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Enzymes

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•  ENZYMES are proteins that

catalyses specific reactions.

• Basic mechanism involved

 binding of the substrate(s)

to the active site on the enzyme.

Active site:: specificregion of the enzyme

which combines with

the substrate.

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Enzymes and chemical catalyst

• They differ in:

1) Higher reaction rates by enzyme

2) Enzymes catalysed reaction happened in

milder conditions

3) Enzymes catalysed specific reactions anddo not produce side products

4) Catalytic activities by enzymes varied by

[substances]

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Descriptions of shapes of enzymes

• Water-soluble globular proteins

•  Active site recognized certain substrate.

• Specificity determined by chemical nature of 

the amino acids R-groups located at the

active site.•  Active site occupies < 5% of enzyme surface

area and involves 3 to 12 amino acids.

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The key and lock model• Proposing by Fischer 

• Enzymes catalysed reactions by binding tosubstrates

• The substrate (key) fits into enzyme (lock)

•

Once the product formed, they leave theactive site and the enzyme is free to

combine with new substrate.

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Competitive Inhibition

• Competitive inhibitors are substrate that

have a similar shape to the substratemolecule.

• Can bind to the active site but cannot take

part in the catalysed reaction.

• Reversible by increasing concentration of 

substrate.

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Non-competitive Inhibition• Most non-competetive inhibitors bind weakly

to the enzyme• [Inhibitors] falls, enzyme-inhibitor complex

falls apart and the functional shape of theenzyme restored.

• Reversible• Eg heavy metal like Ag or Hg can replace H

in one or more  –SH group, it change theshape of the enzyme.

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Factors affecting enzyme activity

1. Temperature  – affect speed, activation

energy of the catalysed rxn, thermalstability of the enzymes and substrate.

0 to 40º C, rate increase with T. Optimum at

40º C, decrease above 40º C. heat

denatured at 65º C.

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2. pH  – extreme pH will denature proteins by

disrupting the precise 3-D arrangement of the protein chains.

Small changes in pH affect the ionisation of 

amino acid side-chains in the active site.

Each enzyme has its own distinct optimum

pH. Eg figure 1.27 pg 23.

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3. Chemical denaturation

High salt concentration changes the ionicenvironment of an enzyme, disrupting the

ionic interactions between different regions

of the chain.

Urea denatures protein by disrupting the H-

bonds that maintain the secondary and

tertiary structure of proteins.

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• Certain chemical inhibitors totally inactivate

enzymes, it means their effect is irreversible.• Eg DFP binds to serine in active sites of 

chymotrypsin. Sarin (nerve gas) is very

similar in structure to DFP.

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•In the secondary structure, hydrogen bonding occurs between amides.

•In the tertiary structure, hydrogen bonding between

"side chains" occurs

•All these bonds are disrupted by the addition of the

 – OH functional group.

ALCOHOL