1. Enzymes

2. Lecture outlines
•Catalysis
•Activation energy & its
profile
•Enzyme & substrate
•How enzymes bind to
substrates
•Lock & Key model
•Induced-fit model
•Enzyme assay

3. Lecture outcomes
• At the end of this lecture, students are
able to:
• Define the catalyst
• Understand how enzymes work as catalysts,
the concept of activation energy and
enzymes-substrate binding
• Explain different theories of the relation
between enzymes and substrates

4. Catalysis
• It is probably the most important function of all
proteins
• Most reactions in biological systems would take
place far too slowly in the absence of catalyst
• The catalysts that serve this function in
organisms are called enzymes
• All other enzymes are globular proteins (with the
exception of some RNAs (ribozymes) that have
catalytic activity)

5. Continue
• Enzymes can increase the rate of a reaction by
factor of up to 1020 over uncatalyzed reactions
• Non-enzymatic catalysts typically enhance the
rate of the reaction by factors of 102 to 104
• Catalysts are substances that speed up the rate
of a chemical reaction
• Biocatalysts or enzymes are biological catalysts
that speed up the metabolic reactions that
occur in the body

6. Catalase
2 H2O2 ⇄ H2 O + O2
3% of hydrogen peroxide in water at 37ºC
a = no catalyst added
b = with Fe3+ salt
c = with enzyme (catalase)

7. Hydrogen peroxide
• Hydrogen peroxide is a waste product of
metabolism, and if it left in the cell, it would
initiate the formation of free radical

8. Activation energy
• The amount of energy that must be acquired by
reactant molecules before they can be
converted to a product
• It shows the intermediate stages of a reaction,
those between the initial and final states.
• It directly affects the rate of reactions.
• It speeds up a reaction by changing the
mechanism and thus lowering the activation
energy

9. Continue

10. Continue
• Plots the energies for an
exergonic, spontaneous
reaction, such as the
complete oxidation of
glucose. At the maximum of
the curve connecting the
reactants and the products
lies the transition state with
the necessary amount of
energy and the correct
arrangement of atoms to
produce products.

11. Continue
• The activation energy can also be seen as
the amount of free energy required to
bring the reactants to the transition state.
• The change in energy corresponds to the
change in elevation, and the progress of
the reaction corresponds to the distance
traveled

12. Continue
• Considerable effort has gone into
elucidating the intermediate stages in
reactions of interest chemists and
biochemists and determining the pathway
or reaction mechanism that lies between
the initial and final states.
• Reaction dynamics, the study of the
intermediate stages of reaction mechanism,
is currently a very active field of research

13. Enzymes and substrates
• In an enzyme-catalyzed reaction, the enzyme
binds to the substrate to form a complex
• The formation of the complex leads to the
formation of the transition-state species,
which then forms the product.

14. Enzymes-substrate binding
• In an enzyme-catalyzed reaction
– Substrate, S (a reactant)
– Active site: the small portion of the enzyme
surface where the substrate(s) becomes
bound by noncovalent forces, e.g.,
hydrogen bonding, electrostatic attractions,
van der Waals attractions
E + S ES
enzyme­substrate
complex

15. Lock and Key Model
• 1890 as envisioned by Emil Fischer
• The enzyme active site (lock) is able to accept only
specific type of substrate (key)

16. Induced –fit model
• Proposed by Daniel Koshland in 1958
• The active site in the absence of substrate is a rather nondescript
region of the enzyme
• The process of substrate binding induces specific conformational
changes in the protein structure especially in the active site region.

17. Continue
• The final shape and charge characteristics
of the active site are not in place until the
substrate is completely bound.

18. Enzyme Assay
• The amount of product formed in a given time
period.
• The concentration of enzyme in a test sample
can be determined by comparing its activity
to a standard curve.