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Enzymology

Objectives:
At the end of this lecture you should:
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Understand the concept of an enzyme
Have a basic understanding of how enzymes are made
Have a basic understanding of how enzymes work
Understand the criteria for optimum enzyme activity

Life is the process of brining order to the chemicals from which we are made
...
Life is the
bringing together of chemicals from our environment to build new cells (increase
order) and enable growth
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Entropy is a term that can be described as a measure of disorder in the universe
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When we bring chemicals together to make cells, entropy decreases and order
increases, which is known as enthalpy
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Therefore we can consider enzymes to be acting to change the entropy of a system
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Enzymes are used in metabolism
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E
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the breakdown of glucose into CO2 and H2O
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E
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the formation of complex fats from glucose
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Their molecular weight can vary from 5,000 to
5,000,000 Daltons (Da, the unit of mass)
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Enzymes are proteins with very special 3 dimensional structures
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Catalysts are molecules that are able to speed up the rate of a chemical reaction
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For
example, the spontaneous breakdown of sucrose (sugar) into its component glucose

and fructose could tae many years
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The job of enzymes is to increase the rate of chemical reactions within living cells
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Enzyme terminology
In enzyme catalysed biochemical reactions, the molecules being acted upon are
called substrates
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Sometimes enzymes need other molecules to support them
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Many of the cofactors are vitamins:
- B1 = Thiamine – helps decarboxylation (a chemical reaction that removes a
carboxyl group and releases carbon dioxide) reactions
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Some other cofactors are metal ions, including magnesium, zinc, iron, copper
manganese, potassium and molybdenum
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The cell, when needed,
can manufacture enzymes, and they can also be regulated (switched on and off)
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Enzymes are able to create conditions where the reaction takes place at the speed
needed
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They are able to increase the speed
of a reaction in some cases by a factor of a billion over the natural reaction speed
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Activation energy makes conditions favourable for the reaction to occur
For a reaction to proceed, the energy in the system has to be favourable to
movement in that direction
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Once started, the wood will continue to burn
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In the left figure, frogs
want to go from a high pool to a low pool but the wall is too high for most
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Enzymes effectively reduce the height
of the barrier
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Another analogy is shown in the picture below
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Once over the lip it can run down easily
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Enzymes are able to lower the activation energy such that the direction of the
reaction is now favourable
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There is a special
part of the enzyme into which the substrate fits – the active site
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Enzymes are very specific as to which substrates they can act upon
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Within the active site, an
enzyme/substrate complex is formed and the product will pop out (as shown in the
picture below)
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The rate at which enzymes are able to convert its substrates into products is affected
by a number of external factors:
- Concentration of substrate
- Temperature
- pH
Concentration of substrate
At low substrate concentrations, collisions between enzyme and substrate will be
infrequent and so products will be made slowly
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As the substrate increases further, a point will be reached where the enzymes are
cycling at their maximum rate and further increases in substrate concentration will
have no effect – the enzyme is saturated
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Inhibitors can bind reversibly to the active site so reduce access by the enzyme to
the substrate
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This is known as competitive inhibition
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This is known as non-competitive inhibition
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Inhibitors that bind irreversibly stop enzyme action
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For example, cyanide and carbon monoxide bind to enzymes of the respiratory path
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As temperature rises further, the weak molecular interactions that maintain the 3D
structure of proteins (enzymes) begin to break and the enzyme will unfold and lose
its critical structure
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Enzymes work best at 37˚C
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Above 55˚C, there will be little activity and proteins tend to become
denatured
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Affecting the activity of critical molecules within the active site
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A few enzymes will operate
at pH values significantly away from this
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Rate of enzyme action
A quantitative measure of the rate of enzyme action can be determined using the
Michaelis-Menten Equation: **



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