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Enzyme InhibitionBesides denaturation, another important form of catalyst deactivation is called‘inhibition’‘inhibition’. (Think of it as the ‘enzyme equivalent’ of catalyst poisoning!)

The following types of inhibition need to be distinguished:

(1) Reversible(1) Reversible inhibitioninhibition

Inhibitors that can reversibly bind and dissociate from enzyme, activity of enzyme recovers when inhibitor diluted out, usually non-covalent interaction

– Competitive:

– Mixed (noncompetitive):

– Uncompetitive

(2) Irreversible(2) Irreversible inhibitioninhibition

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Irreversible Inhibition

Irreversible inhibitionIrreversible inhibition is caused by ‘inactivators’ (we called these ‘poisons’ in heterogeneous catalysis!) that associate irreversiblywith the enzyme at the active site.

Most typically, covalent bonds are formed.

Due to the irreversible nature, the activity of enzyme does not recover with dilution.

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Competitive InhibitionGeneral Scheme:General Scheme: Example:Example:

A substrate & its inhibitorA substrate & its inhibitor

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Effect of Competitive Inhibition on Kinetics

with KI (inhibitor dissociation constant) = kI,b/kI,f

…assuming again ESA for both ES and EI formation, we obtain:

We can re-write this as:

where we have defined an ‘apparent’ Michaelis constant.

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Effect of Competitive Inhibition on Kinetics

–– KKmm is changed:is changed:

-- vvmaxmax is is

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Mixed (Noncompetitive) Inhibition

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Mixed (Noncompetitive) Inhibition

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Effect of Mixed Inhibition on Kinetics

where we have assumed (for simplicity) thatKI ~ K’I , i.e. the dissociation constants for the EI and ESI complexes are (almost) identical.

…assuming again ESA for ES, EI, and ESI formation, we obtain:

We can re-write this as:

where we have defined an ‘apparent’ maximum velocity.

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Effect of Mixed Inhibition on Kinetics–– KKmm is unchanged:is unchanged:

Nevertheless,

-- vvmaxmax isis

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Uncompetitive Inhibition……notnot the same as ‘nonthe same as ‘non--competitive’ inhibition!!competitive’ inhibition!!In uncompetitive inhibition, the inhibitor only binds to the ES complex, but can not bind to the free enzyme!

…again with equilibrium-stepassumption for ES and ESI formation, we can derive:

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Uncompetitive Inhibition

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Generalized InhibitionOne can derive a generalized One can derive a generalized MichaelisMichaelis--MentenMenten equation that accounts for all types of equation that accounts for all types of inhibitioninhibition

We identify the following possibilities:

Non-inhibitedCompetitive Inhibition

Non-competitive InhibitionUncompetitive Inhibition

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Generalized InhibitionOne can derive a generalized One can derive a generalized MichaelisMichaelis--MentenMenten equation that accounts for all types of equation that accounts for all types of inhibitioninhibition

And one other case:

Mixed Inhibition

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Quick Learning CheckIdentify each statement as describing an inhibitor that is

(1) Competitive (2) Noncompetitive (3) Uncompetitive

A. Increasing substrate concentration reverses inhibition

B. Binds to enzyme, but not at active site

C. Binds to enzyme-substrate complex, but not to free enzyme

D. Structure is similar to substrate

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Enzyme Inhibition: ExampleRemember the problem from last class?

You are working for a biochemical company and are given a 4.0 mg/ml solution of an enzyme (M = 40,000 Da) which catalyzes the conversion of a substrate A to a product P. You are requested to determine Km and vmax for this enzyme.

You are now given the additional information that the enzyme is You are now given the additional information that the enzyme is compromised by the compromised by the presence of an inhibitor I. You need to determine Kpresence of an inhibitor I. You need to determine KII and the type of inhibition. and the type of inhibition.

How do you proceed?How do you proceed?

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Enzyme Inhibition: Example

Lineweaver-Burk Plot

05

101520

253035

404550

0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

1/[A]

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Lineweaver-Burk Plot

05

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0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0 4.5

1/[A]

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Solution:

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Enzyme Inhibition: Example

y = 1.2053x + 4.4211R2 = 0.9889

0.0

2.0

4.0

6.0

8.0

10.0

12.0

0.0 1.0 2.0 3.0 4.0 5.0 6.0

[I] mM

Slop

es o

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lot

Solution:

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Co-Factors & Co-EnzymesIn some enzymatic reactions, additional

‘activators’ are needed for the activity of specific enzymes.

Two types of activators are commonly distinguished:

CofactorCofactor::

CoenzymesCoenzymes::

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Major Uses for EnzymesTextile processing

Grain processing

Food processing

Cleaning

Feed enzymes

Diagnostic/pharma

Waste management

Textile processing

Grain processing

Food processing

Cleaning

Feed enzymes

Diagnostic/pharma

Waste management44%

18%

12%10%4%4%4%4%

CLEANING:

FOOD PROCESSING:

GRAIN PROCESSING:

TEXTILES:

FEED ENZYMES:

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Enzyme ImmobilizationJust like for non-biological homogeneous catalysts, separation from the reaction products and subsequent purification is a major hurdle for industrial enzyme use. (Since enzymes are VERY expensive, catalyst re-use is essential!)Just like for non-biological homogeneous catalysts, immobilization of enzymesimmobilization of enzymes is the typical approach to overcome this problem.

Main techniques used for enzyme immobilization:

• surface immobilizationsurface immobilization:

• matrix entrapmentmatrix entrapment:

• crosscross--linkinglinking:

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Enzyme ImmobilizationWhile immobilization of enzymes overcomes problems such as separation, it can result in significant loss of enzyme activityloss of enzyme activity. This is most often due to conformational changes in the enzyme upon immobilization.

methodmethod supportsupport enzyme activity enzyme activity after immobilizationafter immobilization

entrapment polyacrylamide

encapsulation nylon

ionic binding DEAE cellulose

covalent binding iodoacetyl cellulose

cross-linking with glutaraldehyde

AE-cellulose

Effect of immobilization methods on the enzymatic activity of Effect of immobilization methods on the enzymatic activity of aminoacylaseaminoacylase

(source: Wang et al., “Fermentation & Enzyme Technology, Wiley & Sons, 1979)

Immobilization of enzymes also introduces another problem: transport limitationstransport limitations! (Treatment analogous to Het Cat! Thiele modulus, effectiveness factor, etc…!!)

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Enzymes & Mass Transport LimitationsRemember that we claimed that homogeneous catalysis does not usually suffer from mass transport limitations? Due to the extremely high reactivity of enzymes, this does often not hold for enzymatic reactions (although they are ‘homogeneous’) – even when they are not immobilized.

In an aqueous environment (= most enzymatic reactions), the diffusion limited reaction rate is about 108 to 109 mol/(L s). (Even enzymes do not actively ‘hunt down’ substrate (Even enzymes do not actively ‘hunt down’ substrate molecules but rely on reactive collisions by diffusion or convecmolecules but rely on reactive collisions by diffusion or convective transport…!)tive transport…!)