Enzymes are globular proteins that regulate and speed up metabolic reactions in cells. They lower the activation energy needed for reactions to occur at normal body temperatures. Enzymes have an active site that binds specifically to substrates, catalyzing their conversion to products. The substrate fits into the active site like a key in a lock (lock-and-key model), though the induced fit model suggests the active site changes shape to better accommodate the substrate. The rate of enzyme-catalyzed reactions depends on factors like temperature, pH, enzyme and substrate concentrations - each enzyme works best within a specific range of conditions.

1. Enzymes
Topic 2.1

2. What are Enzymes?
The chemical reactions that take place inside an organism, i.e. its’
metabolism, must be controlled. The rate of these metabolic
reactions is controlled by enzymes.
Enzymes are globular proteins (3-D spherical shape). They are usually
named by adding the suffix –ase to the name of the substrate it
helps metabolise.
E.g. Maltase – helps break down the disaccharide maltose
Lactase – helps break down the disaccharide lactose
Amylase – helps break down the polysaccharide starch (amylose)
• Enzymes regulate the metabolic processes that occur in cells
• They are catalysts, they speed up reactions and are not used
up themselves in the reaction
• The energy needed to start a reaction is the ACTIVATION
ENERGY
• Enzymes lower the activation energy, so reactions can take
place at acceptable temperatures in living organisms

3. Activation Energy
Activation energy is the amount of energy required to
start off a chemical reaction, where the reactant(s)
are turned into product(s).
The reaction will not occur unless ‘extra energy’ is
given temporarily to the molecules:
WITH ENZYME – when a WITHOUT ENZYME –
substrate binds to an to change into a product the
Potential Energy
Potential Energy
enzyme, the ActivationActivation
(of molecules)
energy of the substrate must
(of molecules)
Energy Activation
Energy is lowered because be briefly raised by an
Energy
they split the reaction into amount Substrate(s)as the
known
Substrate
Substrate(s) Substrate
smaller stages Activation Energy
Product(s) Product(s)
Time Time
http://www.sumanasinc.com/webcontent/animation

4. Why do enzymes increase the rate?

5. How Enzymes Work
Enzymes are globular proteins, coiled into a precise 3-D shape,
with hydrophilic side chains on the outer parts of the molecule
(making them soluble).
Enzymes have a special feature called an active site, where a
substrate molecule(s) bind to, forming an ‘enzyme-substrate
complex’. The substrate is then converted to product(s).
Substrate (has
complimentary Product
shape)
Enzyme
Active
Site
Here side-chains of
amino acids in the active
site temporarily bind with
the substrate
Enzyme-substrate
Enzyme + Substrate Enzyme + Product(s)
Complex

6. Hypotheses
This leaves the active site unchanged and ready to bind
with another substrate molecule.
This was traditionally called the ‘Lock and Key’
hypothesis because the substrate fits into a rigid
active site like a key into a lock.
This explains why enzymes are very specific (the
substrate must fit the active site exactly to be
catalysed).
However this theory has since been developed into the
‘Induced Fit’ hypothesis. Here the active site is
believed to change shape slightly so that the enzyme
moulds itself around the substrate. (See fig 3, p.43)
Now answer Qu.2 from p.43 in your book)
http://www.sumanasinc.com/webcontent/animations/content/enzymes/enzymes.html

7. Models of enzyme action:
lock-and-key

8. Models of enzyme action:
induced fit

9. Enzymes: true or false?

10. What factors affect enzymes?
The rate of an enzyme-controlled reaction is affected by
several factors:
 temperature
 pH
 enzyme concentration
(you must know in detail!)
 substrate concentration.
Each enzyme works best within a
range of conditions, and this range is
different for each enzyme.
Enzymes are also affected by the
presence of inhibitors.

11. Effect of enzyme concentration on rate

12. Rate of reaction experiment

13. Effect of temperature on enzymes

14. Effect of pH on enzymes

15. Factors affecting rate of reaction