Download ENZYMES - York Catholic District School Board

ENZYMES
Biological Catalysts

Enzymes are complex proteins, usually
having either tertiary or quarternary
structure, and are responsible for
mediating chemical reactions in organisms
Dissolving away
For example, think about hydrolysis
reactions that use water to break apart
major macromolecules
 If hydrolysis reactions were that easy to
initiate, that means you would dissolve
every time you contacted water!

Catalysts
Most reactions in organisms cannot happen
without the help of enzymes
 These enzymes accelerate the rate of
reactions and push them forward
 Therefore, enzymes are known as catalysts –
they lower the energy required to initiate a
chemical reaction, increasing the likelihood
that the reaction occurs

Lock and Key
Enzymes, structurally, are designed to fit
specific SUBSTRATES – the reactants in
the biochemical reactions
 This is known as a “lock and key”
mechanism where each enzyme is
specialized

The substrate
binds on the ACTIVE SITE of the enzyme where the
chemical reaction will occur
Other binding sites might be present on the enzyme that are secondary
to the active site. These are known as ALLOSTERIC SITES and
generally do not result in the formation of a new product via a chemical
reaction
Enzyme + substrate

enzyme substrate complex
Tertiary and Quarternary structure
Notice, therefore, that enzymes are
complex proteins – often having at least
tertiary structure and sometimes
quarternary structure
 The complex possibilities seen in protein
folding produces a large number of
structural shapes necessary to produce
specialized enzymes for each possible
chemical reaction

Specificity is key
Because they are so specific, each enzyme
is designed to accelerate only one type of
chemical reaction
 http://highered.mcgrawhill.com/sites/0072495855/student_view0/
chapter2/animation__how_enzymes_wor
k.html

Enzymes are custom made
On top of being specific to certain
reactions, enzymes are also designed to
function optimally under certain
conditions
 The following factors can affect enzyme
function:

◦ pH
◦ Temperature
◦ salinity
Why the sensitivity?
Enzymes are located everywhere in living
organisms
 Each living organism must survive in
ecosystems that can affect their internal
systems (for example, crustaceans living in
high temperature sea vents)
 Even organisms that have their internal
systems well shielded from the external
enviornment (like us)

Specialists in special systems
For example, within a human being, pH levels
can vary within the digestive system
 The stomach is a highly acidic area (pH = 1)
whereas the duodenum is a more basic area
thanks to the pancreas (pH = 5-6)
 Therefore, enzymes that function in the
stomach must be able to function in low pH
vs. those that function in the duodenum that
must function in higher pH

Why?
This comes back down to understanding
the biochemistry of enzymes
 Remember that functional groups in
amino acids create enzymes that can be
more basic, acidic, hydrophilic or
hydrophobic
 These characteristics help an enzyme to
survive better in certain environments

Coming undone
Enzymes can “denature” - acidic/basic environments
or increased temperatures can interfere with basic
bonds (disulfide bridges, Vanderwaals interactions,
etc.) that will cause the enzyme to unfold
 Remember: a loss of structure = loss of function
 An enzyme that cannot hold its 3D shape loses its
ability to “fit” with a substrate
 http://www.sumanasinc.com/webcontent/animations/c
ontent/proteinstructure.html


http://www.lewport.com/10712041113402793/lib/10712041113402793/Animation
s/Enzyme_activity.html
Pepsin
http://www.biotopics.co.uk/JmolApplet/pe
psin.html
 Pepsin will denature at pH levels of more
than 5.0
 This means that as food moves from your
stomach to your duodenum pepsin is
inactivated by the increase in pH – this
allows other proteases in the duodenum
to take over protein digestion

Temperature
In regard to temperature, the ideal
temperature for most enzymes is
dependent on the average internal
temperature of the organism
 Human enzymes function optimally at
internal body temperature = 370
 However, as mentioned before, there are
some enzymes that can function well at
more extreme temperatures

Enzyme kinetics
Enzymes are used widely in the production
of various chemical substances in
manufacturing, or for laboratory tests
because they are such efficient ways to
speed up chemical reactions
 Enzymes can be used over and over again to
catalyze numerous reactions (they will
degrade eventually though)
 Therefore, the knowledge of optimal ranges
of enzyme function are beneficial for this
reason

Enzyme kinetics
Therefore, enzyme kinetics is the study of
enzymatic function – how fast an enzyme can
catalyze a reaction
 The speed at which an enzyme can catalyze a
reaction is best illustrated by how much
product is produced in the chemical reaction
over time


Substrate + enzyme  product + enzyme
Vmax and Km
Vmax = fastest rate at which substrate
can be produced by the enzyme
 Km is a rate constant - it describes how
much the enzyme “wants” to binds to the
substrate – it usually is related
numerically to half of Vmax

Why does it plateau?
In general it makes sense that if you add
more substrate to a given concentration
of enzymes, you should get more product
 However, you should notice that the
curve plateaus eventually
 Why?

Think about function
Remember that in order to carry out a
reaction, the enzyme must bind to a
substrate
 If a small concentration of substrate is
added to a group of enzymes, it makes
sense that more product can be produced
as the concentration increases – because
you are engaging more and more enzymes
in the reaction process

Think about function
But if the concentration of enzyme is not
increased, and more and more substrate
is added, the rate of product production
starts to slow down
 If each enzyme in the reaction is attached
to a substrate, adding more substrate will
not increase the rate of reaction since
each enzyme is already occupied with a
substrate

Think about function
Therefore it can be said that the enzyme
concentration is the limiting factor
 Increasing the amount of substrate will
not cause the rate of the reaction to in
increase unless enzyme concentration
increases

Inhibition




Inhibition occurs when enzyme function is
tampered with – this is sometimes necessary
to control enzymatic function
This can occur two ways:
COMPETITIVE INHIBITION: another
substance binds to the active site of the
enzyme
NON-COMPETITIVE INHIBITION: another
substance binds to a non-active site – an
allosteric site that prevents the binding of
the main substrate

The most common form of noncompetitive (allosteric) inhibition occurs
when the binding of an inhibitor to an
allosteric site causes a conformational
change in the enzyme making it unable to
bind with the usual substrate
Why does Vmax and Km change?
Binding the active site will make the enzyme
less “desirous” to bind the substrate – since it
is already bound to something else – this
affects Km
 Binding an allosteric site might not change the
enzyme’s desire to bind the substrate, but it
will affect its ability to carry out chemical
reactions, therefore affecting Vmax
