Enzymes
Enzymes, substrates and active sites
 Enzymes are globular proteins that work as catalysts
 Substrates are the substances that enzymes convert into products during
metabolic reactions.
 Active site is the region on the surface of an enzyme to which substrates bind
and which catalyzes the reaction.
http://www.regentsprep.org/regents/biology/units/homeostasis/processes.cfm
Enzyme – substrate complex
 Once a substrate
has been locked
into the active
site, the reaction
is catalyzed
 The products are
released and the
enzyme is used
again.
 Animation: How Enzymes Work
Enzymes are specific to their substrates
 Enzymes are highly
specific and work only
on specific substrates
(enzyme substrate
specificity)
 Two theories explain
the substrate
specificity of enzymes:
Lock and key model
 Induced fit model

Making and testing theoretical models
The German scientist Emil Fisher introduced the lock and key model in
1890. Daniel Koshland suggested the induced-fit model in 1959 in the
US. The conformational changes predicted by Koshland’s model were
subsequently observed using high-resolution X-rays analysis of
enzymes and other newly developed techniques. Although much
experimental evidence has accumulated confirming predictions based
on the induced-fit model, it is still just viewed as a model of enzyme
activity.
 Why has the lock and key model been superseded by the induced-fit
model?
 Could there ever be a situation in which two models are both retained?
 Why is the induced-fit model not now called the Law of Induced Fit?
Enzymes lower the activation energy of a reaction
(HL)
 Activation energy is
the amount of
energy that must be
put into a reaction
to make it occur.
 An enzyme stresses
the bonds in the
substrate(s),
reducing the
activation energy
required for the
reaction to occur.
http://njms2.umdnj.edu/biochweb/education/bioweb/PreK201
0/EnzymeProperties.html
http://home.pacbell.net/lpoli/page0001.htm
 Enzymes are globular proteins with a
tridimentional structure
 Their structure can be altered by several factors
 When the shape of their active site is changed
considerabily they will not function.
 Denaturation is changing the structure of a
protein (enzyme) so that it can not carry out its
function.
http://qwickstep.com/search/
denaturation-of-protein.html
Factors Affecting Enzyme Activity
 Effect of pH on enzyme activity
•Enzymes only operate in a
narrow range of pH values.
The optimum pH
•If there is a deviation from
the optimum pH, hydrogen
bonds in amino acids are
broken an the enzyme is
denatured.
http://www.dentistry.leeds.ac.uk/biochem/lecture/en
zymes/index.html
•Different enzymes have
different optimum pH and
work in different pH
environments.
Factors affecting enzyme activity
 Effect of temperature on enzyme activity
•As temperature increases the rate of
reaction increases. The particles have
more kinetic energy, therefore collide
and react more often.
•Above the optimal temperature
denaturation of the enzymes occur,
the active site changes and looses its
function.
•A thermophile, such as bacteria in
deep sea vents, is an organism that is
able to withstand higher
temperatures before enzymes
denature.
http://staff.jccc.net/pdecell/metabolism/enzy
mes/enzymes.html
Factors Affecting Enzyme Activity
 Substrate concentrations

Increasing the substrate concentration
increases the rate of reaction.

At the optimum concentration of
substrate molecules, all the active sites
are full and working at a maximum
efficiency

Any increase in concentration beyond
the optimum will have no added effect
as there is no extra active sites to be
used.
http://leavingbio.net/ENZYMES.htm
Lactose Intolerance
 Lactose, the sugar found in milk causes allergies in some people
because they are unable to produce enough of the enzyme lactase.
lactose
- lactase--
glucose
+
galactose
Global estimates of lactose intolerance
How can we cope with lactose intolerance
Take a lactase supplement
1.

Produced industrially using the Aspergillus niger fungus
Drink lactose free milk
2.
1.
2.
http://curiosidadmedica11.blogspot.
Milk is treated with lactase (produced by A. niger) and pre-digested
before being
com/
packaged.
Lactase can be either added directly to milk or be inmmobilized on a surface or
beads of a porous material.
http://lactoseintoleranceforum.com/lactase-enzymebiology_lactase-enzyme-biology-lactose-intolerancebegins-here_29.html
Metabolic Pathways (HL)
 Chains or cycles of enzyme catalyzed reactions
 The Krebs cycle (cell respiration) and the Calvin cycle (photosynthesis)
are examples of metabolic pathways that form cycles
http://cuzwearefriends.blogspot.com/2010
_05_01_archive.html
http://www.starsandseas.com/SAS%20Cells/SAS%20c
ellphysiol/SAS%20photosyn/cellcalvin.htm
Metabolic Pathways (HL)
 Protein synthesis is a metabolic pathway that occurs in a
chain.
http://www.cssforum.com.pk/css-compulsorysubjects/everyday-science/everyday-science-notes/11865glossary-biology-2.html
Enzyme Inhibition(HL)
 Enzymes can be inhibited
by other molecules.
 The process can be:
 Competitive

http://www.biologypictures.net/competitive-inhibition-enzym/
Non – competitive
Allosteric interactions
 Allosteric site

http://www.emc.maricopa.edu/faculty/farabee/biobk/biobookenzym.
html
End – product inhibition of metabolic pathways
(HL)
 Metabolic pathways can be switched off completely in cells where there
is an excess of product.
 The end product inhibition prevents a build up of intermediate
products.
http://www.occc.edu/biologylabs/Documents/Homeos
tasis/End_Product_Inhibition.htm
 Feedback Inhibiton of Metabolic Pathways