed by simple

dialysis; or change in buffer or pH.

Irreversible inhibitors:

normally bind covalently with the enzyme; and cannot be removed easily.


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Forms of reversible inhibition:

Three principal forms:

(a) competitive;

(b) non-competitive; (c) uncompetitive.


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The necessity for a close;

if brief, fit between enzyme and substrate:

explains the phenomenon of competitiveinhibition.

One of the enzymes needed for the

release of energy within the cell: succinic dehydrogenase.


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succinic dehydrogenase: catalyzes theoxidation (by the removal of two hydrogenatoms) of succinic acid

If one adds malonic acid to cells,

or to a test tube mixture of succinic acidand the enzyme,


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action of the enzyme succinicdehydrogenase: strongly inhibited.

due to the structure of malonic acid; allows it to bind to the same site on the

enzyme.

But there is no oxidation so no speedyrelease of products.


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The inhibition is called competitive:

because the ratio of succinic to malonic

acid in the mixture increase, it gradually restores the rate of catalysis.

At a 50:1 ratio, the two molecules compete

on roughly equal terms for the binding(=catalytic) site on the enzyme.


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normally strongly resembles the substratefor the enzyme in shape;

and size; and therefore competes with the substrate

for the substrate binding site on theenzyme.


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reduces the rate of reaction:

by lowering the proportion of enzymemolecules that have bound substrate.


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Mechanism of competitive inhibition

No products E + products


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So the more I present, the more EIformed, and the less product produced.

Classic example of this type is action ofmalonate on succinic dehydrogenase.

Malonate has one less methylene groupthan succinate (substrate),

and serves as a very strong competitiveinhibitor of this enzyme.


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It is common to have the product of a reaction;

being a competitive inhibitor of the substrate;

because of the structural resemblance of thetwo,

and in many cases the product of a reaction:

regulates the activity of an enzyme by a

feedback mechanism.


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Competitive inhibition:

Actual level of inhibition caused by acompetitive inhibitor:

is strictly dependent on the relative [I] and[S]

because they compete with each other.


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e.g. at low [I] the inhibition can beovercome by adding large [S];

to swamp out inhibition by I. This increases the amount of ES over EI.


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Because it is necessary to add more;

and more substrate to overcome the

inhibition this effects Km for the substrate.

i.e. Km will be higher.


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the plot for competitive inhibition:

characterised by straight lines of different

slope; all intersecting at a single point on the 1/vaxis i.e. y-axis.


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In competitive inhibition Vmax : notaltered;

but Km : altered, and this is the characteristic of competitiveinhibition.


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Non-competitive inhibition

In this type the S(substrate) and I(inhibitor)bind together on the enzyme,

but at different binding sites, S at active site;

and I at another site.


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The binding of the I exerts an effect on theactive site:

via a conformational change; to reduce the enzyme activity possibly byaffecting the structure of the active site;

so that it does not function efficiently.


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Consequently Vmax: altered;

but not Km (binding of S).


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The typical Lineweaver-Burk plot:

characterised by a series lines meeting on

the x-axis; showing Km constant,

but varying Vmax.


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So for non-competitive inhibition Vmax:altered;

but Km constant.


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Un-competitive inhibition

the I does bind at the active site;

but only AFTER the S has bound to the

active site; so it does not compete with the S.


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Thus even when the [S] =saturating;

and all E is in ES; the inhibitor can still bind to the active site;

and produce an inactive ESI complex.


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I only binds to ES;

so I stimulates formation of ES;

and increases binding of substrate toenzyme;

so Km is reduced.


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However the ESI complex is non-productive;

so Vmax lowered.


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Feedback inhibition

If the product of a series of enzymatic reactions,

e.g., an amino acid, begins to accumulate withinthe cell,

it may specifically inhibit the action of the firstenzyme involved in its synthesis (red bar).

Thus further production of the enzyme: halted.


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Feedback Inhibition


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Differences between feedbackinhibition and precursor activation

In feedback inhibition,

the allosteric effect: lowers the affinity ofthe enzyme for its substrate.

In precursor activation,

the regulator molecule: increases theaffinity of the enzyme in the series for itssubstrate.


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Allosteric inhibition

In the case if feedback inhibition andprecursor activation,

the activity of the enzyme is beingregulated by a molecule which is not itssubstrate.


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In these cases, the regulator moleculebinds to the enzyme at a different site thanthe one to which the substrate binds.


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When the regulator binds to its site,

it alters the shape of the enzyme so that

its activity is changed. This is called an allosteric effect.


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Irreversible inhibition

In contrast to the above types of reversibleinhibition,

where the effects on the enzyme reactiondepend on the concentration of an inhibitor (andits KI),

many examples of compounds that react

chemically with residues in the enzyme activesite. In these cases, enzyme activity: destroyed.


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For example, the "nerve gas" Sarin:

reacts specifically with an active site Ser residueon the enzyme, acetylcholinesterase.


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If acetylcholine cannot be hydrolyzed bythis enzyme:

nerve signals cannot be passed across thesynapses of the nervous system.

On exposure to this compound, death canresult in minutes due to respiratory failure.


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