Download Section 6 – Catalysis

Unit 1
Cell and Molecular
Biology
Section 6
Catalysis
Chemical Reactions (Revision)
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Synthesis (anabolic)
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Condensation reactions
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Removal of water to form a bond
Degradation (catabolic)
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Hydrolysis reactions
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Addition of water to break a bond
Enzymes
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Proteases
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Nucleases
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Hydrolyse phosphodiester bonds
Break down nucleic acids into nucleotides
ATPases
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Hydrolyse peptide bonds
break down proteins into amino acids
Hydrolyse ATP
Break ATP into ADP and Pi with the release of energy
Kinases
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Catalyse the transfer of a phosphate group onto a
molecule such as a carbohydrate or a protein
Specificity of enzymes
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Induced-fit model
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When substrate combines with the enzyme
it causes a change in shape of the active
site
The change in shape results in an optimal
fit for the substrate-enzyme interaction
Once the product diffuses away, the
enzyme returns to its original shape
Cyanide
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Cyanide is found is a gas (sometimes liquid)
Used / found in
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House fires
Apricot stones
Suicide pills
Gas chambers (both US and Nazi Germany)
Stock piled by US and Soviet Union in 50’s and 60’s
Mining
Photography
Electroplating
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Binds to iron atom in the enzyme
cytochrome C oxidase
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This changes the shape of the enzyme
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Knowing how this works has important
applications for
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Detection of poisoning
Treatment
Control of enzyme activity
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Competitive Inhibitors
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Decrease the rate of reaction
Inhibitor is similar in structure and electrical
charge to substrate
It binds to the active site
An increase in the substrate can result in an
increase of product formation (inhibitor is out
competed)
Competitive inhibition can be reversible or
irreversible (depending on mechanism of binding)
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Non-competitive inhibitors
 Decrease the rate of reaction
 Inhibitors have no similarity to the substrate
 Inhibitor binds to part of the enzyme (other than the active site)
distorting the shape of the enzyme
 Increase in substrate concentration does not increase product
formation
 Can also be reversible
Control of enzymes – Enzyme
modulators
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Allosteric enzymes
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Allosteric enzymes have at least one other
binding site than the active site (called an
allosteric site)
Allosteric enzymes have 2 forms – active and
inactive
When a substance binds to an allosteric site it
changes the shape of the active site.
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Positive modulation
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The modulator changes the active site so the enzyme
becomes active (substrate fits)
Positive modulators are activators
Negative modulation
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The modulator changes the active site so the enzyme
becomes inactive
Negative modulators are inhibitors
Control of enzymes – Covalent
modifications
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Addition, modification or removal of a variety of
chemical groups
Changes the shape of the enzyme
Phosphorylation and dephosphorylation
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Kinase enzymes add phosphate
Phosphatase enzymes remove phosphate
Some enzymes are activated by phosphorylation,
others are inactivated (and vice versa for
dephosphorylation)
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Proteolytic cleavage
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Conversion of an inactive enzyme to an active one
Example Trypsinogen – Trypsin
Trypsinogen is synthesised in the Pancreas
Activation occurs when trypsinogen has amino acids
removed in the duodenum by another protease enzyme
This changes the trypsinogen into the active form trypsin
Trypsin then helps to activate more trypsinogen molecules
Control of metabolic pathways
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End product inhibition
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Chemical reactions are normally organised into
metabolic pathways with enzymes controlling
each chemical reaction
The end-product can act as a negative modulator,
binding to the first enzyme preventing the
metabolic pathway from proceeding because
intermediary substrates are not produced
This is a process of negative feedback
Activity
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Read and take notes from DART pg 61-68
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Scholar 6.3 and 6.4
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Check out http://highered.mcgrawhill.com/sites/0072437316/student_view0/chap
ter8/animations.html#
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Find examples for each type of enzyme control