YEAR 12 - AS Biology
29th November 2005
Enzymes
Mr Potter
Lesson Objectives
• Enzyme unit overview
– What are they?
– How they work
• Activation energy
– What controls their activity
• Rates of reaction
• Substrate/enzyme concentrations
• Temperature, pH
• Enzyme inhibitors
– Practical to demonstrate “Catalase” activity in different tissue
samples
Previous related topics covered?
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Enzyme controlled reactions?
Proteins?
Lipase, protease, pectinase, amylase etc?
“Lock & Key” molecular structures?
By the end of the unit you should
be able to:• Explain enzymes as Globular Proteins which act
as catalysts
• Explain their catalytic action in terms of lowering
activation energy
• Describe examples of enzyme-catalysed
reactions
• Discuss factors affecting reaction rates and
inhibition
• Describe how to investigate these effects
experimentally
• Enzymes:–
Are defined as a BIOLOGICAL catalyst i.e. something that speeds up a
reaction. Up to 1012 fold
– Usually end in ‘…ase’.
– Discovered in 1900 in yeasts. Some 40,000 in human cells
– Control almost every metabolic reaction in living organisms
– Are globular proteins coiled into a very precise 3-dimentional shape with
hydrophilic side chains making them soluble
– Possess an active site such as a cleft in the molecule onto which other
substrate molecules can bind to form an enzyme-substrate complex
– Once the substrate has been either synthesised or split, enzymes can be re-
used.
– Do not ‘create’ reactions
– Widely used in industrial cleaning
– Often require co-factors (co-enzymes) to function – metal ions, or vitamin
derivatives
Amylase + starch substrate
How do enzymes work?
• Reaction Mechanism
– In any chemical reaction a substrate is
converted into a product.
– In an enzyme catalysed reaction the substrate
first binds to the active site of the enzyme to
form the enzyme-substrate complex
Molecule Geometry
• Substrate molecule fits into the enzyme
like a lock & key.
• Enzyme shape distorts or it changes other
factors to make the reaction happen
“Activation Energy”
• In a ‘natural’ reaction the product has a lower
energy than the substrate so equilibrium will
take it in the direction of the product.
• However there is an energy ‘barrier’ to be
overcome
• Enzymes lower the activation energy required to
bring about a reaction.
• EG catalase reduces the activation energy for
the reduction of H202 86-fold
Reaction rate factors
• Substrate
concentration
– Initially rate increases
with substrate conc.