Enzymes 2007
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Transcript of Enzymes 2007
7/28/2019 Enzymes 2007
http://slidepdf.com/reader/full/enzymes-2007 2/14
• 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.
7/28/2019 Enzymes 2007
http://slidepdf.com/reader/full/enzymes-2007 3/14
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]
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
http://slidepdf.com/reader/full/enzymes-2007 6/14
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.
7/28/2019 Enzymes 2007
http://slidepdf.com/reader/full/enzymes-2007 8/14
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.
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
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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.
7/28/2019 Enzymes 2007
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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