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Enzymes
Enzymes are catalysts that:
 Participate in reactions but are not changed by the
reaction
 Increase the rate of a reaction.
 Can be used over and over again.
 Bring substrate molecules together for reaction to
occur
Why do cells need enzymes?
 Brownian motion (random motion) controls most
reactions.
 Result is reactions move very slowly.
 Usual way to increase rate is to increase temperature
(thus increase motion of particles and chances of
two substrate molecules meeting and reacting)
 Heat increase will destroy the cell
Enzyme Theories:
 Lock and Key Theory: (old way of thinking)
 A site on enzyme where substrate attaches. (exact fit
like a lock and key)
Each enzyme is specific to
one substrate
Current Theory:
 Shape of active site on enzymes do not match the
shape of the substrate
 What actually happens is the substrate binds to the
enzyme, which causes enzyme to change shape such
that it forms an exact fit for the substrate
 Called the Induced Fit Theory
Like a “hand in a glove”
How enzymes work:
1. One or more substrate molecules bind to active site
on enzyme – form substrate enzyme complex
2. The interactions of the substrates and the active site
stress (weaken) bonds in substrate
3. Stresses allow the substrate molecules to
chemically bind – forming a new product
4. New product is released, enzyme is ready to work
again
Animation:
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/chapter2/ani
mation__how_enzymes_work.html
Naming Enzymes”
 International guidelines – name is based on the
reaction they catalyse, and “ase” is added at end.
 Transferase:
 transfer of a molecular group from one molecule to
another
 Hydrolase:
 bond cleavage by the introduction of water
Factors that affect rate of
reaction:
 Changes in”




temperature
pH
concentration of substrate
concentration enzyme
Denaturing:
 Inactivation of a protein by changing the
structure/shape
 Commonly caused by changes in temperature or pH
 Examples
 Cooked egg whites
 Vinegar in milk
Temperature and enzymes
 Enzymes are sensitive to temperature
 Have an optimum activity/efficiency near normal
temperature of organism the function in
pH and enzymes
 A similar relationship to temperature
Other factors:
Substrate concentration
Enzyme concentration