Enzymes are proteins that act as catalysts to speed up chemical reactions in cells. They work by lowering the activation energy of reactions. Enzymes specifically bind to substrates at their active sites via lock-and-key or induced fit mechanisms. They speed up reactions without being used up. Enzyme activity is optimized at certain temperatures, pH levels, and substrate/enzyme concentrations and can be inhibited. Examples are named after their substrates except for exceptions like pepsin and trypsin.

1. Ian Anderson (2013)
Saint Ignatius College Geelong
05. Enzymes.

2. What are enzymes?
Enzymes are proteins (globular).
Act as organic catalysts.
 Enzymes speed up the rate of a chemical reaction
without themselves being used up (consumed) during
the reaction.
Are essential to the functioning of all cells.
 Without enzymes, metabolism would be too slow &
insufficient energy would be available to maintain life.

3. How do enzymes work?
Enzymes being catalysts work by lowering the
activation energy (activation energy = the amount of
energy required to ‘kick start’ the chemical reaction).
Source: http://www.emc.maricopa.edu/faculty/farabee/BIOBK/BioBookEnzym.html

4. How do enzymes work?
Enzymes don’t
Change the direction of a reaction, or
Alter the amount of product formed (or substrate
used).
Enzymes speed up the rate at which a chemical reaction
occurs without being used up during the reaction.

5. How do enzymes work?
Enzymes are very specific in their action, although
specificity varies from enzyme to enzyme.
There is a specific region of the enzyme (the active
site) where the compound being acted on (the
substrate molecule ) binds, allowing the enzyme to
catalyse the reaction.
When a substrate binds to an enzyme’s active site, an
enzyme-substrate complex is formed.
While attached to the substrate, the enzyme causes a
weakening of certain chemical bonds in the substrate
molecule thus allowing the reaction to proceed more
readily.

6. How do enzymes work?
Two models – Lock and key & Induced Fit.
Lock and key model.
 The configuration of the active site is complimentary to
the shape of the substrate molecule.
Source:
http://neurobio.drexel.edu/GalloWeb/loudon_enzymes.htm

7. How do enzymes work?
Induced fit model.
 The shape of the active site changes when the substrate
attaches to it. This ensures a good fit.
Source:
http://neurobio.drexel.edu/GalloWeb/loudon_enzymes.htm

8. How do enzymes work?
Catabolic reactions.
 Involve the breakdown of larger molecules into smaller
molecules, with the release of energy (exergonic).
 e.g. respiration and digestion
Anabolic reactions.
 Smaller molecules are joined together to form larger
ones. Requires the input of energy (endergonic).
 e.g. protein synthesis, photosynthesis

9. How do enzymes work?
Sometimes an enzyme’s active site is not a perfect fit and
the enzyme remains inactive until it can bind with
other molecules or ions that change the shape of the
active site (allowing them to achieve the perfect fit and
capture substrate molecules). Two classes of substance
play this role:
Cofactors (small inorganic substances such as Zn & Mg
ions), &
Coenzymes (non protein, organic substances such as
vitamins).

10. Examples of enzymes.
There are over 2000 know enzymes.
Enzymes are typically named for their substrates
 Maltase is the enzyme that acts on the disaccharide
maltose.
 Lipases act on lipids.
 Amylase acts on amylose.
Exceptions to the naming rule include pepsin &
trypsin.

11. Factors affecting enzyme activity.
Enzymes are very sensitive to the conditions in which
they work. They usually have a narrow range of
conditions under which they operate properly.
Factors affecting enzyme activity
 Temperature.
 pH.
 Enzyme concentration.
 Substrate concentration.
 Enzyme inhibitors

12. Factors affecting enzyme activity.
Temperature
 Enzymes usually work best in the temperature of the
environment in which they are found in.
 e.g. most human enzymes have an optimum
temperature of ~37ºC (= body temperature).
 At high temp enzymes are permanently denatured.
 At low temp enzyme activity slows down, however no
permanent damage is done to the enzyme.

13. Factors affecting enzyme activity.
Temperature
Source:
http://www.bbc.co.uk/scotland/education/bitesize/standard/biology/investigating_cells/e
nzymes_and_aerobic_respiration_rev3.shtml

14. Factors affecting enzyme activity.
pH
 Most human enzymes have an optimum pH between 6
and 8. e.g. Trypsin has an optimum pH of 8.0.
 Enzymes that are secreted into the stomach have a much
lower optimum pH. e.g. Pepsin has an optimum pH of
1.5.
 If pH is too high or low, enzyme can become denatured.

15. Factors affecting enzyme activity.
pH
Source:
http://employees.csbsju.edu/hjakubowski/classes/ch331/transkinetics/olinhibition.html

16. Factors affecting enzyme activity.
Enzyme concentration
Rate of reaction
continues to increase
with an increase in
enzyme concentration
(assuming non
limiting amount of
substrate)
Source:
http://www.rsc.org/education/teachers/learnnet/cfb/enzymes.htm

17. Factors affecting enzyme activity.
Substrate concentration
Rate of reaction
increases up to a point.
After this rate levels off
because all enzyme
molecules are working
at their maximum
capacity (i.e. active
sites are saturated). Source:
http://resources.edb.gov.hk/biology/english/health/
enzyme/enzyme_factors_subconc.html

18. Factors affecting enzyme activity.
Enzyme inhibitors
Two types
 Competitive inhibitors.
 Non-competitive inhibitors.

19. Factors affecting enzyme activity.
Competitive inhibitors – block the active site and stop
the substrate from getting in there.
e.g. poisons such as arsenic

20. Factors affecting enzyme activity.
Non-competitive inhibitors – bind to enzyme (but not
the active site) and cause it to change shape and
become less effective.