AP Biology Chapter 6:
An Introduction to Energy and
Enzymes
Metabolism
 Totality of an organism’s
reactions
 (from Greek metabole, to
change)
 An emergent property from
interactions between
chemicals within the
environment of the cell
 Concerned with managing the
material and energy resources
of the cell
Metabolism
 Two types of reactions:
 Catabolic Pathways: release energy by
breaking down complex molecules into
simpler compounds (e.g. cellular respiration)
“downhill” reactions
 Anabolic Pathways: consume energy to
build complicated molecules from simpler
ones (e.g. synthesis of proteins from amino
acids)
“uphill” reactions
 These reactions are coupled together
Bioenergetics
 The study of how organisms manage their
energy resources
 Energy: the capacity to do work—ability to
rearrange a collection of matter
Kinetic: energy of motion
Potential: stored energy
Chemical: form of potential energy stored in
molecules as the result of the arrangement of
atoms
Thermodynamics
 Study of the energy transformations that
occur in a collection of matter
 First Law of Thermodynamics: energy is
constant—energy can be transferred and
transformed, but it cannot be created nor
destroyed
 Second Law of Thermodynamics: every
energy transfer makes the universe more
disordered or random, a.k.a. increases the
entropy (measure of disorder)
Entropy
 In most energy transformations, some of the
energy stored is converted to heat (the most
random - entropic - form of energy)
 Organisms are open systems and exchange
energy and materials with the surroundings—
taking in both organized and unorganized
forms of matter and energy and releasing
both into the environment
 Depletions of energy in organisms is due to the
loss as heat.
Put more simply…
Entropy
 “Living organisms preserve their internal order
by taking from their surroundings free energy,
in the form of nutrients or sunlight, and
returning to their surroundings an equal
amount of energy as heat and entropy.” Albert Lehninger
Cells maintain their orderliness by taking in highly
ordered things like light photons or polymers, and
discharging disorderly things.
Life makes its environment more disorderly, in
order to be orderly.
Free Energy
Free energy: The portions of a system’s
energy that is available to perform work when
temperature is uniform throughout the system
Not all of the energy in a system is available
for work
G= H – T S
G = Free Energy
H = Total Energy (of the system)
T = Temperature (in Kelvin)
S = Entropy
Discussion
∆G= ∆H - T ∆S
 ∆G= change in free energy, ∆H = change in total
energy, T = temperature, ∆S = change in entropy
 What happens to the amount of free energy
available if we…
 Increase the total amount of energy in the system?
 Increase the temperature of the system?
 Increase the entropy of the system?
 So, how can organisms use free energy to reduce
their entropy?
Energy Changes
∆G= ∆H - T ∆S
 For spontaneous, “downhill” reactions, ∆G
must be negative (∆G < 0). In other words, a
loss of free energy.
Free Energy and Metabolism
 Exergonic Reactions: (“energy
outward”) proceeds with a net
release of free energy. ∆G is
negative. Reactions are
spontaneous.
 Endergonic Reactions: (“energy
inward”) absorbs free energy from
its surroundings, stores free energy
in molecules.
∆G is positive. Reactions are
nonspontaneous. Require energy
to drive the reaction.
Energy Changes
Metabolic Disequilibrium
 Equilibrium: ∆G = 0. There is no net change in
the system. No work is being performed
 Reactions in closed systems will eventually
reach equilibrium
Living organisms are open systems, and maintain
disequilibrium by constantly flowing materials into
and out of the cell
Discussion
 Together with a partner, draw a picture or
pictures summarizing metabolism, energy,
and entropy
Catalysts
 Catalyst = chemical agent that changes the
rate of a reaction without being consumed
by the reaction
 Enzymes are biological catalysts, most often
made of protein (there are a few ribozymes
made of RNA)
Without enzymes, most bio reactions (even
spontaneous, exothermic reactions) proceed
VERY slowly.
Example: Leave a cracker out on the counter.
How long will it take for all the starch to turn to
sugar?
Activation Energy Barrier
 Chemical reactions involve forming and breaking of
bonds. Existing bonds in reactants must be broken
and new bonds of products formed.
 Breaking bonds requires
an input of energy
 The initial investment of
energy for starting a
reaction—energy required
to break bonds– is called
the activation energy or
free energy of activation
(EA)
Enzymes and Activation Energy
 Enzymes speed up reactions by lowering the
activation energy, i.e. the EA barrier, so the
transition state is within reach at moderate
temperatures.
 They do not change the ΔG of the reaction
Analogy: They don’t help the high jumper up,
they lower the bar
Enzymes are Substrate Specific
 The reactant a specific enzyme works on is
called a substrate
 Enzymes bind to their substrate(s) allowing
the catalytic action of the enzyme to create
the products
Substrate
Enzyme
Product
 Enzymes can distinguish their substrate by
shape. The substrate must “fit” into the
active site of the enzyme, a groove or
pocket in the protein

http://highered.mcgraw-hill.com/sites/0072495855/student_view0/chapter2/animation__how_enzymes_work.html
Enzyme-Substrate Cycle
Induced Fit
 Active sites are not rigid like a “lock-and-key”
but instead change shape slightly to fit snugly
around the substrate—like a handshake
 Induced fit brings chemicals together into
positions that enhance their ability to
catalyze
http://highered.mcgraw-hill.com/sites/0072507470/student_view0/chapter25/animation__enzyme_action_and_the_hydrolysis_of_sucrose.htm
Discussion
• When making jello with fruit in it, you
must be careful as it will not “gel” if
fresh pineapple is used, but it will gel
with canned pineapple. Fresh
pineapple contains the enzyme
bromelain which prevents proteins
from arranging into tertiary and
quaternary structures.
• Explain!
Discussion
• Papain is a hydrolytic enzyme that is
present in papaya. It is sold as a
component in powdered meat
tenderizer available in most
supermarkets.
• How might such powders make meat
more tender?
Environmental Effects on
Enzymes
 Enzymes have optimal conditions where they
work “best.” These tend to match the
environment (think evolution)
http://www.kscience.co.uk/animations/model.swf
Environmental Effects on
Enzymes
 Temperature: thermal agitation
can disrupt conformation.
Optimal temp allows greatest
number of molecular collisions
without denaturing
 pH: H+ concentration can also
disrupt conformation.
Denaturation
 Denaturation = The loss of a protein’s
secondary (tertiary, quaternary) structure by
the application of an external stress
Strong acids, strong bases, and high
temperatures cause denaturation
Warped protein shape -> Substrate cannot bind
to active site -> Function reduced or eliminated
http://highered.mcgrawhill.com/sites/0072507470/student_view0/chapt
er2/animation__protein_denaturation.html
Discussion
• Pepsin is a digestive enzyme that
functions in the stomach to break
down proteins, while salivary amylase is
an enzyme that functions in the mouth
to break down carbohydrates. Using
the following information, discuss the
answers to these questions…
Discussion
• What is the optimal pH for
pepsin? How does this
relate to its environment?
• What is the optimal pH for
amylase? How does this
relate to its environment?
• (Note: amylase breaks down
starch starting in the mouth,
continuing with the food bolus
through the esophagus,
stomach, and small intestine.)
Discussion
• Would you expect
carbohydrate breakdown
to be ongoing in the
stomach? Why/why not?
• Would you expect pepsin to
work in the intestine?
Why/why not?
Discussion
• When fruits & veggies are frozen, the water in
the vacuoles tends to expand and cause it
to burst. This releases a number of hydrolytic
enzymes and can cause the fruit to become
mushy.
• Fruits & veggies are often blanched (placed
in boiling water for a short time) before being
frozen to prevent this.
• Why does blanching help at all?
Discussion
• When slicing fruit, an enzyme called
catecholase causes a reaction
between catechol and oxygen. The
products formed by this reaction are
benzoquinone and water; since
benzoquinone has a brown color, this
results in the fruit browning.
• Browning can be prevented by adding
lemon juice to cut fruit. Why?
Cofactors
 Many enzymes require non-protein helpers
for catalytic activity, called cofactors which
are bound to the active site
They can be permanent or bind reversibly with
the substrate
Cofactors are inorganic such as iron, zinc, or
copper
 Coenzymes are organic cofactors
http://highered.mcgrawhill.com/sites/0070960526/student_view0/chapter
6/animations.html
Enzyme Inhibitors
 Certain chemicals selectively inhibit
the action of an enzyme by
covalently bonding to the active
site. Usually irreversibly.
 Competitive Inhibition: bind with the
active site, competing with the
substrate for access to the active site
 Can be overcome by increase the
concentration of the substrate
 Noncompetitive Inhibition: bind with
the enzyme outside of the active
site, changing the enzyme’s
conformation and impeding the
substrate binding
 Examples: poisons, antibiotics
Allosteric Regulation
 Reversible noncompetitive inhibitors are in
charge of most of the control of metabolism
 Regulatory molecules (activators or inhibitors)
bind at an allosteric site away from the
active site to turn on/off an enzyme’s activity
 Allosteric enzymes have multiple subunits
(polypeptide chains)
Feedback Inhibition
 Products of a pathway
can act as the allosteric
inhibitors and switch off
an enzyme in the
catabolic process
 Example: ATP is the
allosteric inhibitor for the
ATP-generating
catabolic pathway
http://highered.mcgrawhill.com/sites/0072943696/student_view0/chapter2/animation__feedba
ck_inhibition_of_biochemical_pathways.html
Cooperativity
 Substrate molecules can stimulate an
enzyme. Binding a substrate can induce the
enzyme to change into a shape which is
more favorable for binding at other sites
 Amplifies the response of enzymes to
substrates
Localization of Enzymes
 Organisms are more efficient
because they can keep all
the enzymes required for a
pathway in one place, organ
or organelle.
 Metabolic pathways can be
assembled together into a
multienzyme complex to keep
everything organized and
efficient
Discussion
 Work together with a partner to invent an
enzyme.
Determine what species it’s in
Determine its optimum environment
Determine what reaction it catalyzes, substrates
and products
Determine how it’s regulated. How does the
organism ensure that it’s only carrying out the
reaction when needed, and that it does so
efficiently?