Enzymes
Every chemical reaction between molecules involves
bond breaking and bond forming
ACTIVATION ENERGY =
amount of energy required to get chemical reaction started
Activation energy is often supplied in the form
of heat from the surroundings
Free energy animation
IT’S LIKE PUSHING A
SNOWBALL UP A HILL . . .
Once you get it up there,
it can roll down by itself
http://www.chuckwagondiner.com/art/matches.jpg
http://plato.acadiau.ca/COURSES/comm/g5/Fire_Animation.gif
LE 8-14
The Activation Energy Barrier
A
B
C
D
Free energy
Transition state
A
B
C
D
EA
Reactants
A
B
DG < O
C
D
Products
Progress of the reaction
CATALYST = a chemical agent that speeds up a
reaction without being consumed by the
reaction
ENZYMES = biological catalysts
Most enzymes are PROTEINS
Exception = ribozymes (RNA)
Free energy
Course of
reaction
without
enzyme
EA
without
enzyme
EA with
enzyme
is lower
Reactants
Course of
reaction
with enzyme
DG is unaffected
by enzyme
Products
Progress of the reaction
ENZYMES work by LOWERING ACTIVATION ENERGY;
ENZYMES LOWER ACTIVATION
ENERGY BY
– Orienting substrates correctly
– Straining substrate bonds
– Providing a favorable microenvironment
Enzymes change
ACTIVATION ENERGY
but NOT energy of
REACTANTS or PRODUCTS
http://sarahssureshots.wikispaces.com/Focus+on+Proteins
http://www.ac-montpellier.fr/sections/personnelsen/ressources-pedagogiques/education-artistique/consultation-avis-du
ENZYMES
•
•
•
•
•
•
Most are proteins
Lower activation energy
Specific
Shape determines function
Reusuable
Unchanged by reaction
Image from: http://www.hillstrath.on.ca/moffatt/bio3a/digestive/enzanim.htm
Enzyme Basics:
• The REACTANT that an enzyme acts on
= SUBSTRATE
• Enzyme + substrate =
ENZYME-SUBSTRATE COMPLEX
• Region on the enzyme where the substrate
binds = ACTIVE SITE
• Substrate held in active site by WEAK
interactions (ie. hydrogen and ionic bonds)
TWO MODELS PROPOSED
• LOCK & KEY
Active site on enzyme
fits substrate exactly
• INDUCED FIT
Binding of substrate causes change
in active site so it fits substrate
more closely
http://www.grand-illusions.com/images/articles/toyshop/trick_lock/mainimage.jpg
http://commons.wikimedia.org/wiki/File:Induced_fit_diagram.png
Enzyme Activity can be affected by:
– General environmental factors, such as
temperature, pH, salt concentration, etc.
– Chemicals that specifically influence the
enzyme
See a movie
Choose narrated
http://www.desktopfotos.de/Downloads/melt_cd.jpg
http://www.nealbrownstudio.com/adm/photo/163_nb_fried_egg.jpg
TEMPERATURE & ENZYME ACTIVITY
Each enzyme has an optimal temperature at which it
can function (Usually near body temp)
http://www.animated-gifs.eu/meteo-thermometers/001.htm
http://www.uic.edu/classes/bios/bios100/lectures/chemistry.htm
Increasing temperature increases the rate of an
enzyme-catalyzed reaction up to a point.
Above a certain temperature, activity begins to
decline because the enzyme begins to denature.
pH and ENZYME ACTIVITY
Each enzyme has an optimal pH at which it can
function
http://www.wissensdrang.com/media/wis9r.gif
COFACTORS
= non-protein enzyme helpers
• EX: Zinc, iron, copper
COENZYMES
= organic enzyme helpers
• Ex: vitamins
http://www.elmhurst.edu/~chm/vchembook/595FADcoq.html
SUBSTRATE CONCENTRATION
& ENZYME ACTIVITY
←
Adding substrate increases activity up to a point
V MAX