Enzymes
What is it??
• Enzymes are PROTEIN molecules.
• Protein molecules are composed of one or
more amino acid chains, folded into
uniquely shaped globs.
 Enzymes act as CATALYSTS!
 Catalysts are chemicals that regulate the
rate of chemical reactions.
 Are not consumed or altered during the
reaction
Activation Energy
 Activation Energy is the energy input required to
initiate any reaction.
Activation Energy
 Activation Energy is the energy input required
to initiate any reaction.
 Enzymes regulate cell activities (metabolism)
by lowering the activation energy
 reactions, therefore, occur more rapidly and at
lower temperatures.
Activation Energy
Activation Energy
Activation Energy
FUNCTION vs. SHAPE
TWO THEORIES
1) LOCK & KEY THEORY
• Each chemical reaction requires its own enzyme
therefore “one reaction = one enzyme” concept
• The enzyme forms a temporary bond with a
special molecule called a SUBSTRATE
• substrate a molecule on which an enzyme works
– A substrate is always…
» the substance acted upon
» the substance which is changing
• Active Site the area of an enzyme that
combines with the substrate
Get’in Together
• When the substrate and the enzyme
combine or “join” at the active site, the
tandem is called an Enzyme-Substrate
Complex.
Lock & Key (Con’t)
– The ENZYME-SUBSTRATE COMPLEX then
separate into product(s) and enzyme
Important
• Note that:
– The enzyme remains unchanged and ready to react
again with a new substrate.
Important
• The substrate has been turned into
products.
INDUCED FIT MODEL
• Improved Theory – 1973
– suggests that the shape of the active site
does NOT exactly fit the shape of the
substrate
– The substrate forces its way into the
enzyme
– This makes for a tighter fit
– The orientation of the substrate molecules
in the ENZYME-SUBSTRATE COMPLEX
helps speed up the chemical reaction by
 adding stress to bonds more easily
 bringing reactive sites physically closer
together
Induced Fit (Cont’d)
 Once a bond is formed (or broken) in
substrate(s) then products are released and the
ENZYME REMAINS UNCHANGED and may be
REUSED!
 A single enzyme can catalyze several million
reactions in one minute
 The same enzyme may also catalyze the reverse
reaction
 The net result is that a one step reaction is
converted into a multi-step reaction, therefore,
lowering the activation energy – the minimum
amount of energy required to initiate a chemical
reaction.
Naming Enzymes
 Enzymes are named after the substrate which it acts
upon
 To name an enzyme, usually, the suffix “ase” is
added to the end of the substrate name.
 For example:
Substrate
Sucrose
Lactose
Peptide Bonds
-Ketoglutarate ...
Enzyme
Sucrase
Lactase
Peptidase
?????
-Ketoglutarase
Regulation of Enzyme Activity
 METABOLIC PATHWAYS
 cellular processes that involve many steps are
controlled by enzymes
 one enzyme for each step.
Allosteric Activity
a change in an enzyme caused by the binding
of a molecule
 Some enzyme’s shape may be altered by a
“moderator molecule”.
 can be a cofactor (mineral)
 Coenzymes (organic molecules)
 sometimes even the product molecule.
A Moderator Molecule
Cofactors
Regulation of Enzyme Activity
 FEEDBACK INHIBITION
 Stops a metabolic pathway
 the product of an metabolic pathway acts as
a moderator on an enzyme in the series,
thereby altering its shape (active site)
 the enzyme cannot combine with the
substrate
 Once the moderator molecule is removed
from the moderator site, the active site
snaps back to its original shape.
Feedback Inhibition
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
Glucose
feedback inhibition
• feedback inhibition the inhibition of an
enzyme in a metabolic pathway by the
final product of that pathway
feedback inhibition
Factors Affecting Enzyme
Reactions
• There are four factors that affect the rate
at which an enzyme can work.
1) Temperature
2) pH
3) Substrate Concentration
4) Competitive Inhibitor Molecules
TEMPERATURE
in order for a reaction to occur
molecules must collide
 as temperature increases,
collisions increase
DOES RATE OF REACTION
INCREASE WITH
TEMPERATURE???
• NOT NECESSARILY!!
Enzymes have an optimal
temperature at which the reaction
is fastest.
 Beyond this temperature, the rate of reaction
decreases
 This is because at high temperatures, the unique
shape begins to change – denaturation.
 This results in a loss of the active site
 Each enzyme has its own optimal temperature
 Human body approx. = 370C
 Sperm producing enzymes = 340C
 This explains why fevers and colds are dangerous
pH
 acidity or alkalinity
 the lower the number the more acidic
 the higher the number the more alkaline
 Enzymes have an optimal pH at which the
reaction is fastest
 Just like with temperature, pH’s out of
the optimal range will cause a decrease
in rate of reaction
 shape changes = enzyme denatures.
CONCENTRATION
 Since molecules must collide for a reaction to
occur, it is only logical that the more substrates
you have, the greater the chance the enzyme will
have of combining and reacting with it.
 The rate does not continue to rise as you add
more and more substrate.
 There is a limit to the amount of enzyme available
 A substrate cannot join with the active site of an
enzyme until it is free.
 Therefore, once the number of substrate molecules
exceeds the number of enzyme reaction sites, the
reaction rate levels off.
Competitive Inhibitor Molecules
 Competitive Inhibitor molecules interfere
with the enzyme combining with its
substrate.
– Competitive Inhibitor
 shaped like substrate
 COMPETES for active site
 fits into active site
 = physically blocks substrate from entering active
site
 enzyme becomes useless
Competitive
Inhibition
Competitive Inhibitor Molecules
 Examples:
 Cyanide – binds to enzyme in the Electron
Transport Chain preventing formation of
ATP.
 Carbon Monoxide – binds to hemoglobin
irreversibly, therefore, no oxygen can be
carried
 Penicillin – binds to enzyme that allows
bacteria to make its protective covering,
therefore, bacteria becomes susceptible to
the immune system and other drugs